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Identification of atmospheric boundary layer height over a tropical

station using high-resolution radiosonde refractivity profiles:

Comparison with GPS radio occultation measurementsGhouse Basha1 and M. Venkat Ratnam1

Received 31 December 2008; revised 1 April 2009; accepted 29 May 2009; published 20 August 2009.

[1] In this study, long-term (2.5 years) observations of a high vertical resolutionradiosonde are used for the first time to identify the atmospheric boundary layer (ABL)height over a tropical station, Gadanki (13.5N, 79.2E). An alternative method ofdetecting ABL height from refractivity (N) profiles is proposed, which includes bothtemperature and water vapor information, and several advantages were found. Theidentified height using Nis compared with that detected by traditional methods likepotential, virtual potential temperature, and mixing ratio during different backgroundmeteorological conditions. Very good correlations in all weather conditions indicate thatN

can also be used as an indicator for detecting the ABL height. The ABL height thusobtained is compared with independent measurements ofN from the ConstellationObserving System for Meteorology Ionosphere and Climate GPS radio occultation (RO),and very good correlation is found between the two. ABL height is found to be higherduring premonsoon, followed by monsoon and postmonsoon, and is minimum in winter.In addition, radiosondes launched four times a day during different seasons have beenused to study the diurnal variation. These results were compared with GPS RO datacollected during different times in a day for a given season, and very strong diurnalvariation was found. For studying the global distribution of ABL height from GPS ROdata, it is suggested that one considers the GPS RO data for a fixed time or range of time(as RO data are very sparse in tropical regions) and the profiles reaching down to 0.5 km,particularly during nighttime.

Citation: Basha, G., and M. V. Ratnam (2009), Identification of atmospheric boundary layer height over a tropical station using high-

resolution radiosonde refractivity profiles: Comparison with GPS radio occultation measurements,J. Geophys. Res., 114, D16101,doi:10.1029/2008JD011692.

1. Introduction

[2] The atmospheric boundary layer (ABL) is the lowestlayer of the troposphere that is directly influenced by theEarths surface [Garratt, 1992]. Substances emitted into theABL are gradually dispersed horizontally and verticallythrough the action of turbulence and finally become mixedover this layer, and it has become essential to monitorABL height on a day-to-day basis for air pollution meteo-rology [Seibert et al., 2000]. It is important to determine the

ABL height for understanding the transport process in thetroposphere, weather prediction, and climate monitoring[Garratt, 1993].

[3] Several remote sounding systems like light detectionand ranging (lidar), sound detection and ranging (sodar),radio acoustic sounding system (RASS), and wind profilingradars are used to determine the ABL height. Lidars allowthe direct measurement of aerosols and trace gas profiles,

since the top of the well-mixed layer is often associated withstrong gradients of aerosols concentration. This stronggradient in aerosols is taken to mark the top of the ABL.However, interpreting aerosol data from lidars is notstraightforward because the detected aerosol profile is notalways the result of ongoing vertical mixing but mayoriginate from advection transport [Russell et al., 1974].

[4] In the case of sodar [Beyrich, 1997] the maximumdetectable height is limited to about 1 km and is alsosensitive to environmental noise. Wind profiling radar can

provide a direct and continuous measurement of convectiveboundary layer height [Kumar and Jain, 2006] but havelimited height and range resolution, and also the demarca-tion of ABL height may not be clear at all times in the wind

profile. RASS [Gorsdorf and Lehmann, 2000] can providevirtual temperature and requires information on humidity

profile for identifying ABL height from it.[5] ABL height can be determined from direct measuring

techniques like radiosondes and tethered balloons. Tetheredballoons can provide information on turbulence and tracegas concentration profile, but are limited to field campaignsonly and the range is limited to below 500 m. In case of

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radiosonde, the ABL height can be determined by temper-ature inversions, sharp decreases in humidity, wind speed,and rate of ascent [Johansson and Bergstro, 2005]. A clear

jump in ascent rate at the ABL height in case of strongturbulence was reported. Several studies have shown itsability to determine the height of the ABL [Sempreviva andGryning, 2000;Gryning and Batchvarova, 2002;Johanssonet al., 2001]. Although many definitions exists to determine

the ABL height using radiosonde, the most common way todetermine the ABL height is from potential temperature (q),virtual potential temperature (qV), and/or mixing ratio (r).

[6] Seibert et al. [2000] and Hennemuth and Lammert[2006] have shown that the parcel method is the mostreliable method for deriving the convective boundary layer(CBL) height. This method relies on determining the heightof the intersection ofq with dry adiabatic ascent starting atnear-surface temperature. This method is generally appliedduring stable conditions. The gradient Richardson methodidentifies a layer in which turbulent mixing can occur andABL height can be specified to be the height at which theRichardson number exceeds critical value.

