Clouds and Water Vapor in the Tropical Tropopause Transition Layerover Mesoscale Convective Systems

KATRINA S. VIRTS AND ROBERT A. HOUZE JR.

Department of Atmospheric Sciences, University of Washington, Seattle, Washington

(Manuscript received 1 May 2015, in final form 5 August 2015)

ABSTRACT

Observations from A-Train satellites and other datasets show that mesoscale convective systems (MCSs)

affect the water vapor and ice content of the tropical tropopause transition layer (TTL). The largest MCSs

with radar reflectivity characteristics consistent with the presence of large stratiform and anvil regions have

the greatest impact. Most MCSs are associated with clouds in the TTL. Composites in MCS-relative co-

ordinates indicate enhanced cloudiness and ice water content (IWC) extending toward the cold-point tro-

popause (CPT), particularly in large and connectedMCSs.Widespread anvils in the lower TTL are evident in

the peak cloudiness diverging outward at those levels. Upper-tropospheric water vapor concentrations are

enhanced near MCSs. Close to the centers of MCSs, water vapor is suppressed at TTL base, likely because of

the combined effects of reduced moistening or dehydration at the higher TTL relative humidities and sub-

sidence above cloud top. Weak moistening is observed near the CPT, consistent with sublimation of ice

crystals at the tops of the deepest MCSs. In the outflow region, moistening is observed in the lower TTL near

the largestMCSs. Enhanced water vapor in the upper troposphere and lower TTL extends beyond the area of

substantially enhanced cloudiness and IWC, in agreement with the observed radial outflow, indicating that

MCSs are injecting water vapor into the environment and consistent with the possibility that MCS devel-

opment may be favored by a premoistened environment.

1. Introduction

Deep convective clouds in the tropics sometimes ag-

gregate and develop extensive regions of stratiform pre-

cipitation and nonprecipitating anvil. Large, organized

cloud features of this type are called mesoscale convective

systems (MCSs; Houze 2014, chapter 9). Over half of

tropical precipitation and latent heating is produced by

MCSs (Yuan and Houze 2010, hereafter YH10). The la-

tent heating in MCSs is top heavy as a result of their sig-

nificant stratiform precipitation areas (Houze 1982, 1989),

and this top-heavy heating is important in forcing the

large-scale tropical circulation (Hartmann et al. 1984;

Schumacher et al. 2004). The net radiative heating inMCSs

is a positive maximum in the upper troposphere, with the

strongest heating near anvil cloud top (Houze 1982).

Observations from the CloudSat satellite, launched in

2006, have permitted the analysis of the vertical structure

of clouds around the world. Cetrone and Houze (2009)

and Yuan et al. (2011, hereafter YHH11) have used

CloudSat to examine characteristics of tropical MCS an-

vils. The distribution ofCloudSat reflectivities varies with

anvil thickness and distance from the MCS center and is

consistent with the detrainment of ice particles from the

MCS updrafts into the anvils and the sedimentation, or

gravitational settling and eventual removal, of the larger

particles as the anvil age increases. Both studies noted

contrasts between the anvil structures of land and oceanic

MCSs. LandMCSs exhibit broader and higher reflectivity

distributions consistent withmore intense convection and

less extensive stratiform rain areas, while oceanic MCSs

exhibit narrower distributions with modal intensity

strongly decreasing with height, consistent with the de-

velopment of broad stratiform areas.

MCS vertical structure in the troposphere has impli-

cations for the systems’ impacts on the overlying tropical

tropopause transition layer (TTL). The TTL extends

Corresponding author address: Katrina Virts, Department of

Atmospheric Sciences, University of Washington, 408 ATG Bldg.,

Box 351640, Seattle, WA 98195-1640.

E-mail: kvirts@uw.edu

Denotes Open Access content.

DECEMBER 2015 V IRT S AND HOUZE 4739

DOI: 10.1175/JAS-D-15-0122.1

� 2015 American Meteorological Society

from around 14km, just above the level of main convective

outflow, to the highly stable stratosphere above 18km

(Fueglistaler et al. 2009; Randel and Jensen 2013). Air be-

low the TTL undergoes net radiative cooling as it subsides

toward the ground, while air in the TTL tends to ascend

slowly while undergoing net radiative heating. The TTL

links the rising branch of the tropospheric Hadley circula-

tion, whose ascent is concentrated in areas of tropical moist

convection, and the broader rising branch of the strato-

spheric Brewer–Dobson Circulation. The TTL is charac-

terized by extremely low temperatures (Anthes et al. 2008;

Kim and Son 2012), and air ascending through the TTL

undergoes freeze drying, producing the low stratospheric

water vapor concentrations noted by Brewer (1949).

The mean level of neutral buoyancy (LNB) in con-

vective regions of the tropics lies between 12 and 14km,

just below TTL base (Takahashi and Luo 2014). Di-

vergent outflow from regions of tropical deep convection

occurs in the upper troposphere and at TTL base

(Highwood and Hoskins 1998; Folkins and Martin 2005).

Based on CloudSat observations of anvil characteristics,

Takahashi and Luo (2012) concluded that maximum

mass detrainment from convection is around 11km, just

below the LNB determined from atmospheric soundings.

The geographic distribution of cirrus in the upper tro-

posphere strongly resembles the distribution of tropical

precipitation, consistent with cirrus forming in and

spreading out from the upper portions of deep convec-

tion (Virts et al. 2010). Convectively generated cirrus

also occur in the TTL (Massie et al. 2002; Luo and

Rossow 2004), although in situ formation in ascending

layers becomes increasingly important at higher alti-

tudes (Riihimaki andMcFarlane 2010; Virts et al. 2010).

Trajectory analysis suggests that cirrus can persist up to

2 days after convection weakens and be advected up to

1000km from the region of the parent convection (Luo

andRossow 2004). As this advection occurs, sublimation

of the smaller ice crystals adds vapor to the environment

(Soden 2004).

