Features of cloud types over the Bay of Bengal using split window
measurements and TRMM satellite data.
*Tomotsugu Inoue1, Hiroaki Ueda 1, Toshiro Inoue2(1: University of Tsukuba, 2: Meteorological Research Institute)*Graduate School of Life and Environmental Sciences, University of Tsukuba,Tsukuba, Ibaraki 305-8572, Japan. e-mail: s0430629@ipe.tsukuba.ac.jp
The Asian monsoon is characterized the enhanced convection which is composed of diurnal cycle as well as intrasesonalvariation, besides the basic seasonal evolution. Although several studies have used Outgoing Longwave Radiation (OLR) andBlack Body Temperature (TBB), these data sets are not necessarily represented deep convection area attended latent heating.
Therefore, the main objective of the present study is to reveal spatiotemporal structures of cloud properties over the Bay ofBengal in summer derived from split window measurements. In this study, split window data from Advanced Very High
Resolution Radiometer (AVHRR) on board NOAA-9 is utilized during the period 1986 to 1993. Optically thin clouds (cirrustype clouds) can be identified by inspection of the image constructed from the brightness temperature difference (BTD) between
11ȣm and 12ȣm (split window) data. Cloud type classification based on a threshold technique in the TBB-BTD diagram. Inboreal summer, optically thick clouds (cumulonimbus type clouds) associated with deep convection appear principally over theeastern Bay of Bengal, while cirrus type clouds including anvil clouds are found over the whole Bay of Bengal. With respect toseasonal evolution, cirrus type clouds observed nearby the tropic abrupt northward expand in the monsoon onset. Its expanded
cirrus type clouds reduce gradually after the monsoon season. Although this annual cycle is similar to that of OLR,
cumulonimbus type clouds exhibit different variability from OLR. Finally, comparison of cloud type derive from split
window measurements and Tropical Rain fall Measuring Mission (TRMM) Precipitation Radar (PR) data is conducted forconvective activity area. Very good agreement is obtained for the distribution of cumulonimbus type clouds and large
precipitation area over the Bay of Bengal.
Keyword: Split Window, Cloud classification, TRMM, Bay of Bengal

1. Introduction
Clouds are considered to be important component
controlling Earth’s radiation budget. Therefore theInternational Satellite Cloud Climatology Project(ISCCP) has collected satellite-measured radiation andthe cloud properties including cloud cover (Schiffer andRossow 1983). The first global radiance dataset wasarchived in 1984 (Schiffer and Rossow 1985), and thepreliminary cloud data products were released in 1988
(Rossow and Schiffer 1991). These datasets are used for
estimation of Earth’s radiation budget (e.g., Zhang et al.
1995; Rossow and Zhang 1995), hence new datasets
have been composed until recently (Rossow andSchiffer 1999). The ISCCP datasets are utilized toevaluate radiation budget of Earth as well as to calculate
radiative effects of cloud type variation (Chen et al.
2000).
Inoue (1987; 1989) have revealed that advantage ofcloud classification by use of split window (11 and 12ȣm) technique, especially in the high level clouds, ascompared with conventional scheme including visible
and infrared radiation method (Luo et al. 2002; Lutz etal. 2003).
Convective activities in the tropics play an importantrole as a driving force of general atmospheric circulation.
Outgoing Longwave Radiation (OLR) and Black Body
Temperature (TBB) have been generally used as a proxyof convective activity in numerous studies (e.g., Zhang
1993; Waliser et al. 1993; Matsumoto and Murakami
2000; Ohsawa et al. 2001, and many others). Especially,
the Asian monsoon is characterized the enhanced
convection which is composed of diurnal cycle (Nittaand Sekine 1994) as well as intraseasonal variation (e.g.,Madden and Julian 1972; Yasunari 1979, and manyothers), besides the basic seasonal evolution (Ueda andYasunari 1998).
Generally, the low value of OLR and TBBindicate enhanced convection including upper
level cirrus clouds. In other words, these
indicators are not suitable for estimatingcondensation heating caused by deep cumulus
type convection. Therefore, this paper compares
spatiotemporal features in view of cloud types such as
cumulonimbus type dense cirrus. Additionally, TropicalRainfall Measuring Mission (TRMM) Precipitation
Rader (PR) is used validate three- dimensional structureof precipitation system.

