A preliminary study on Planetary Boundary Layer and Precipitation overIndia and China using Lower Atmospheric Wind Profilers
K.Krishna Reddy1, Biao Geng1, Hiroyuki Yamada1, Hiroshi Uyeda1, 2,Kenji Nakamura2, Toshiaki Kozu3, and Atsushi Higuchi2
1.
2.
3.

Frontier Observational Research System for Global Change (FORSGC), Yokohama
Hydrospheric-Atmospheric Research Center (HyARC), Nagoya University, Nagoya
Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue, Shimane

1. IntroductionLower atmospheric wind profiler (LAWP) operating around 1.3 GHz is a good tool forinvestigation of the convective boundary layer (CBL) and vertical structure of theprecipitating cloud systems (Krishna Reddy, 2002). The FORSGC installed a LAWP
long with automatic weather station, the Institute of Low Temperature Sciences,
Hokkaido University X-band Doppler radar and micro rain radar in the premises of theDongshan Meteorological Observatory, in Suzhou prefecture, about 120 km west ofShanghai, PR China [Figure 1(a)]. The LAWP has been operated continuously from 4
June to 16 July 2001 during the campaign. Under Indio-Japan collaboration,Communication Research Laboratory installed a LAWP [Figure 1(b)] at Gadanki, India(Krishna Reddy, 2001). June and July 1999 data has been utilized for the present study.

(a)

(b)

Fig.1 LAWP with Radio acoustic sounding System (RASS) Antenna assembly installed (a)at Dongshan, China and (b) at Gadanki, India

2.
Convective Boundary Layer Characterization
It is now widely recognized that the dynamics of the CBL in tropics and sub-tropics arevery important to understanding of global climate, short- range forecasts andmeteorological now casting, including the prediction of boundary-layer evolution andpollution dispersion. The wind profiler can measure the height of the CBL (with good
spatial and temporal resolutions), also described as the mixing depth or boundary layer
depth during non-precipitating condition. Figure 2(a) shows the midday boundary layerheight for all the days with well-formed CBL over Dongshan and Gadanki. The height
shown for each day is the average over four hours in early afternoon (12:00 – 15:00 LocalTime) of the boundary layer heights determined for each hour by subjective examination

of the results of a peak-finding algorithm run on the reflectivity. The results suggest that
CBL height at Dongshan varies between 1 and 1.5 km and whereas at Gadanki theboundary layer height various between 1.05 and 2.05 km. The CBL evolution depends
on variety of factors and is not simply related to any local surface meteorologicalvariables. The low boundary heights at Dongshan (Gadanki) during July (June) areprobably related to low Bowen ratios (ratio of sensible to latent heat flux at the surface)and very high humidity. Figure 2(b) shows statistics of the daytime boundary layerwinds during 2001-IOP. These are averages of winds over all heights within the CBL.
The boundary layer wind speed distribution at Dongshan (Gadanki) was sharply peaked
at approximately 5 m/s (8 m/s). The wind direction within CBL at Don Shan is found tobe easterly or southeasterly (westerly or northwesterly) during wet season. These results
suggest that the low (moderate) wind speeds are favorable for the CBL development atDongshan (Gadanki) during wet period.
CBL observations during wet (05June - 30 July) season
over Dongshan and Gadanki (averaged 11:00 – 15:00 LT)

Boundary layer height, km

2.5(a)

Dongshan(2001)
Gadanki (1999)

21.5
10.5
0
155

160

165

170

175

180 185 190
Day of the year

195

200

205

210

Percent of Occurrence (hr. aveg)

25(b)Dongshan
20215

Gadanki

15
10
50
12

3

4

5

6

7

8

9

10

11

12

13

14

15

Wind Speed, m/s

Fig.2 (a) Boundary layer height during wet season over Dongshan and Gadanki.
(b) Boundary Layer averaged wind speed statistics obtained during wet season
over Dongshan and Gadanki.

3. Application of LAWP to diagnostic studies of precipitation
To improve algorithms for the remote sensing of rainfall, which are crucial for meso- andlarge-scale circulations studies and climate applications through better determinations ofprecipitation type and latent heating profiles. Toward this end wind profiler-derived
quantities, namely, equivalent reflectivity, Doppler velocity and spectral width from
Doppler spectra were used to classify precipitation type in three categories: Convective,Transition (mixed convection-stratiform) and Stratiform (William et al 1995).
22 - 24 June 2001 (Dongshan WP)
817 & 18 May 1999 (Gadanki WP)(a)8.85 (b)7.65

5322.85

11.65Height, km

6Height, km

76.45
5.25

44.05

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

141822263034384246

Local Time, hrs

0 10 20 30 40 50-10 0 10 20 30 40 50

Equivalent Reflectivity, dBZe

Reflectivity, dBZ
20(c)

40

Rain rate (dBR)

Rain Rate, mm/hr

5030
2010(d)1510
50-5-1000 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Local Time, hrs

Fig.3141822263034384246

Local Tim e, hrs

Time-height cross-section of reflectivity observed by the vertical beam of the (a)Dongshan and (b) Gadanki wind profilers on 22 and 24 June 2001. Time series ofrain rate observed (c) at Dongshan using AWS and (d) at Gadanki using
Disdrometer.

Figure 3 illustrates Dongshan and Gadanki wind profilers potential for diagnosing thevertical structure of precipitating cloud systems. The reflectivity in the panel (a) andpanel (b) show a distinct episode of convective activity, and a bright band of highreflectivity associated with a melting layer is visible. The ability of Dongshan andGadanki wind profilers to clearly resolve the melting layer when it is present also
provides a means of differentiating between stratiform and convective precipitation.
5(a)Convection
Transition
Stratiform

10

percentage of Occurrence

Percentage of Occurrence

12864
2004

81216
Local Time, BST

2024

4

Convective
Transition
Stratiform(b)32
100:00

6:00

12:00
18:00
Local Time, IST

0:00

Fig.4 Diurnal variation of Precipitating Clouds over (a) Dongshan and (b) Gadanki duringBaiu/Meiyu/Monsoon.

Figure 4 shows diurnal variations of precipitating cloud occurrence at Dongshan andGadanki as observed by the wind profilers. Diurnal variation of convection seems tooccur within 19:00 – 05:00 Local time at Dongshan whereas at Gadanki during lateafternoon and evening. The peak of stratiform precipitating cloud has smaller value andcomes later than the convective clouds. The time delay between the peak of the stratiform
and convective precipitating cloud corresponds to the life cycle of the mesoscaleconvective system. Several Precipitation events/storms at Gadanki are short lived aroundone hour with high intensity of the rain. Dongshan most of the precipitation events arelong lasting and mostly associated with heavy rainfall or tropical cyclones or both.
Preliminary results from Gadanki confirm that at least half of tropical rainfall is
stratiform in nature being associated with mesoscale convective systems. However, thisresearch needs further detailed investigations, which are in progress.
Reference:
Krishna Reddy. K., T.Kozu, K.Nakamura, Y.Ohno, P.Srinivasulu, V.K.Anandan,
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Williams, C.R., W.L.Ecklund, and K.S.Gage, Classification of precipitating clouds in thetropics using 915 MHz wind profilers, J. Atmos.Ocean. Tech., 12, 996-1012,1995.