[7] The GPS radio occultation (RO) technique [Kursinski

et al., 1997] provides a new method of remote sensing ofboundary layer height. The ABL height from this techniquecan be detected from cutoff height [von Engeln et al., 2005]defined in the full-spectrum inversion (FSI) method [Jensenet al., 2003], radio refractivity [Sokolovskiy et al., 2006],and directly from a bending angle (BA) profile [Sokolovskiyet al., 2007]. Although detection of ABL height using a BA

profile have the simplest errors but are most complicated ininterpretation. Conversely, the retrieved temperature andhumidity profiles are easy to identify ABL height but havelarge errors. In this study we show that the detection of ABLheight from refractivity is an easy approach that includes

both temperature and humidity information.[8] The intention of the present study is to identify the

height of the ABL using a parameter, namely, radio refrac-tivity (N), which involves both temperature and water vaporinformation using radiosonde. Using theNprofile we haveshown that it is possible to determine the ABL height easilyin different background conditions. This study is first of itskind in three ways. For the first time long-term high-resolution radiosonde data are used to study the seasonalvariation of ABL height over the Indian subcontinent.Second, we propose an alternative method to identify theABL height directly from the refractivity profile of radio-sonde which can be used to compare with ABL height fromindependent measurements from GPS RO. Finally, several

precautions are advised while studying the global distribu-tion of ABL height using GPS RO data.

2. Database

2.1. Radiosonde Data

[9] High-resolution ground-based radiosonde (VaisalaRS-80, RS-92, Meisei RS-01GII) balloons were launchedalmost regularly over Gadanki (13.5N, 79.2E) from 19April 2006 to 31 August 2008. Most of these radiosondeswere launched around 1200 UT (1730 LT). In addition,radiosondes were launched four times a day during differentseasons. In total, 744 profiles of temperature (T), pressure(P), relative humidity (RH), and horizontal wind are

obtained in different seasons. Major data gap exists duringthe month of December 2006 (only six balloons werelaunched) and April 2007. All the atmospheric parameterswere collected with a height resolution of 2530 m (sam-

pled at 5-s intervals) from RS-80 type (April 2006 to March2007) and 10 m (sampled at 2-s intervals) from RS-92 (from17 July 2006 to 31 August 2006) and Meisei (May 2007 toAugust 2008). Note that there is no difference in the

atmospheric parameters retrieved from different receiversused in the present study. Later, the entire data set has beeninterpolated to 100 m so as to remove outliers arising fromrandom motions of the balloon. Quality checks were thenapplied to remove further outliers arising due to variousreasons followingTsuda et al. [2006] to ensure high qualityin the data, which otherwise contaminate the entire results.

2.2. GPS Radio Occultation Data

[10] The six-satellite Constellation Observing System forMeteorology Ionosphere and Climate (COSMIC) GPS RO[Kursinski et al., 1997] provides an opportunity to measureABL height globally. There were total 431 overpasseswithin 2 latitude and longitude from Gadanki during July

2006 to August 2008. A further time constraint of 2 h iskept for selecting overpasses over Gadanki so as to comparethe ABL height detected by radiosonde which has reducedthe number of overpasses to 95.

2.3. Satellite Observations of Equivalent BlackbodyBrightness Temperature and Other Supporting DataSets

[11] We also make use of the simultaneous hourly cloudtop equivalent blackbody temperature, called brightnesstemperature (BT) from Multifunctional Transport Satellite(MTSAT-1R) data provided by the Japan MeteorologicalAgency (JMA) through Kochi University, Japan. Data wererecorded in longitude/latitude grids of 0.05 between 11

15N latitude and 77 81E longitude covering the locationof Gadanki. The equivalent blackbody temperature data,averaged from the pixel data, are used to examine thecharacteristics of mesoscale cloud systems. These data have

been used as a proxy for tropical deep convection.[12] Besides this, we have also made use of rainfall data

collected from a colocated optical rain gauge (ORG). Itprovides accurate measurement of precipitation with adynamic range of 0.1 500 mm h1. In addition, skyconditions (available from September 2007) monitored(visually) during launch of the radiosonde are also used tostudy ABL characteristics during different backgroundconditions.

3. Background Conditions

3.1. Topographical Conditions

[13] Before going into the details of the observed results,the background topography and atmospheric conditionsover the study region are presented, which will be usefulwhile interpreting the ABL characteristics. Radiosondeswere launched regularly from Gadanki, which is in a ruralenvironment about 120 km northwest of Chennai (Madras)on the east coast of the southern peninsula. The locationand topography of the Indian subcontinent are shown inFigure 1a. More detailed topography of Gadanki and the

D16101 BASHA AND RATNAM: ABL HEIGHT OVER A TROPICAL STATION

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surrounding places are shown in Figure 1b. It can be seenthat the station is surrounded by hills with a maximum

height of 350400 m, and the station is at a height of 375 mabove mean sea level (hereinafter all heights are mentionedabove mean sea level only). The local topography iscomplex, with a number of small hillocks and an irregularmix of agriculture and small population centers. However,there also exists a high hill of 1 km about 30 km from theradar site in the northeast direction. The influence of the seaon the east may not be present at the Gadanki location as itis located far inland.