Observations across the tropics indicate that some

deep convection overshoots its LNB and penetrates into

the TTL.Overshooting occurs over both ocean and land,

although the most vigorous convection is concentrated

over land (Alcala andDessler 2002; Liu andZipser 2005;

Zipser et al. 2006). Alcala and Dessler (2002) found that

approximately 5% of convective elements observed by

the Tropical Rainfall Measuring Mission (TRMM) ra-

dar penetrated into the TTL. Takahashi and Luo (2012)

reported that mean cloud-top height of the deepest

convective cores observed by CloudSat is 15 km, within

the TTL. However, less than 1% of deep convective

clouds reach the cold-point tropopause (CPT; Alcala

and Dessler 2002; Gettelman et al. 2002; Dessler et al.

2006). These observations suggest that tropical convec-

tion affects the moisture budget of the TTL.

Analysis of the impact of deep convection on water

vapor profiles in the TTL has addressed the fundamental

question of whether the net contribution is to moisten or

dehydrate the environmental air. Relative humidity with

respect to ice increases with height in the upper tropo-

sphere and TTL because of the decrease in saturation

mixing ratio in the ascending cold air, and supersatura-

tion frequently occurs in the TTL (Luo et al. 2007;

Fueglistaler et al. 2009). Previous studies have empha-

sized the importance of the ambient relative humidity

in determining the impact of convection: convection in

subsaturated regions moistens the environment, while in

supersaturated regions net dehydration is observed, be-

cause excess water vapor is deposited onto the ice parti-

cles, which may be removed via sedimentation (Jensen

et al. 2007; Wright et al. 2009; Hassim and Lane 2010).

Chae et al. (2011), analyzing water vapor anomalies from

the Microwave Limb Sounder (MLS) below the cloud-

top heights identified in data from the Cloud–Aerosol

Lidar and Infrared Pathfinder Satellite Observations

(CALIPSO) satellite, observed strong moistening in the

upper troposphere and decreased moistening above

TTL base, shifting to dehydration above 16km if the

tops of clouds were above this level, consistent with less

effective moistening with increasing background hu-

midity. They furthermore emphasized that the mecha-

nism they posited is not restricted to convective towers

and cirrus outflow but could apply to any cirriform

cloud; the mechanism could occur in situ or in the upper

reaches of stratiform or anvil tops of MCSs.

As mentioned above, a fraction of deep convective

clouds penetrate the CPT. The penetrating towers con-

tain extremely cold, dry air and have been hypothesized

as a mechanism for lower-stratospheric dehydration

(Danielsen 1993; Sherwood and Dessler 2000). How-

ever, modeling studies of such clouds indicate net hy-

dration because they leave behind small ice particles

that undergo sublimation in the relatively warmer

stratosphere (Chaboureau et al. 2007; Jensen et al.

2007). These modeling results have been confirmed by

observational evidence (e.g., Corti et al. 2008; Khaykin

et al. 2009). However, the horizontal scale of the pene-

trating cores of active convection is very small, and it

seems unlikely that they alone can account for the water

vapor properties of the TTL.

Rossow and Pearl (2007) concluded that most con-

vection penetrating near the CPT is associated with or-

ganized (i.e., large) MCSs; however, to date little

analysis has been attempted of MCS characteristics in

the TTL or how MCSs might affect the moisture content

of the TTL. Yet MCSs dominate much of the upper-level

4740 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

cloud coverage of the tropics. The stratiform portions of

MCSs are orders of magnitude larger in scale than the

convective towers embedded in the MCSs, and the strat-

iform regions are buoyant and have very cold tops rising

and diverging over wide areas, suggesting that they might

have a substantial effect on the moisture of the TTL. In

this paper, we investigate the water vapor and ice content

of MCSs in the upper troposphere and TTL to determine

the impact of these larger convective entities on the water

content at upper levels.

To identify MCSs, we use the dataset of YH10, who

introduced a methodology for identifying MCSs in ob-

servations from NASA’s A-Train satellites (L’Ecuyer

and Jiang 2010): high, cold cloud tops observed by the

Moderate Resolution Imaging Spectroradiometer

(MODIS) and precipitating areas observed by the Ad-

vanced Microwave Scanning Radiometer for Earth

Observing System (AMSR-E; see section 2 for details).

Other instruments aboard A-Train satellites observe

cloud vertical structure (CloudSat), including that of

optically thin cirrus layers (CALIPSO), and measure

profiles of ice water content (IWC) and water vapor

concentration in the TTL (MLS). Collectively, these

observations offer a unique opportunity to analyze the

vertical structure of MCSs and their contribution to

clouds and moisture in the TTL. We analyze these ob-

servations of water vapor and ice in the vicinity of each

MCS in the YH10 database.

The remainder of the paper is organized as follows: the

YH10 MCS identification scheme is described in section

2, along with the other satellite datasets used in this study.

The climatological distributions of MCSs and of clouds

and moisture in the TTL are shown in section 3. MCS

vertical structure and microphysical characteristics in

the troposphere are discussed in section 4, while the

distribution of clouds, ice, and water vapor in MCSs in

the TTL is presented in section 5. Conclusions are in

section 6.

2. Data

a. Observations from A-Train satellites

NASA’s A-Train constellation operates in a sun-

synchronous orbit, crossing the equator around 1:30

and 13:30 LT. The A-Train instruments used in this

study, including the YH10 MCS database derived from

observations from two of those instruments, are de-

scribed in this section.

1) MCS DATABASE

Clouds and precipitating areas are two prominent

MCS components readily observed by satellites. The

YH10 methodology begins with the identification of

these components in data from two instruments on the

Aqua satellite: MODIS 10.8-mm brightness tempera-

tures (TB11) and the AMSR-E AE-Rain product

(Kummerow et al. 2001; Wilheit et al. 2003; Kummerow

and Ferraro 2006). Their methodology is summarized in

Table 1.

