NOTES AND CORRESPONDENCE. A Pronounced Continental-Scale Diurnal Mode of the Asian Summer Monsoon

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1 462 MONTHLY WEATHER REVIEW VOLUME 128 NOTES AND CORRESPONDENCE A Pronounced Continental-Scale Diurnal Mode of the Asian Summer Monsoon T. N. KRISHNAMURTI AND C. M. KISHTAWAL Department of Meteorology, The Florida State University, Tallahassee, Florida 4 December 1998 and 3 May 1999 ABSTRACT A pronounced continental-scale diurnal mode of the Asian summer monsoon is mapped using data from recent satellites Meteosat-5 and TRMM. These datasets were available at high temporal resolutions. A result that stands out is the diurnal divergent circulation that in the afternoon hours has an ascending lobe over north-central India and has a descending lobe that reaches out radially toward central China, the southern part of China, the equatorial Indian Ocean, and the western Arabian Sea. The reverse circulation is clearly seen during the early morning hours. This diurnal pulsation of continental-scale divergent circulation appears to be an integral part of the monsoon. Another finding relates to the diurnal slowing down and speeding up of the Tibetan high circulations, especially in the southern flanks where the tropical easterly jet resides and exhibits a pulsation of intensity. The amplitude of pulsation was found to reach up to 7 m s 1. Thus this continental-scale change appears to be a pronounced feature. The phase and amplitude of various satellite datasets derived from the 90-min datasets are also displayed to confirm this major mode, that is, the diurnal oscillation of monsoon. 1. Introduction This paper presents an observational analysis of a continental-scale diurnal change over the Asian monsoon belt that is seen from datasets from geostationary satellites. The newer datasets include the fields of outgoing longwave radiation (OLR), and high density cloud and water vapor tracked winds from Meteosat-5 with a temporal resolution of 1.5 h. Meteosat-5 is a European meteorological satellite on a geostationary orbit and is presently located at 63 E over the equator. The National Aeronautics and Space Administration (NASA) Tropical Rainfall Measuring Mission (TRMM) is a nonsunsynchronous polar-orbiting satellite that carries a variety of sensors including microwave imager at several frequencies (10.7, 19.4, 21.3, 37, and 85.5 GHz). Algorithms have been developed by the TRMM science team to obtain quantitative estimates of rainfall rates from its microwave instrument TMI. A detailed description of the basic framework of these algorithms can be found in Kummerow et al. (1996). Since the satellite does not pass over the same region at the same time each day, a collection of data over several days is necessary to obtain the diurnal change of the monsoon environment. Ananthaknishnan (1977) has examined the diurnal var- Corresponding author address: Dr. T. N. Krishnamurti, Dept. of Meteorology, 3034, The Florida State University, Tallahassee, FL tnk@io.met.fsu.edu iation of surface and tropospheric winds for selected sites over India. He noted that pronounced surface wind speed variations were evident at coastal as well as inland sites. His work shows that the amplitude of the surface wind speed oscillations were as large as 7 m s 1 at the coastal sites and 3 m s 1 at inland sites on the diurnal timescale. He also examined differences of the uppertropospheric winds at 1200 UTC (roughly 1730 local time) and 0000 UTC (0530 local time). Figure 1 illustrates the seasonal time history of this diurnal difference for four selected sites. Three of these are coastal sites (Bombay, Madras, and Calcutta) and one is an inland site (Delhi). Madras has a general lack of cloud cover or rain during the summer months and the land heating influences a strong sea breeze component in the lower troposphere. Bombay and Calcutta are strongly affected by monsoon rain and cloud cover and the sea breeze is much weaker over these sites. At upper-tropospheric levels ( 14 km), the data over Delhi and Bombay show a diurnal strengthening and weakening of easterlies, which is consistent with the broader findings of present study. Figure 1 denotes that the easterlies at 1200 UTC are stronger than those at 0000 UTC, such that the wind difference (1200 minus 0000 UTC) shows a remnant easterly component. Ananthakrishnan (1977) was among the first who noted this diurnal variation of upper-tropospheric winds over the monsoon domain. His study, however, was limited because he had access to upper-air data at only selected sites. A number of other studies have examined the diurnal variability of cloud American Meteorological Society