3.2. Weather Conditions

[14] In the Indian subcontinent, mesoscale phenomenadominate, and they include convective activity. On the basisof the meteorological conditions over the observational sites,

seasons are divided into winter (December February),summer/premonsoon (March May), monsoon/progressingsouthwest monsoon (June August), and postmonsoon/retreating southwest (SW) monsoon (September November).The onset of the monsoon will be about the first or secondweek of June, and withdrawal will be during the last week ofOctober. Besides these monsoons, a peculiar monsoon due tonortheast circulation (northeast monsoon) will occur over thesouthern part of India. Normal date of onset of thesemonsoons is about the middle of October, with a deviationof about a week on either side.

3.3. Meteorological Conditions

[15] In this section, background meteorological condi-tions prevailing over the observational site are brieflydiscussed. The monthly mean contours of zonal, meridionalvelocities, virtual potential temperature, and relative humid-ity are shown in Figures 2a 2d. Monthly mean zonalvelocities were westerly during winter (with a maximumvelocity of 68 m/s), and premonsoon (with a maximumvelocity of 12 m/s), with some variation during transitionfrom one season to the other. During monsoon season, low-level westerlies exist. In general, meridional velocities arevery small and are southward in all the seasons, exceptduring monsoon, with peak velocity of 5 m/s. Virtual

potential temperature show significant variations up to theheight of about 3.5 km. In general, high and low virtual

potential temperatures can be noticed during premonsoonand winter months, respectively. There exist significantvariations in the relative humidity from winter to monsoonseasons. Humidity is very low (about 50%) during winter,and it occurs only in the first few kilometers and almostnegligible above it. However, during other seasons, partic-ularly in monsoon season, high humidity concentrations(60 70%) are noticed up to the middle troposphere. Aclear annual oscillation with some interannual variationscan be seen in all of the above mentioned meteorological

parameters.[16] Daily accumulated rainfall obtained from the colo-

cated ORG is shown in Figure 2e. Mostly rainfall at this

Figure 1. (a) Topography of Indian subcontinent plotted using global 30 arc sec elevation data providedby the EROS Data Center, National Mapping Division, U.S. Geological Survey. The location of Gadankiis shown with a circle. (b) High-resolution topography mapping within a 50-km radius from Gadanki.


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location occurs due to both southwest and northeast mon-soon between June and December, although some spells ofrainfall can also be noticed during other months. Dailymean satellite brightness temperature is shown in Figure 2f.During evening times, mesoscale convection dominates atthis location particularly during premonsoon and postmon-soon months (not shown). All of this information has beenused to study the ABL characteristics during different

background conditions in section 5.

4. Methodology to Identify AtmosphericBoundary Layer Height From Refractivity Profile

[17] Earlier studies have shown that ABL height can bedetermined traditionally either from temperature inversions(potential temperature (q), virtual potential temperature(qV)), or water vapor (mixing ratio (r)) from radiosondemeasurements in tropical latitudes. Figure 3 shows typical

profiles ofq, qV, and robserved on 6 July 2006 (monsooncase) and on 24 February 2007 (winter case). Note thatinversion in q, qV, and sharp reduction inroccur at a heightof3 km (1.9 km) during the monsoon (winter) case. Foreasy identification of sharp changes taking place near ABLheight, we have used the gradient of each parameter whichis plotted in Figures 3d and 3h for the monsoon and winter

cases, respectively. We have taken the negative maximumgradient in r and positive maximum gradient in q, qV foridentifying ABL height. Good agreement between theheights of ABL observed by q, qV and r can be seen.Although qV contains both temperature and water vaporinformation, we have used refractivity (N), which alsocontains both temperature and water vapor information foridentifying ABL height, and found several advantages

over q, qV, or rprofiles alone which will be discussed insection 6.

[18] It is well known thatNdepends mainly on temper-ature and water vapor gradients in the lower troposphereand is given by the following equation [Thayer, 1974]:

N n 1 106 77:6p

T 3:73 105

e

T2; 1

where N is refractivity, n is the refractive index, p isatmospheric pressure in hPa, Tis atmospheric temperaturein kelvins, and e is water vapor partial pressure in hPa. It isto be noted that the ionospheric term is not shown here as it

is irrelevant for the studies below 5 km. The profiles ofNobserved on the same days mentioned above are depicted inFigures 3c and 3g. Note that a sharp change also occurred intheNprofile near inversion in q, qV, and reduction inr. ABLheight from N is around 3 km (1.9 km) during monsooncase (winter case), exactly same as that observed by q, qV,andrprofiles. Note that gradient in Nis significantly higherwhen compared with q, qV, and r and thus allows us todetect ABL height even in weak inversion cases. We haveextensively compared in the following the ABL heightsdetermined from q, qV, r, and Nprofiles.