YH10 then divide MCSs into two categories. Sepa-

rated MCSs (SMCSs) are those in which the pre-

cipitation feature (PF) containing the largest raining

core (RC) contains at most one other dominant RC of

any MCS. Long-lived MCSs in close vicinity sometimes

merge to form larger precipitating systems (Williams

and Houze 1987; Mapes and Houze 1993). This type of

system is designated by YH10 as a connected MCS

(CMCS), in which a rain area contains the largest RC of

at least three MCSs. SMCSs occur more frequently and

TABLE 1.Methodology used byYH10 to identify cloud and precipitation features andMCSs, based onMODISTb11 andAMSR-EAE_

Rain data. The cloud-top minimum temperature Tb11RC1min is defined as the mean Tb11 of the coldest decile of the largest raining core

(RC1). Adapted from YHH11.

Abbreviation Name Definition

HCC High cloud complex Region of MODIS Tb11 contained within a single 260-K isotherm

HCS High cloud system Portion of HCC associated with a particular minimum value of Tb11PF Precipitation feature Region of AMSR-E AE_Rain parameter surrounded by 1mmh21 contour

RC Raining core Portion of any PF overlapping and/or located within an HCS

HRA Heavy rain area Portion of PF with rain rate greater than 6mmh21

MCS Mesoscale convective

system

Any HCS whose largest RC satisfies the following criteria:

1) Exceeds 2000 km2 in total area

2) Accounts for more than 70% of the total area with rain rate greater than 1mmh21 inside

the HCS

3) Minimum cloud-top temperature above the RC (indicated by Tb11RC1min) is less than 220K

4) More than 10% of RC is occupied by HRAs

SMCS Separated MCS The largest RC of the MCS is part of a PF that contains less than three dominant RCs of any

MCS

CMCS Connected MCS The largest RC of the MCS is part of a PF that contains dominant RCs of at least three MCSs

DECEMBER 2015 V IRT S AND HOUZE 4741

exhibit a greater range of sizes than CMCSs, so they are

further subdivided into small SMCSs (the smallest 25%

by area, with HCSs less than approximately 11 000 km2)

and large SMCSs (the largest 25%, with HCSs greater

than approximately 41 000km2). In this study, we ana-

lyze all MCSs identified in the tropics (308N–308S)during 2007–10.

2) CLOUDSAT REFLECTIVITY

CloudSat carries a 94-GHz cloud profiling radar

(CPR) that measures backscatter from clouds (Stephens

et al. 2002; Marchand et al. 2008). The backscatter

profiles are available every 2.5 km along track and

have a vertical resolution of 240m at nadir. A cloud

mask is provided in the CloudSat geometric profile

product (2B-GEOPROF). In this study, observations

within clouds are identified by screening for cloud mask

values greater than or equal to 20.

TheCloudSat profiles analyzed in this study are those

identified by YHH11 as sampling some portion of an

MCS. In that study and in YH10, it was noted that

clouds with tops above 10 km identified by CloudSat

fall into two categories: precipitating clouds with bases

near the surface and elevated anvils with bases pri-

marily above the freezing level. Accordingly, they de-

fined anvil clouds as having base above 3 km and top

above 10 km. The same definition is used in this study.

We will also present statistics on CloudSat profiles

sampling nonanvil regions of the MCS. This category is

dominated by profiles sampling the deep convective or

stratiform precipitating areas of the system and is re-

ferred to herein as the precipitating category.

3) CALIPSO CLOUD OCCURRENCE

The Cloud–Aerosol Lidar with Orthogonal Polar-

ization (CALIOP) is a two-wavelength polarization

lidar carried aboard CALIPSO. Cloud layer base and

top heights are identified from the lidar backscatter

and are provided at a resolution of 60m in the vertical

in the TTL and 5 km along track. Following Fu et al.

(2007), opaque features (those that completely at-

tenuate the lidar signal) are assumed to be deep

convective clouds and assigned a cloud base at

Earth’s surface. While CloudSat’s estimated opera-

tional sensitivity (approximately 232 to 230 dBZ) is

insufficient to see clouds with small IWC, CALIPSO

can detect cloud layers with optical depths as low as

0.01 or less (Winker et al. 2007). In this study, we

take advantage of the complementary capabilities of

these instruments, using CloudSat to examine the

vertical structure of the deep convective clouds and

CALIPSO to analyze the extent of cirrus shields in

the TTL.

4) MLS ICE WATER CONTENT AND WATER VAPOR

The MLS aboard the Aura satellite observes thermal

microwave emissions in five spectral regions. We ana-

lyze MLS version 3 vertical profiles of IWC and water

vapor mixing ratio, whose validations are described by

Read et al. (2007) and Wu et al. (2008), respectively.

Profiles are provided at 1.58 intervals along track and

have vertical resolutions of 4 and 3 km, respectively,

and horizontal resolutions of approximately 7 km

across track and approximately 300 km along track.

The useful altitude ranges are from 215 to 83 hPa for

IWC and above 316 hPa for water vapor. The un-

certainties for water vapor are 20% below 100 hPa and

10% at 100 hPa and above, while the uncertainty for

IWC is a factor of 2, largely owing to the uncertainty

from particle size assumption (Jiang et al. 2012). The

data are screened following the recommendations in

Livesey et al. (2011).

