Tropical Cloud Systems [In the ‘tradition’ of GCSS Plenary talks, a broad and misleading title]

Ed Zipser

University of Utah

AcknowledgementsResearch Associates:• Chuntao Liu (U of Utah); Baike Xi (U of Utah, now UND)Current and Former Students:• Haiyan Jiang (U of Utah, now GSFC/UMBC)• Yaping Li (U of Utah)• Steve Nesbitt (Texas A&M and U of Utah, now CSU),• Dave Yorty (U of Utah, now modifying the weather) • Dan Cecil (Texas A&M, now NASA MSFC/UAH)• Rick Toracinta (Texas A&M, now Ohio State U)• Karen Mohr, Chris Lucas, Brad Ferrier, Gary McGaughey, Robbie

Hood, Gerry Heymsfield, +participants in numerous field programsTRMM Program Scientists: Joanne Simpson, Chris Kummerow, Bob

AdlerErich Stocker and TSDIS Funding mostly NASA TRMM Science, also NASA CRYSTAL-FACE,

NASA Earth System Science Fellowships

Precipitation Features

What is a precipitation feature?

Contiguous area at least 4 pixels in size (75 km2) with:

PR “near surface” reflectivity ≥ 20 dBZ

(identify near surface rain)

orTMI 85 GHz PCT ≤ 250 K

(identify anvils and be consistent with previous work using the SSM/I, e.g. Mohr and Zipser

1996)

The TRMM Database (5 yr; 12M PFs)can be used as observational validation of

properties of rain-producing systems

• Where are the MCSs and how much rain do they contribute?• Where do small PFs contribute most? :Large PFs?• How much rain is contributed by PFs reaching 12 km? 15 km?

Where? • How much rain is contributed by PFs containing lightning?• How much rain falls from mesoscale PFs with strong convection?• How much rain falls from mesoscale PFs without strong convection?• What is the convective vs. stratiform partitioning of rain and how

does it vary around the globe?

• [….and how well do GCMs do answering these questions….?]

Fractional rain (%) contributed by large MPFs vs. small PFs

Very large

Small

Fractional rain (%) contributed by PFs with specified properties

Fractional rain (%) contributed by MPFs with 40 dBZ height </> 6 km

Fractional rain (%) contributed by MPFs with convective fraction </> 50%

Proxies for Convective Intensity

• Lowest IR brightness temperature - gives indication of cloud top height but poorly related to convective vertical velocity- might even be only thin cirrus

• Highest altitude with >20 dBZ radar echo - good• Highest altitude with >40 dBZ radar echo - better• Lightning flash rate - generally good (but indirect and also

may be related to aerosol properties of environment)• Lowest brightness temperature at 85 GHz or 37 GHz - good

How good? Which are best?

Next slides show global distribution of proxies for convective vertical velocity

Logarithmic color scale by rarity of event (i.e., black 10 X rarer than purple, purple 10 X rarer than orange, etc. (Cecil et al)

CDF of PFs by flash rate

Green: > 2/minuteOrange: > 30/minute Purple: > 120/minuteBlack triangle: > 300/minute

Each color step signifies that the event is 10 X rarer!

Why does convective vertical velocity matter?

• It matters a great deal in the microphysics of the mixed phase region

• Weak updrafts => low graupel mixing ratios, absence of large graupel, little electrification, low dBZ aloft

• Rain over tropical oceans comes from weak updrafts…..

• Really strong updrafts (undilute, adiabatic cores) may exhaust huge numbers of small ice crystals, with huge ice water content, at and above the tropopause

Data from oceans

Data from land

20°N-20°S Intensity Proxies: Deep tropics--no influence of intense subtropical systems

Total rainfall is not closely related to flash rate or other intensity proxies (but it is, of course, closely related to convective mass flux)

Challenge

• Challenge to modelers: Can we simulate convective intensity as well as rainfall?

• Opportunity: Seven years of TRMM observables are now available. Why not use them to evaluate models?

• Example: Distribution of overshooting radar tops

Overshooting PFs are defined using 5 different reference heights

(I): Constant 14 km (Alcala and Dessler, 2002)

(II): Potential temperature > 380 K

(III): NCEP reanalysis tropopause height

(IV): Level of Neutral Buoyancy (LNB) from NCEP sounding (using e surface) (V): Level of Neutral Buoyancy (LNB) from NCEP reanalysis

(using maximum of e 925mb and e 1000mb)

(Tropopause height and LNB for each one of 5 million PFs in 5 years are interpolated from 2.5o x2.5o, 6 hour interval NCEP reanalysis data.)

 

Location of overshooting towers(total # of PFs with 4 pixels between 20N-20S in 5 years: 5,208,813)

Overshoot distance > 2km (blue) >3km (red)

Diurnal variation of overshooting tower occurrence

Land

Ocean

Overshooting area distribution from 5 reference heights

14km

=380k

Tropopause

LNB (e 925&1000mb)

LNB (e surface)

Summary

• TRMM database is now 7+ years and may be as valuable for convective properties as for rainfall

• The longer TRMM stays in orbit, the more valuable the statistics become; e.g., more stable diurnal cycles, more sampling of rare events, …

• All proxies for convective intensity [w] show land much stronger than ocean, with intriguing regional differences

• Why are equatorial Africa storms stronger than Amazon storms?

• Stratospheric penetration of convection is greatest over Africa. Why? Significance for transport of (name it!)?

January 19, 2005 Department of Meteorology Seminar Series, University of Utah

Thank you!Thank you!

January 26, 2005 Department of Meteorology seminar series, University of Utah

Time-height maximum radarreflectivity and vertical velocity Kwajalein MCS of11-12 Aug. 1999

Max reflectivity

Max upward speed

dBZ

m/s

Time-height maximum radar reflectivity and vertical velocity

Time-height maximum radar reflectivity

Graupel is major contributor of reflectivityin the ice region

Max reflectivity from Rain Water

From Graupel

From Snow

Max reflectivity

dBZ

Kwajalein MCS of11-12 Aug. 1999

Time-height maximum graupelmixing ratio and vertical velocity

Max upward speed

Max Graupel mixing ratio g/kg

m/s

Kwajalein MCS of11-12 Aug. 1999

Comparison of the simulated results with actual observations

20 dBZ

40 dBZ

Maximum height of

Storm total CFADs of convective region reflectivity

(From Yuter et al. 2005)

Research radar observations

Kwajalein MCS of 11-12 August 1999

Comparison of the simulated results with actual observations

Maximum reflectivity at Minimum PCT of

85 GHz

37 GHz

6 km

9 km

Comparison of the simulated withthe actual TRMM observation statistics

Comparison of simulated & actual TRMM observation statistics