5. Results and Discussion

5.1. Statistical Comparison Between ABL HeightsDerived Fromq, qV, r, and NProfiles

[19] In order to check how good different techniquescompare, we have separated the radiosondes launched onlyat 1730 LT, which comes to 613 profiles. Profiles are furtherseparated according to the background weather conditions

prevailing during the launch and are monitored on visualbasis just before, during, and after the radiosonde flight; thisinformation is available since September 2007. On the basisof background weather conditions, launches are furtherclassified to clear-sky (188), cloudy (75), and rainy (30)cases. Out of 613 radiosonde profiles available at 1730 LT,in 10 cases no sharp gradients are found and six cases arerejected due to bad data quality; hence a total of 16 cases

were not considered for further analysis. In order to see thecorrelation between the heights of ABL detected from q, qV,andr, a statistical comparison between them has been madeand is shown in Figures 4a4c. Excellent correlation (R =0.92) can be found between q and qV, as expected (since

both have almost the same information except for having rinformation in the latter case), with standard deviation (SD)of only 0.33 km. Good correlation between the heightdetected by q and rand qVand r is found, although SD isslightly more than the first case, suggesting that there existsexcellent correlation between the traditional methods.

[20] A statistical comparison between ABL heightderived from N and q, N and qV, and N and r is shown

Figure 2. Time-height section of monthly mean (a) zonalvelocity, (b) meridional velocity, (c) virtual potentialtemperature, and (d) relative humidity observed duringApril 2006 to August 2008 over Gadanki using radiosondeobservations. (e) Daily accumulated rainfall recorded atGadanki using colocated optical rain gauge, and (f) dailymean brightness temperature observed at Gadanki (5 5 kmgrid).


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Figure 4. Scatterplots showing the correlation of atmospheric boundary layer (ABL) height (abovemean sea level) determined from (a) q and qV, (b) q and r, (c) qVandr, (d) q and radiosondeN, (e) qVandradiosondeN, and (f)rand radiosonde N. (gi) Same as Figures 4d4f, but while using only significantvalues. Their respective correlation coefficients (R), standard deviations (SD), and numbers of points (N)are given on top. The red line shows the best fit. All the data correspond to 1200 UT (1730 LT) only.

Figure 3. Typical profiles of (a) potential temperature (q) and virtual potential temperature (qV),(b) mixing ratio (r), (c) refractivity (N), and (d) their respective derivatives observed on 6 July 2006

(monsoon case). (e h) Same as Figures 3a 3d, but observed on 24 February 2007 (winter case). Note thatthe axes for the derivatives ofq, qV, andrare given at the top.


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in Figures 4d4f, respectively. Note that the good

correlation is again seen (Figure 4d) between the pro-posed new method (N) and the traditional method (q),suggesting that the proposed method can also be used asan indicator to identify the ABL height. Interestingly,relatively less correlation (0.76) between N and qV can

be noticed (Figure 4e). This reveals that although both Nand qV have information on temperature and watervapor, their individual contributions are different. Wefurther studied this issue and will discuss it in section6. Note that excellent correlation is seen between N andr with the smallest SD, suggesting that contribution ofwater vapor will be more to N than T (or q,) in thelower troposphere. Note that ABL height is detectedwith different parameters for all the profiles from their

gradients, however small in magnitude they are. Furtheranalysis is made by considering the significant gradientsonly; that is, enhancement in q, qV should be more than0.5 K, and reduction in r (N) should be more than 0.5g/kg (2.5 N units). While doing so a number of profilesreduced to 558. In this case, correlation increasedsignificantly (Figures 4g 4i) to 0.99 (SD = 0.09), 0.99(SD = 0.09), and 0.99 (SD = 0.03) between q, qV, r, and

N, respectively.[21] A study of the correlation between the traditional and

proposed method during different background meteorolog-ical conditions (clear sky, cloudy, and rainy) was also done,

and the results are shown in Figure 5. A good correlation

between N and q and N and qVcan be seen during rainyconditions followed by clear-sky conditions. Note that SD issmaller in the case of rainy conditions than in the clear-skyconditions. Correlation is slightly less during cloudy con-ditions with more SD. During cloudy (rainy) conditions, 8%(16%), 12% (20%), 33% (13%), 17% (16%), and 2% (6%)

profiles have ABL heights between 0 and 1 km, 2 3 km,34 km, and 45 km, respectively. In general, excellentcorrelation during different background conditions can beseen between Nand traditional methods, suggesting thatNcan also be used as a parameter to detect ABL heightaccurately. Interestingly, although both N and qV haveinformation on temperature and water vapor together, theircorrelation is less particularly during cloudy conditions.