MLS also observes atmospheric temperature, with

recorded biases of about 1.5K in the TTL (Schwartz

et al. 2008). For this paper, we apply an offset at each

pressure level based on comparison with global posi-

tioning system (GPS) temperature observations, as

given by Chae et al. (2011). The water vapor and cor-

rected temperature observations are then used to cal-

culate relative humidity with respect to ice (referred to

herein as relative humidity). Because it is dependent

upon the resolution of the temperature observations, the

vertical resolution of the relative humidity is coarser

than that of water vapor: approximately 5 km in the TTL

(Schwartz et al. 2008; Livesey et al. 2011).

b. ERA-Interim

The horizontal winds (u, y) and vertical velocities in

pressure coordinates (v) in this study are from the

European Centre for Medium-Range Weather Fore-

casts (ECMWF) interim reanalysis (ERA-Interim;

Dee et al. 2011). ERA-Interim fields are available four

times daily at 1.58 resolution for five pressure levels in

the upper troposphere and TTL (200, 175, 150, 125, and

100 hPa). The u and y fields are used to calculate the

radial wind—that is, the component of the horizontal

wind directed toward or away from the center of

each MCS.

c. Compositing technique

For the results presented in section 5, eachMCS in the

YH10 database is assigned a separate coordinate sys-

tem, where the origin corresponds to the center of the

largest raining core. Observations from CALIPSO,

MLS, and ERA-Interim are extracted and stored in

system-relative coordinates—that is, as a function of

4742 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

vertical height or pressure level and distance from the

MCS. For MLS and ERA-Interim, the dataset pressure

levels and horizontal sampling frequency (every 1.58)are retained. As described earlier, CALIPSO observa-

tions are available at higher spatial resolution; however,

in order to reduce noise, we calculate cloud occurrence

for 0.25 km vertical and 18 horizontal bins. No further

spatial filtering is applied to the data. When indicated,

anomalies are calculated by subtracting the climatolog-

ical monthly mean at each location prior to compositing.

3. Climatology of MCSs and the TTL

The annual distribution in Fig. 1 indicates that all

three types of MCSs are observed throughout the deep

convective regions of the tropics—the tropical conti-

nents, the intertropical convergence zones (ITCZs), and

ubiquitously over the western Pacific warm pool. How-

ever, as shown by YH10, both large and small SMCSs

are more frequently observed over land or near-coastal

regions, while the merging of systems to produce

CMCSs occurs mostly over the oceans, especially the

warm pool.

Annual-mean distributions of clouds and moisture at

147hPa, near TTL base, are shown in Fig. 2. Maxima in

each field are located over the regions of intense

convection over the tropical continents and the warm

pool. Clouds and IWC are more strongly focused in

areas with high annual-mean precipitation and frequent

MCS occurrence (Fig. 1). Water vapor and relative hu-

midity exhibit more diffuse distributions, indicating the

advection of water vapor beyond the areas of frequent

deep convection as well as the effect of the large-scale

temperature field.

The climatological, zonal-mean vertical structure of

tropical clouds and moisture is shown in Fig. 3. In the

upper troposphere, clouds and ice are most frequent

around 78N—the latitude of the rising branch of the

annual-mean Hadley circulation. A secondary maxi-

mum in IWC is observed south of the equator, associated

with convection in the austral summer Hadley cell. IWC

concentrations decrease with height above ;170hPa,

while cloud occurrence broadens and becomes more

equatorially symmetric in the TTL, where cirrus can

form either through the shearing off of convective anvils

or in situ in rising, adiabatically cooling layers.

Water vapor concentrations decrease by about three

orders of magnitude from the upper troposphere to the

lower stratosphere (Fig. 3c). This strong decrease with

height reflects the temperature dependence of the

saturation mixing ratio, particularly at the low tem-

peratures observed in the TTL. The decrease in the

FIG. 1. Density of (top) small SMCSs, (middle) large SMCSs, and (bottom) CMCSs for the

years 2007–10, expressed as the number of systems observed in some portion of a 18 3 18 gridbox per year (note that the color scales differ).

DECEMBER 2015 V IRT S AND HOUZE 4743

infusion of water vapor into the environment by con-

vection also contributes to the water vapor distribution at

higher altitudes. Despite the low water vapor concen-

trations in the TTL, relative humidities are high (Fig. 3d),

with zonal-mean values in excess of 80% because of the

extremely low temperatures. The high relative humidities

in the TTL promote cirrus occurrence.

Before considering the contribution of MCSs to

clouds and water vapor in the TTL, it is useful to analyze

their characteristics in the troposphere, specifically their

vertical structure and microphysical attributes. In the

following section, these characteristics are investigated

using CloudSat observations.

4. Vertical structure of MCSs in the troposphere

The distribution of CloudSat reflectivities in MCSs

as a function of height can be represented using con-

toured frequency by altitude diagrams (CFADs; Yuter

and Houze 1995; Masunaga et al. 2008). Cloud micro-

physical processes can then be inferred from the re-

flectivity distributions. While YHH11 analyzed CFADs

of anvil characteristics as functions of geographic re-

gion, anvil thickness, and distance from theMCS center,

we examine reflectivity characteristics for both the

precipitating portions and the nonprecipitating anvils of

each category of MCS (Fig. 4). The primary features of

the CFADs are shared by each type of MCS: in the

precipitating areas, peak reflectivities of 10–15 dBZ are

observed in the melting layer (around 4–5 km). Re-

flectivities decrease below the melting layer as a result

of signal attenuation by the precipitation. High re-

flectivities dominate the distribution through most of

the troposphere, indicating the presence of large drop-

lets in the convective updraft below the melting layer

and the formation of graupel and large ice particles

above. In the upper troposphere, above 9–10 km, re-

flectivities generally decrease with height because of

FIG. 2. Annual-mean (a) CALIPSO cloud fraction, (b) MLS IWC (mgm23), (c) MLS water

vapor mixing ratio (ppmv), and (d) MLS-relative humidity with respect to ice (%) at 147 hPa.

Relative humidity values have been corrected for MLS temperature bias [see section 2a(4)].

4744 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

lower concentrations of particles of generally smaller

dimension.