2. Data and method
The major data used in this study is TBB obtained by
Advanced Very High Resolution Radiometer (AVHRR)on board NOAA-9 with a 0.1
0.1 horizontal
resolution. Especially, we utilize the TBB of channel 4(11μm) and channel 5 (12μm) for the period from1986 through 1993. It is known that optically thinclouds, mainly composed of ice, show large value of theTBB Difference (BTD) due to emissivity difference
between 11μm and 12μm (BTD=TBB11μm−TBB12μm).
For example, Inoue (1985) has shown that optically thinclouds (cirrus-type clouds) can be identified byinspection of the image constructed from BTD.

NOAA satellite is utilized as a traditional measure ofconvective activities over the tropics. Because thepresent study is to reveal basically seasonal evolution,
we use pentad mean data for the analysis. Additionally,
we use TRMM PR product designated 2A25 (0.050.05 ) between 1998 and 2001 to compare with spatial
distribution of Cu type clouds with the rain structure indetail.
Features of enhanced convection over the Bay ofBengal
Although Dc type without enhanced convection
dominates over the Bay of Bengal, optically thick clouds,
corresponding to Cu type, are observed to the east of theBay of Bengal in boreal summer (Fig.2).
3.

Fig. 1. Cloud type classification by the split window method
based on Inoue 1989.

Figure 1 shows cloud type classification based on athreshold technique of TBB-BTD diagram. According toInoue (1989), the height of the cloud top is classified by
two thresholds. If the TBB value was below (above)-20℃, then the cloud type could be classified as high(low) level cloud. In generally, -20 ℃ roughly
corresponds to the air temperature around 400 hPa in thetropics. Clear TBB is determined by subtracting monthly
mean clear-sky TBB minus 3℃ to distinguish cloudy orclear sky (Inoue 1987). The classification of optical
thickness is separated by two threshold values for the.
Theoretically, the BTD of optically thick clouds,
corresponding to cumulonimbus type cloud, features
0℃. However, We set 0.75℃ as the threshold value for
optically thick clouds, because the temperature
resolution is low for colder TBB. Clear-BTD is computedby multiplying 0.5 on monthly mean cloudy-sky TBB todivide clear sky from cloudy condition (Inoue 1987).
We have classified TBB into six cloud types including
low-level cumulus, cumulus, thin cirrus, thick cirrus,
dense cirrus, and cumulonimbus type by adopting
so-called split window method. Lutz et al. (2003)
reconfirms the capability of concerning the detection ofhigh-level clouds, thus the present study extracts densecirrus (Dc) and cumulonimbus (Cu) type. This data set
is converted pentad and monthly mean within 1
1latitude/longitude area.
Daily OLR data of 2.5-degree grid derived fromFig. 2. Climatological distribution of dense cirrus type (a) andcumulonimbus type (b) during June to August (JJA).

Figure 3 displays the seasonal evolution of Dc andCu type eastern Indian Ocean (80rE-100rE) along
90rE. Two cloud types including Dc and Cu typeexhibit different annual cycle respectively. Dc type,appearing around the equator during winter to spring,
rapidly expands to northward in monsoon onset. The
Bay of Bengal is characterized by large amount of
optically thin clouds, corresponding to anvil cloud,during June-August. In fall, appearance frequency of
Dc type gradually decrease over the Bay of Bengal.
However, A large amount of Cu type, appearing only
eastern Bay of Bengal, is not observed for thesummer monsoon period. Figure 4 shows seasonalevolution of each cloud appearance frequency greaterthan 5% and OLR smaller than 240 W/m-2. Annual
cycle of Dc type is roughly consistent with that ofOLR, while Cu type reveals different variation from
Dc and OLR. Finally, comparison of cloud typederive from split window measurements and TropicalRain fall Measuring Mission (TRMM) Precipitation
Radar (PR) data is conducted for convective activity
area. In result, very good agreement is obtained forthe distribution of cumulonimbus type clouds andlarge precipitation area over the Bay of Bengal.

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Fig. 3. Latitude-time section of dense cirrus type (a) andcumulonimbus type (b) from their climatological mean values
along 90rE.

Fig. 4. Climatological seasonal march of two cloud types,
including dense cirrus type and cumulonimbus type, and OLR in
the Bay of Bengal (20 S-20 N, 80 E-100 E).

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