5.2. Daily, Monthly, and Seasonal Variation of ABLHeight

[22] For getting monthly and seasonal variation of ABLheight, ABL height on each day is detected separately fromeach parameter and averaged over a month, which is shownFigure 6. Figure 6a shows monthly mean variation of ABLheight along with SD observed during April 2006 to August2008 using q, qV, r, and N. Standard deviation for a givenmonth represents the day-to-day variability of ABL heightin that month. Number of days used for each monthdepicted in Figure 6b shows that on average more than

Figure 5. Scatterplots showing the correlation of ABL height (above mean sea level) determined from(a) q and radiosonde N, (b) qVand radiosonde N, and (c) r and radiosonde N observed during clear-skyconditions. (d f) Same as Figures 5a 5c, but for cloudy conditions. (g i) Same as Figures 5a 5c, butfor rainy conditions, respectively. Their respective correlation coefficients (R), standard deviations (SD),and numbers of points (N) are given on top. The red line shows the best fit.


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25 profiles have been obtained. Note that large SD isobserved in the height detected during premonsoons and

postmonsoons followed by monsoon and minimum inwinter months. Similar month-to-month variability is ob-

served in all the parameters. It can be clearly noticed thatABL height is maximum (minimum) during MarchMay(DecemberFebruary). Details of ABL height and ampli-tude variations in q, qV, r, and Nare shown in section 5.3.

Figure 6. (a) Monthly mean variation of ABL height (above mean sea level) derived fromq, qV,r, andNfrom April 2006 to August 2008. The vertical bars show the standard deviation in a given monthrepresenting the day-to-day variability in that month. (b) Histogram showing the number of profiles used

each month. All the data correspond to 1200 UT.

Figure 7. Histograms showing the distribution of percentage occurrence of ABL (above mean sea level)at different heights observed from q, qV, r, and Nduring (a) March-April-May (premonsoon), (b) June-July-August (monsoon), (c) September-October-November (postmonsoon), and (d) December-January-February (winter) integrated over 2 years. (eh) Same as Figures 7a7d, but showing the distribution ofenhancement in q and qVand the reduction in rand N. All the data correspond to 1200 UT.


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5.3. Statistics of ABL Height

[23] Histograms showing the distribution of ABL heightdetected from q, qV, r, andNsorted according to the seasonobserved during April 2006 to August 2008 are shown inFigures 7a7d. Most striking feature to be noticed is thatABL height is higher during premonsoon (3.05 0.27 km)

followed by monsoon (2.94 0.08 km), postmonsoon(2.75 0.19), and minimum in winter (2.22 0.38 km).It is worth mentioning that there are few case studies[Praveena et al., 2003] carried out over this station usingthe lower atmospheric wind profiler (LAWP). During pre-monsoon the ABL height observed (51 days) from LAWP is2.1 km 0.4 km, which is within the range (24 km)observed in the present study. More or less similar featurescan be noticed in all the parameters; that is, either less ormore height is detected in maximum cases by consideringany parameter. ABL heights between 0 (ideally 0.5 km) and1 km exist for less than 2% irrespective of the season. Note

that for these statistics, data collected only at 1730 LT areconsidered. However, percentage occurrence (PO) ofABL height between 0 and 1 km will be high for 1800and 0600 UT, which will be shown in detail in section 5.5.During winter, percentage occurrence of ABL height athigher heights (45 km) is very small unlike other seasons.For 19 cases, ABL height is detected above 5 km, which arenot considered for the present analysis. An interesting

feature is that occurrence of ABL height detected from Nis closely following by that detected by q, qV, and r,suggesting thatN is also a suitable parameter to detect theABL height preciously. In latter parameter (N), an additionaladvantage is that we can verify with independent observa-tions of GPS RO measurements, which are available globally.

[24] The distribution of enhancement in q and qV andreduction in rand Nnear the ABL height is also estimatedfor different seasons and is shown in Figures 7e7h. Thisexercise will bring out the most significant enhancements to

be considered in future analysis. Note that in general, thereis an enhancement of about 5 K/100 m in q and qV, and areduction about 10 g/kg per 100 m and 25Nunits per 100 minrand Nin all the seasons, respectively. It is important to

mention here that these statistics were constructed withoutputting any restrictions on the enhancement/reduction in theamplitudes. Note that the number of cases having the qandqVenhancements within 01 K and the reduction in rand

N within 02 g/kg and 2.5 N units are very high. Forgetting reliable statistics, we have considered the ABLheight if all of the q, qV are more than 0.5 K (more thanthe accuracy of the measurement) and reduction in rand Nis more than 0.5 g/kg and 2.5Nunits. Although the overallstatistics remain the same, it is always better to talk abovethe accuracies of measurements.