As air from the MCS raining cores is incorporated

into the anvils, microphysical processes modify the

characteristics of the ice particles. Reflectivities in MCS

anvils are lower, generally below 10dBZ (Fig. 4). Peak

reflectivities are observed in the lower portions of MCS

anvils, around 6–10km, and the median reflectivity de-

creases with height to approximately 220dBZ near

13 km. Vertical velocities in anvils are generally weak,

such that ice particles tend to drift downward (Houze

2014, chapter 6). The presence of larger particles near

anvil cloud base reflects this sedimentation, as well as ice

particle growth by aggregation (McFarquhar and

Heymsfield 1996). YHH11 further noted weaker re-

flectivities and a narrowing of the reflectivity distribu-

tion in anvils as distance from the MCS core increased,

consistent with aging.

While the CFADs for the three MCS categories share

these basic similarities, key differences also exist. In the

raining cores of small SMCSs, the reflectivity distribu-

tion exhibits a more vertical orientation, with median

reflectivities remaining high (greater than 0dBZ) up to

13 km. In contrast, in large and connected MCSs, re-

flectivities decrease more rapidly with height. To more

clearly contrast the MCS types, difference plots are

shown in Fig. 5. These plots were constructed by sub-

tracting the CFADs for small SMCSs from those for

large SMCSs, such that blue shading indicates the re-

flectivities proportionately more likely to be observed in

small systems. In Fig. 5, above the melting layer, high

reflectivities are more commonly observed in small

SMCSs, indicative of strong convection and less de-

veloped stratiform regions. The reverse is observed

above the melting layer in large SMCSs, where the

narrower reflectivity distribution is consistent with the

presence of stratiform rain areas. A narrower distribu-

tion is also observed in the anvils of large SMCSs com-

pared to those in small SMCSs, as shown by the pattern

of blue shading in Fig. 5b. The greatest contrast is ob-

served in the occurrence of high reflectivities above

9 km, which are much more common in the small

SMCSs, as the larger particles formed in the intense

convective updrafts find their way into the anvils of the

small MCSs.

5. MCS impacts on the TTL

Having examined MCS vertical structure in the tro-

posphere, we now investigate the impact ofMCSs on the

overlying TTL. In this section, we present analyses of

clouds, wind, IWC, and water vapor in the vicinity of the

MCSs, with the variables composited in system-relative

coordinates as described in section 2c.

a. Clouds and wind

Cross sections of composited cloud fraction in the

upper troposphere and TTL near MCSs are shown in

FIG. 3. Annual-mean, zonal-mean (a) CALIPSO cloud fraction,

(b) MLS ice water content (mgm23), (c) MLS water vapor mixing

ratio (ppmv; note the log scale), and (d) MLS-relative humidity

with respect to ice (%). Relative humidity values have been cor-

rected for MLS temperature bias [see section 2a(4)]. For (a), the

vertical coordinate is given in both pressure (hPa) and altitude (km,

rounded to the nearest 0.5 km).

DECEMBER 2015 V IRT S AND HOUZE 4745

the left column of Fig. 6. The basic cloud distribution is

similar for all MCS types: while the largest cloud frac-

tions are observed lower in the troposphere, cloud oc-

currence is frequent in the upper troposphere near the

MCS center. About 74% of CALIPSO profiles within

100 km of the center of an MCS observed a cloud top

above 150 hPa, at TTL base, and about half of such

layers were opaque to the lidar (not shown). A cloud

top above 120 hPa was observed in about 47% of the

profiles. Above that level, cloud occurrence decreases

more rapidly with height. Beginning within about

200 km of the MCS center and extending farther away,

an outward bulge in cloud fraction indicates the

spreading of cirrus anvils. This bulge is centered

;170 hPa near the MCS core and shifts upward to

;150 hPa at greater distances.

Large and connected MCSs are associated with sub-

stantially greater cloud fractions in the upper tropo-

sphere and TTL than small SMCSs, particularly in the

area within a few hundred kilometers of theMCS center,

where the probability of CALIPSO observing a cloud in

the upper troposphere is almost double near a large

SMCS compared to a small SMCS. These differences

lessen with distance, and at 1500 km from the MCS,

composite cloud fractions are;0.2 in each case. At such

large distances, CALIPSO is likely observing neigh-

boring clouds. To reduce the impact of clouds that may

not be part of the MCSs, we subtract the climatological

monthly mean cloud fraction at each grid point prior to

compositing. Anomalous cloud fractions for each MCS

type are shown in the right column of Fig. 6. Here, the

contrast between small SMCSs and large and connected

MCSs is even more striking. Enhanced cloudiness near

small SMCSs is observed through the depth of the TTL

but is confined to within about 200 km of the MCS

center, and the magnitude of the anomalies is small: less

than 0.1. In large and connected MCSs, however,

cloudiness anomalies are larger and extend higher in the

TTL. This result is in agreement with Rossow and Pearl

(2007), who reported an increasing likelihood of con-

vective intrusions of the TTL with increasing cloud

system size and that such intrusion was observed in al-

most all systems with radii . 500km. We also observe

enhanced cloud occurrence to distances of 600 km and

FIG. 4. CFADs of CloudSat reflectivities in (a),(d) small SMCSs, (b),(e) large SMCSs, and

(c),(f) CMCSs, using profiles sampling (top) the precipitating portions of the MCSs and

(bottom) the anvil portions. Each CFAD is normalized such that its maximum value is 1.

4746 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

beyond in large and connected MCSs. The outward

bulge in the anomaly patterns is at a slightly higher al-

titude than that in the composites in Fig. 6a, indicating

that the MCS anvil cirrus and outflow are slightly higher

than what is typically observed in the tropics.

As discussed in section 1, various studies have ana-

lyzed how frequently deep convection reaches into the

tropical stratosphere. Our analysis indicates that 11%

of CALIPSO profiles within 100 km of the center of an

MCS indicate a cloud top above 95 hPa, near the CPT,

and approximately 1% indicate a cloud top above

80 hPa, near the top of the TTL. Of the cloud layers

with tops above 95 hPa within 100 km of the center of

an MCS, 65% have a sufficiently low optical depth that

they are not opaque to the lidar. These nonopaque

layers have a median thickness of 1.8 km (not shown).

Thus, while some portion of convection in MCSs ex-

tends into the lower stratosphere, the majority of the

extremely high-topped clouds observed by CALIPSO

near MCSs are thin anvil clouds or cirrus layers over-

lying the deep convective clouds, as discussed in

Garrett et al. (2004).

The overlaid vectors in Fig. 6 show the radial winds

and vertical velocities fromERA-Interim. Strong ascent

is observed in the convective core of the MCS, and its

magnitude is underestimated as a result of the relatively

coarse resolution of the reanalysis data. The vertical

velocities weaken rapidly with height in the stably

stratified TTL. Upper-level divergence and outflow

from the MCS are reflected in the outward direction of

the composite radial winds. A similar pattern is ob-

served in the anomaly wind fields. Large and connected

MCSs are associated with both stronger and broader

ascent than small SMCSs, and their anomalous di-

vergence extends into the cloud-free environment.

b. Ice

Ice associated with MCSs can be derived from MLS

observations, and IWC anomalies calculated as de-

scribed in section 2c are shown in Fig. 7. IWC anomalies

for the subset of levels above TTL base are shown in the

right column. The largest ice concentrations and

anomalies in these cross sections are in the upper tro-

posphere near the MCS convective cores and extend

upward into the TTL. Large and connected MCSs inject

larger quantities of ice into the upper troposphere and

TTL than small SMCSs. IWC decreases with distance

from the MCS core, indicating the removal of larger ice

particles via sedimentation and/or sublimation in the

divergent airflow associated with the stratiform and

anvil regions. While peak enhanced cloudiness in the

MCS outflow lies at or above TTL base, as illustrated by

the overlaid cloud fraction anomalies, the largest IWC

values are at the bottom of the cross section. This is

partly a result of the relatively coarse vertical resolution

of the IWC data. In addition, CALIPSO’s sensitivity

enables it to detect cirrus layers with IWC too small to

be detected by MLS.

c. Water vapor

As shown in Fig. 2, water vapor near TTL base is more

broadly distributed than ice. The strong decrease of

water vapor concentrations with altitude in the TTL

dominates cross-sectional composites of water vapor

relative to MCS location (not shown). It is more in-

formative to present the composites as anomalies from

the climatological monthly mean (Fig. 8). Each MCS

type is associated with enhanced water vapor in the

upper troposphere. The anomalies decrease with dis-

tance from theMCS but extend radially outward beyond

the area of substantially enhanced cloudiness or IWC

(cf. Fig. 7). As noted above, the anomalous upper-

tropospheric divergence also extends beyond the

cloudy region, particularly in large and connected

MCSs. The true strength and extent of the MCS outflow

can only be estimated from the reanalysis winds. Be-

cause A-Train orbits repeat only every 16 days, we also

cannot determine whether premoistening of the upper

troposphere occurred prior to MCS formation. How-

ever, the results in Fig. 8 indicate that water vapor

spreads laterally from the MCSs and into the sur-

rounding cloud-free region prior to MCS dissipation.

Previous studies have shown that diverging cirrus can

persist for up to 2 days after convective dissipation and

FIG. 5. Difference between CFADs for large and small SMCSs,

using profiles sampling (a) the precipitating portions of the MCSs

and (b) the anvil portions. Blue shading indicates where reflectivity

values are proportionately more likely to be observed in

a small SMCS.

DECEMBER 2015 V IRT S AND HOUZE 4747

be advected up to 1000km and that elevated upper-

tropospheric water vapor concentrations persist longer

than the cirrus (Luo and Rossow 2004).

Water vapor values in the TTL are too small to be seen

in the left column of Fig. 8, which is dominated by the

moister upper troposphere. Anomalies for the subset of

altitudes in the TTL (above ;150hPa) are shown in the

right column of Fig. 8. Contrasting characteristics are

observed near the MCS centers (within a radius of about

200km) and at distances greater than about 400km. Near

the convective core, water vapor concentrations are be-

low the climatological mean in the lowermost TTL, par-

ticularly in large and connected MCSs. This altitude can

be above, at, or below the cloud top, as indicated by the

overlaid cloud fraction anomalies and the discussion in

section 5a. Chae et al. (2011) noted rapidly decreasing

moistening within clouds above about 14km and weak

dehydration just below cloud-top level when the cloud

extended above 16km, as the environmental relative

humidity became supersaturated. They also noted strong

dry anomalies in the lower TTL when those altitudes lay

just above the convective cloud tops, where subsidence

occurs in response to strong divergence in the cloud

outflow. They point out that these water vapor patterns

are observed with any type of cloud in the TTL: deep

convective, convectively generated cirriform, or in situ

cirriform. It is therefore reasonable that the processes

they describe should be associated with the MCSs dis-

cussed herein, thus producing the negative water vapor

anomalies in the lowerTTL inFig. 8. The cirriformupper-

level clouds of MCSs and their anvils are produced by the

combination of convective and mesoscale dynamics that

characterize MCSs (Houze 2004, 2014, chapter 9). We

further observe weak moistening in the upper TTL above

theMCS centers, consistent with the observation of Corti

et al. (2008) that the deep clouds that extend upward

beyond the CPT leave behind ice particles which sub-

limate and moisten the environment.

Outside the convective core, at distances of about

400 km from the MCS center and extending outward,

enhanced water vapor concentrations are observed in

the lower TTL as well as the upper troposphere (Fig. 8),

indicating that outflow from the MCSs and evaporative

processes are moistening the environment. The water

FIG. 6. (left) Composite CALIPSO cloud fraction [colors and contours; contour interval (CI) 5 0.1] and ERA-

Interim vertical velocity and horizontal radial wind (vectors) in the upper troposphere and TTL as a function of

distance from MCS center, for (a),(b) small SMCSs, (c),(d) large SMCSs, and (e),(f) CMCSs. (right) Anomalies of

cloud fraction (CI 5 0.05) and wind relative to the MCS center.