5.4. Comparison of ABL Height With GPS ROMeasurements

[25] From the above, it is clear that ABL height can bedetected precisely from the profile of N from radiosondemeasurements. In order to check how good it is able todetect, it is also compared with traditional methods like q,qV, and rin different background conditions. In this sectionthe ABL height detected with N from radiosonde is com-

pared with independent observations from COSMIC GPSRO. As mentioned in section 2, the overpasses of COSMICover Gadanki during July 2006 to August 2008 within 2latitude and longitude and 2 h separation were kept so as tocompare the ABL height detected by radiosonde. The totalnumber of overpasses was 95, out of which 84 profiles havereached below 5 km and three profiles were rejected due to

bad quality in corresponding radiosonde data. Thus a total

of 81 profiles have been used for comparing with the ABLheight detected by radiosonde.

[26] Figure 8a shows typical profiles of N observed byradiosonde and COSMIC GPS RO on 19 January 2007. Thenumber of profiles reaching down to 0.5 km is also shownin Figure 8a. Note the sharp changes that are found near2 km in both the profiles, which is clearer from Figure 8bshowing the profiles of gradients in N. A very goodcomparison can be seen between the ABL height detected

by radiosonde-calculated N and COSMIC GPS ROmeasured N. Statistical analysis has been further per-formed with the ABL height detected from q, qV, r, and

Figure 8. (a) Typical example showing the comparison ofrefractivity (N) profiles between the Constellation Obser-ving System for Meteorology Ionosphere and Climate(COSMIC) and radiosonde launched from Gadanki on 19January 2007. The number of profiles reaching down to thesurface in the case of COSMIC is also shown (with axis on

top). (b) Their corresponding gradients used to detect theheight of ABL (above mean sea level). Scatterplots showingthe correlation of ABL height determined from (c) radiosondeqand COSMICN, (d)qVand COSMICN, (e)rand COSMIC

N, and (f) N and COSMIC N. Their respective correlationcoefficients (R), standard deviations (SD), and numbers of

points (N) are given. The red line shows the best fit.


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N from radiosonde with that detected from COSMIC GPSRO and is shown in Figures 8c8f, respectively. In spiteof completely different techniques, very good correlations(R 0.86) with mean difference of 0.41 km and RMSerror of 0.005 can be noticed from all the comparisons,although a slightly higher correlation (R = 0.90) isnoticed between ABL height detected from q (radiosonde)and N (COSMIC GPS RO). This suggests that ABLheight can be detected preciously with the profiles of N.

5.5. Diurnal Variation of ABL Height

[27] In order to study the diurnal variation in ABL height,radiosonde launched four times a day during differentseasons has been considered. Similar criteria mentioned insection 4 were followed for detecting ABL height fromvarious parameters (q, qV,r, andN). Figures 9a9d show thediurnal variation of ABL height detected during premon-soon, monsoon, postmonsoon, and winter from q, qV,r, and

Nby radiosonde observations, respectively. A clear diurnalvariation with higher and lower heights during 1200 UT(1730 LT) and 0000 UT/1800 UT (0530/2330 LT) can benoticed in all the seasons. However, large diurnal variation(low to high) can be noticed in premonsoon and postmon-

soon followed by monsoon and minimum diurnal variationin winter. Although all the parameters show more or lesssimilar ABL heights, during monsoon season they differ

particularly during 1200 UT where slightly higher heightscan be noticed from q and qV than from r and N. Oftenduring nighttime a few inversions are noticed (figure notshown) between 0.8 km and 2 km in Nprofile but not ineitherq orqV. For our analysis, only the first inversion fromground is considered which is the stable boundary layer(nighttime ABL); we ignored the second inversion, which isa residual layer. This reveals that we are able to distinguishdifferent layers in the ABL using profiles ofN.

[28] Similar analysis has been done using COSMIC GPSRO overpasses over Gadanki. As mentioned in section 2,there were a total of 431 overpasses within 2 latitude andlongitude from Gadanki during July 2006 to August 2008.All these overpasses were sorted according to the time 0000UT3h,0600UT3h,1200UT3h,and1800UT3hforeach season and are superimposed in Figure 9. A very goodcorrelation in the ABL heights can be noticed even in the

diurnal variation between two independent techniques.[29] It is worth mentioning a few important findings in

COSMIC GPS RO measurements from Figure 9. First of all,note that ABL height can reach as high as 4 km and as lowas 0.8 km. Global distribution of ABL height provided by

previous investigators [von Engeln et al., 2005] has fixedthe altitude at 3 km, which is not correct many times. It wasonly shown up to 3 km, which occurred mainly due toaveraging the ABL height without considering the timeissue. Second, note that ABL height can reach as low as0.8 km during 0000 UT and 1800 UT. Yet if all profiles fromGSP RO are not reaching down to 0.5 km, the globaldistribution of ABL height will give wrong statistics. Inaddition, ABL height shows large diurnal variation, and

hence averaging all the occultations over a given region willalso result in a wrong interpretation. Thus it is suggested toconsider the time or range of time (as GPS RO data are verysparse in tropical latitudes) and also profiles reaching asdown as 0.5 km (particularly during nighttime) while dealingwith the global distribution of ABL height.