4748 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

vapor anomalies in this region decrease more rapidly

with height than the environmental water vapor (cf. with

Fig. 3), consistent with decreased convective influence

and less effective moistening in the TTL. The moisten-

ing appears to extend upward to ;120hPa; however,

observations with finer vertical resolution would be

needed to examine the precise vertical extent of the

moistening. As in the upper troposphere, enhanced

water vapor in the TTL extends laterally into cloud-free

regions. Weak dry anomalies are observed near the

CPT, near the tops of the highest clouds (cf. with Fig. 6).

The water vapor anomaly pattern near small SMCSs

exhibits different characteristics than that for large and

connected MCSs. Primarily, it is less robust, indicating

that it is the largest MCSs that are important to the

moistening of these upper levels. Positive anomalies

associated with the small SMCSs are observed through

most of the TTL, with peak values at 121 hPa. Laterally,

the anomalies extend beyond the narrow core of en-

hanced cloudiness and several hundred kilometers into

the region of suppressed cloudiness (i.e., beyond the red

line in Fig. 8b). As previously noted, the radial outflow

associated with small SMCSs is weak; hence, it is un-

realistic to interpret the anomaly pattern in Fig. 8b as

moistening of a broad region by the smallest MCSs.

Rather, it appears that small SMCSs tend to form in

large-scale environments with a slightly moistened TTL.

6. Conclusions

In this study, the vertical structure and microphysical

composition of MCSs, as well as their role in detraining

ice particles and water vapor in the TTL, have been

investigated using a suite of satellite and reanalysis ob-

servations. We have examined three types of MCSs, as

identified in observations from MODIS and AMSR-E

(YH10): small and large separatedMCSs and connected

systemsmostly likely formed via themerging of multiple

MCSs. MCSs are ubiquitous in the convective regions of

the tropics (Fig. 1), including East Asia, the western

Pacific warm pool, and Central and South America,

which have been identified as major source regions

for air parcels eventually entering the stratosphere

(Schoeberl et al. 2013). SMCSs occur more frequently

over the tropical landmasses, while the merging of

longer-lived systems to form CMCSs is more common in

the oceanic ITCZs and over the western Pacific warm

pool (Fig. 1).

FIG. 7. (left) CompositeMLS ice water content anomalies (colored shading; mgm23) as a function of distance from

MCS center, for (a),(b) small SMCSs, (c),(d) large SMCSs, and (e),(f) CMCSs, overlaid with cloud fraction (contours;

CI5 0.05) and circulation anomalies (vectors) from Fig. 6. (right) As in (left), but for subset of altitudes in the TTL.

DECEMBER 2015 V IRT S AND HOUZE 4749

CloudSat reflectivity distributions in the precipitating

regions of active MCSs indicate high reflectivities in the

midtroposphere, indicative of large ice and graupel

particles forming in the convective updrafts and evolv-

ing into stratiform precipitation as the updrafts weaken

or being detrained into neighboring stratiform regions

of theMCSs (Fig. 4). Reflectivities in theMCS anvils are

generally lower and decrease rapidly with altitude in the

upper troposphere, consistent with sedimentation and

aggregation within the anvils and the removal of larger

graupel prior to air entering the anvil, as previously

shown by Cetrone andHouze (2009) and YHH11. Small

SMCSs exhibit characteristics of intense, young con-

vection and are consequently more likely than large and

connected MCSs to contain large ice and graupel par-

ticles in the upper troposphere (Fig. 5). Larger MCSs

exhibit narrower, more strongly tilted reflectivity dis-

tributions in the upper troposphere, consistent with the

‘‘size sorting’’ processes characteristic of mature anvils

(McFarquhar and Heymsfield 1996).

Composites of cloud occurrence observed by

CALIPSO near MCSs (Fig. 6) reveal that cloudiness

anomalies in small SMCSs are primarily vertically ori-

ented and confined near the MCS cores, while large and

connected MCSs exhibit not only broader core regions

but also a prominent outward bulge in cloud occur-

rence near TTL base associated with anvil cirrus. Most

MCSs are associated with clouds in the TTL: 74% of

CALIPSO profiles within 100 km of the center of an

MCS exhibited a cloud top above TTL base (about

150 hPa; Fig. 6). Large and connected MCSs are asso-

ciated with more frequent cloud occurrence in the TTL

than small SMCSs, in agreement with the inference of

Rossow and Pearl (2007) that the likelihood of cloud

penetrating upward into high levels increases with

system size. Rossow and Pearl (2007) and Takahashi

and Luo (2014) both report that penetration of the TTL

tends to occur in the growing stage of the cloud system,

although the latter noted significant penetration during

the mature stage. Our results associate the most ex-

tensive cloudiness in the TTL specifically with large

MCSs, in which anvils and overlying cirrus occur in

greater quantities. ‘‘Overshooting’’ convective ele-

ments are actually less likely in these more widespread

MCSs; for example, Virts and Houze (2015) showed

that the larger MCSs have less vigorous convective

elements, as indicated by lightning occurrence. It is the

combination of convective and stratiform dynamics of

MCSs that make them robust in affecting cloudiness in

the TTL.

FIG. 8. As in Fig. 7, but colored shading is MLS water vapor mixing ratio anomalies (ppmv).

4750 JOURNAL OF THE ATMOSPHER IC SC IENCES VOLUME 72

MCSs inject ice into the upper troposphere and TTL,

as shown by composites of anomalous MLS IWC with

respect to MCS location (Fig. 7). The largest ice quan-

tities are near the centers of the MCSs, but elevated ice

concentrations also extend laterally into the stratiform

and anvil regions. Large and connected MCSs contrib-

ute the greatest quantities of ice to the TTL. These

systems contain predominately smaller ice crystals and

less graupel than the small SMCSs (Fig. 5). The less

rapid sedimentation rates of the smaller particles allow

them to remain aloft in the stratiform and anvil regions.