6. Summary and Conclusions

[30] This paper delineates the characteristics of ABLheight over tropical station Gadanki using long-term highvertical resolution radiosonde observations for the first time.Several parameters are used to determine ABL height in the

past, and now we show that N also can be used for this

purpose. Temperature (stability) information can be used tofind the capping inversion, whereas water vapor can be usedas an indicator for mixing. In general, there will be moremixing within the ABL and a sharp reduction above it.Moreover, tropical latitudes are rich in moisture content andoften sharp changes near ABL height will be noticed. Thusit is easier to detect from water vapor mixing ratio than fromtemperature (T). In addition, it is well known that detectingABL height from T profiles is not possible at least fortropical latitudes, as it may include inversions like tradewind inversions. Although temperature and/or humidity

profiles can be used for detecting ABL height, sometimeswe could notice that they do not occur simultaneously (thereis some height shift). Thus we strongly believe that one

cannot use them independently at least in the tropicallatitudes but that they should be used as a combination oftwo, and hence we have gone to the refractivity (N) methodwhich contains a combination of two. Note that there isanother method which uses combination of temperature andhumidity, i.e., virtual potential temperature (qV). As dis-cussed in detail in Appendix A, we noticed that thecontribution of water vapor in qV is only 0.6 times andremainder of it is coming through temperature but in N itvaries from 0.7 to 2 times depending upon thee value. Thusinversions will be more clearly observed in N than in qV.Moreover, in order to estimate qV over ocean we need

Figure 9. Diurnal variation of ABL height (above meansea level) observed by radiosonde in q, qV, r, and Nduring (a) MAM (premonsoon), (b) JJA (monsoon), (c) SON(postmonsoon), and (d) DJF (winter) integrated over

2 years. The diurnal variation observed by COSMIC overGadanki is also superimposed. The number of profiles forradiosonde (COSMIC GPS radio occultation) used is alsoshown.


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accurate temperature and humidity profiles, which are notpossible from normal radiosonde. Our idea is to see whetherthere is any other parameter that includes both temperatureand humidity and measures directly at various places onglobe (particularly in the tropical latitudes). Since GPS ROgivesNdirectly, in the present study we have tested whetherwe can use this N to detect ABL height preciously. It isworth mentioning here that N is a moisture-dependent

parameter, and at least in the tropical altitudes (30latitudes) where large gaps in the measurements exists(due to large ocean coverage), this technique will provide

better results.[31] ABL height detected with the profile of N is com-

pared with the traditional methods of q, qV, and r duringdifferent background meteorological conditions. In general,excellent correlation is seen between N and traditionalmethods in all weather conditions, suggesting that N canalso be used as a parameter to detect ABL height accurately.Interestingly, although both N and qVhave information ontemperature and water vapor together, their correlation isless, particularly during cloudy conditions. Since excellentcorrelation is found between N and r in all the cases, we

suggest that the contribution ofrto theNis more thanT(orq) in the lower troposphere. ABL height is higher during

premonsoon followed by monsoon, postmonsoon, and min-imum in winter. During monsoon, the boundary layer will

be rich in moisture, and thus most of the radiation from thesurface will be utilized for the evaporation process, whichwill result in a shallow ABL. Enhanced soil moisture

probably contributes to increases in the latent heat fluxesand thus decreases in the surface sensible flux locally andsuppresses the ABL growth. One more reason for theshallow ABL during the monsoon days may be increasedcloudiness, which can reduce the incoming solar radiation.On a given day in the monsoon, ABL height is found athigher heights but on average it is less than premonsoon and

postmonsoon. In general, about 5 K per 100 m enhancementin the q, qV, and up to 10 g/kg per 100 m (25 Nunits/100 m)reduction inr(N) is noticed. Interestingly, the reduction in

Nis more during winter and monsoon seasons followed bypremonsoons and postmonsoons. This reveals that most ofthe water vapor is prohibited from entering into higheraltitudes as Nis more directly influenced by water vapor.

[32] The idea behind using N is that it is the directlymeasured parameter using the GPS radio occultation tech-nique and thus can be helpful in studying the ABL on aglobal scale. A very good correlation between the twoindependent techniques (radiosonde and GPS RO) has beenfound. A strong diurnal variation in the ABL height is foundfrom the radiosonde launched four times a day during all theseasons. However, the difference between the lower heightand higher height is more in premonsoon and postmonsoonfollowed by monsoon and minimum in winter. Similardiurnal variation is also seen in COSMIC GPS RO over

Gadanki, suggesting that while studying the global distri-bution of ABL height, a fixed time or range of time (as GPSRO data are sparse in tropical regions) needs to be consid-ered. In addition, it is also advised to consider only the

profiles reaching down to 0.5 km particularly during night-time. In the future work, global distribution of ABL heightfrom GPS RO measurements will be studied by consideringthese limitations. It is to be noted that in the present studythe use of the Nprofile for detecting ABL height is testedfor Gadanki location, which has a number of small hillocksand an irregular mix of agriculture and small populationcenters. However, it is to be tested how well this hypothesiswill represent at different regions, over oceans, arid zones,hilly terrain etc., which is beyond scope of the present studyand will be done separately using GPS RO data.

Appendix A

[33] The contribution of water vapor to the virtual poten-tial temperature (qV) and refractivity (N) is estimated in thefollowing.

A1. Case 1: Virtual Potential Temperature (qV)

[34] Virtual potential temperature can be represented as

qv q 1 0:6r ; A1

where q is potential temperature in kelvins and ris mixingration in kg/kg. Differentiating above equation, it reduces to

Dqv

qv

Dq 1 0:6r

q 1 0:6r

q 0:6Dr

q 1 0:6r

Dqv

qv

0:6Dr

1 0:6r: A2

The first term in the above equation is very small and hencecan be neglected. In general, considering minimum (10%)and maximum (50%) reduction in r at ABL height, thedenominator of the second term reaches close to unity (i.e.,

1.12) and 7. Thus the maximum contribution of water vaporin qV is roughly about 0.6 times only and the rest of it iscoming through temperature.

Table A1. Original Minimum and Maximum Values ofe,p, andT

at 2 km and 3 km

e p T

Minimum Maximum Minimum Maximum Minimum Maximum

2 km 0.3452 29.05 566.02 837.45 278.46 297.153 km 0.0140 21.69 521.03 744.17 273.44 288.31

Table A2. Minimum and Maximum Enhancement and Reduction

in e, p, and Tat Boundary Layer Heighta

Maximum Minimum

e 47.3 0.0044eR 69.36 0.2545

p 958.2 535.1PR 968.84 548.7T 302.4 272.4TE 304.8 271.7

aSuffixes R and E denote reduction and enhancement values,respectively.

Table A3. Maximum and Minimum Values of Different Terms in

Equation (4) at 2 km and 3 km

1/(N1 + N2)N1(Dp/p)

N2(De/e)

(DT/T) (N1 + 2N2)

At 2 km maximum 0.0029 110.4 200.8 46.9At 3 km maximum 0.0033 113.8 213.3 41.2At 2 km minimum 0.0062 117.9 228.7 17.3At 3 km minimum 0.0067 120.0 237.2 16.2


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A2. Case 2: Refractivity (N)

[35] Refractivity (N) can be represented as

N 77:6 p

T 3:75 105

e

T2 : A3

For simplicity let us assume N1 = 77.6 P/Tand N2 =3.75 105 (e/T2):

DN1

N

1

1

N2

N1

8>:

9>=>;

Dp

p

DT

T

;

DN2

N

1

1 N1

N2

8>:

9>=>;

De

e

2DT

T

DN

N

DN1

N

DN2

N

DN

N

1

N1 N2

N1

Dp

p N1

DT

T

N2De

e N2

2DT

T

;DN

N

1

N1 N2

N1 Dp

p N2

De

e

DT

T N1 2N2

: A4

Original minimum and maximum values of e, p, and Tat

2 km and 3 km are obtained from 613 profiles and areshown in Table A1. Similarly, minimum and maximumenhancement and reduction values of e, p, and T at

boundary layer height are obtained from 613 profiles andare shown in Table A2. The values calculated based on themaximum and minimum values at 2 km and 3 km areshown in Table A3. The values calculated based on theenhancement of reduction values at boundary layer heightare shown in Table A4. From Tables A1A4, it can benoticed that contribution ofe to Nvaries from 0.7 times to2 times depending up on the e values unlike that observed inqVwhere its contribution is only 0.6 times. Thus inversionswill be more clearly observed in N than qV.

[36] Acknowledgments. We are grateful to the National AtmosphericResearch Laboratory (NARL), Gadanki, for providing necessary data forthe present study. One of the authors (G.B.) is thankful to NARL for

providing a fellowship and other necessary facilities to carry ou t this work.We thank B. V. Krishnamurthy for his fruitful discussion on this topic. Wealso thank three anonymous reviewers for providing detailed comments andsuggestions for improving this manuscript.

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G. Basha and M. V. Ratnam, National Atmospheric Research Laboratory,

Gadanki, Post Box 123, Tirupati 517 502, India. ([email protected])

Table A4. Maximum and Minimum Values of Different Terms in

Equation (4) Obtained Based on the Enhancement of Reduction

Values at 2 km and 3 km

1/(N1 + N2)N1(Dp/p)

N2(De/e)

(DT/T) (N1 + 2N2)

At 2 km maximum 0.0029 109.7 293.4 51.8At 3 km maximum 0.0033 113.0 311.7 45.4At 2 km minimum 0.0062 117.0 334.2 19.1At 3 km minimum 0.006 119.2 346.5

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