We note that the IWC anomalies decrease nearly

monotonically with height in the upper troposphere and

TTL, while the cloud occurrence exhibits an outward

bulge associated with the spreading anvils. This differ-

ence is largely due to the coarser vertical resolution of

the IWC data (approximately 4 km in the TTL;Wu et al.

2008) but is also consistent with lower ice concentrations

in the higher cirrus layers observed by CALIPSO.

Composites based on MLS water vapor mixing ratios

(Fig. 8) demonstrate that MCSs moisten the upper tro-

posphere. Moreover, stronger moistening is associated

with the large and connected MCSs. In the TTL, a

complex structure is observed that is consistent with

previous studies of dehydration and hydration at upper

levels. Near the MCS cores, water vapor concentrations

are suppressed near TTL base. Higher relative humidity

in the TTL (Fig. 3) reduces the possible amount of

moistening, and subsidence above cloud top has been

observed to produce strong drying in the lower TTL, as

shown by Chae et al. (2011). Weak moistening is ob-

served in the upper TTL, near the CPT, as the deepest

penetrating convection leaves behind small ice crystals

that sublimate andmoisten the environment. A reversed

pattern is observed from approximately 400km out-

ward, where the spreading anvils of large and connected

MCSs moisten the lower TTL, and weak drying is ob-

served at the CPT. The water vapor and radial wind

anomalies in the upper troposphere and lower TTL ex-

tend into the cloud-free areas, indicating that the MCS

outflow is moistening the environment. We also em-

phasize the possibility that a premoistened environment

favors the development of mature MCS stratiform and

anvil regions, but this cannot be investigated here owing

to the limitations of satellite data.

Our conclusion that large and connected MCSs more

strongly moisten the upper troposphere and lower TTL

than small SMCSs contrasts with the notion that

moistening of the TTL is primarily caused by isolated

overshooting convective cells. The area covered by

overshooting convective cells is many orders of magni-

tude smaller than the area occupied by MCSs. Our re-

sults indicate that the largest, mature MCSs are of

greater importance than isolated overshooting cells be-

cause they are large enough in horizontal scale to be

highly impactful on the water vapor field. Quantification

of the comparative contributions of convection of vari-

ous sizes and organization to the total water vapor

content of the TTL and lower stratosphere is needed but

is not straightforward given the water vapor data cur-

rently available.

MCS evolution cannot be examined using A-Train

satellite observations, which provide only snapshots in

time. However, the results presented in this paper sug-

gest that small SMCSs are primarily young, developing

systems, while large and connected MCSs represent the

mature stage of the MCS life cycle (Houze 2004, 2014,

chapter 9). Small SMCSs contain vigorous convection

that produces large ice particles, especially graupel.

These systems have limited stratiform and anvil regions,

but the anvils that do exist contain large particles de-

trained from the convective core. Water vapor also de-

trains from the convective region, moistening the upper

troposphere, while reduced moistening in convection

penetrating the TTL combined with subsidence above

cloud top produces negative water vapor anomalies near

TTL base. As MCSs mature (and in some cases merge),

their convection becomes less vigorous, resulting in re-

duced ice and graupel size. This ice is incorporated into

the extensive stratiform and anvil regions (either by

detrainment or by cells weakening and becoming strat-

iform), where it undergoes sedimentation and aggrega-

tion, such that smaller ice particles dominate near cloud

top. At this stage, MCSs are characterized by extensive

upper-level divergence. Anvils extend radially outward

via this divergence, with peak cloud occurrence near

TTL base shifting to slightly higher altitudes with dis-

tance from the MCS core. The MCSs thus moisten the

upper troposphere via sublimation, and water vapor is

detrained laterally into the cloud-free air. At larger

distances from the convective core, where the above-

cloud subsidence is weaker, this moistening extends into

the TTL.

Our discussion has focused on contrasts between

small SMCSs and large and connected MCSs. More

subtle differences exist between large SMCSs and

CMCSs. As shown in Fig. 1, large SMCSs occur more

often over or near land, while most CMCSs are observed

over the oceans. Thus, in a broad sense, differences

between these two MCS categories represent the dif-

ferences between the largest MCSs over land and

oceans. Examining Figs. 4 and 6–8, it can be seen that

compared to large SMCSs, CMCSs are slightly larger

(Yuan and Houze 2013), exhibit somewhat stronger

outflow in the upper troposphere, and are associated

with larger ice and water vapor anomalies. However,

DECEMBER 2015 V IRT S AND HOUZE 4751

these differences are slight, and our results indicate that

mature MCSs, whether they form over land or ocean,

are major contributors to water vapor in the upper tro-

posphere and TTL.

Acknowledgments. The authors thank three reviewers

for their helpful suggestions and Beth Tully for processing

the graphics. This work was supported by the National

Aeronautics and Space Administration (Grants

NNX10AM28G and NNX13AQ37G) and the U.S. De-

partment of Energy (Grant DE-SC008452). CALIPSO

data are available from the NASA Langley Research

Center Atmospheric Science Data Center (http://www-

calipso.larc.nasa.gov/), CloudSat data are available from

the CloudSat Data Processing Center (http://www.

cloudsat.cira.colostate.edu/), MLS data are available

from the Goddard Earth Sciences (GES) Data and In-

formation Services Center (DISC; http://mirador.gsfc.

nasa.gov/), ERA-Interim fields are available from the

European Centre for Medium-RangeWeather Forecasts

(http://ecmwf.int), and MODIS and AMSR-E data are

available from the NASA data centers (http://ladsweb.

nascom.nasa.gov/ and http://nsidc.org/data/amsre).

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