2 FEBRUARY 2000 NOTES AND CORRESPONDENCE 463 FIG. 1. Diurnal variation of upper winds (1200 minus 0000 UTC) at Madras (13.1 N, 80.2 E), Bombay (18.9 N, 72.8 E), Calcutta (22.5 N, 88.3 E), and Delhi (28.9 N, 77.2 E). iness and convection from the hourly fields of the OLR from geostationary satellites. Murakami (1983) used datasets from the Japanese Geostationary Meteorological Satellite to examine the phase and amplitude of the diurnal activity of convection. He defined a threshold of OLR for defining what he called highly reflective clouds to represent deep convection. He noted that during the summer monsoon months, the diurnal convection over the Tibetan plateau is enhanced during the afternoon hours and is suppressed during the early morning hours. Over the eastern foothills of the Himalayas, Murakami noted an out-of-phase relationship, that is, heavy convection in the early morning hours and suppressed convection during the afternoon hours. Over most of the

3 464 MONTHLY WEATHER REVIEW VOLUME 128 FIG. 2. Time and duration of major weather systems during the monsoon season of 1998, based on Indian Daily Weather Report (IMD 1998). Figure also shows the period selected for the present study. rest of the Asian monsoon landmass of south Asia and southeast Asia, the diurnal mode of convection is dominated by late afternoon convection and suppressed conditions during the early morning hours. We feel that most of these features noted by Ananthakriahnan and Murakami and many others are closely related to a pronounced continental-scale diurnal mode of oscillation that is not just at surface levels but that influences most of the troposphere as well. Diurnal variation of rainfall and convection over the different tropical land areas, including the monsoon domain, have been reported by many authors (Ramage 1964; Haldar et al. 1991; Nitta and Sekine 1994). Differences between morning and evening temperatures of cloud tops over tropical continents and oceans were examined by Riehl and Miller (1978) using longwave radiation measurements. They found that over land, the frequency of very low values of outgoing infrared (which can be associated with the population of high cloud tops) was greater during the evening than during the morning. Several recent datasets, especially those obtained from recent satellites Meteosat-5 and TRMM have provided a unique opportunity to examine the diurnal change issues in greater detail than had been previously possible. In the following sections we shall examine the datasets 1.5-h resolution from the satellites toward mapping the amplitude, phase, and geographical extent of the diurnal change of the Asian summer monsoon. 2. Data Infrared cloud images at 30-min intervals and cloud motion winds at 90-min intervals from Meteosat-5 were available for the entire monsoon season of Cloud images were filtered to retain the high clouds (with cloud-top temperature 40 C) and were used to study the diurnal behavior of deep convection. Cloud motion winds were interpolated to uniform 2 lat 2 long grids for statistical and spectral analysis. In order to emphasize the characteristics of the diurnal mode independent from the other disturbances present during the monsoon season, we have selected a period for our analysis (15 28 July 1998) that was free from typical monsoon disturbances. Figure 2 shows the time and duration of major weather disturbances during the monsoon season of 1998 (IMD 1998). This figure shows that July was the longest clean period available to study the exclusive behavior of the diurnal mode, during the middle of the monsoon season. Rainfall observations from the TMI sensor aboard the TRMM satellite during the same period have been used to study the diurnal behavior of precipitation fields. Due to its unique orbital characteristics, TRMM observes a given tropical area at different local times and thus has the capability of detecting the diurnal variation of rainfall. For the present analysis, the orbital TRMM observations were sorted as per their observation times, to represent the precipitation fields at different local hours. 3. Results a. The OLR field and the diurnal convection OLR data were available every 30 min from the Meteosat-5. A film was first prepared using the dataset for the summer of Features shown in Fig. 3 were typical of the diurnal progress of deep convection. Here the deep convection was identified from threshold values of cloud-top temperature less than 40 C. The four panels of Fig. 3 show, respectively, the fields at 0000, 0600, 1200, and 1800 local time (LT) for 27 July The contrast between 1800 and 0600 LT is clear. The time lapse film exhibits continental (rather than a local sea breeze) scale oscillations. Between midnight and

4 FEBRUARY 2000 NOTES AND CORRESPONDENCE 465 FIG. 3. Diurnal fields of OLR for deep convection (for cloud-top temperatures less than 40 C) for 27 July The four panels show OLR fields for 0000, 0600, 1200, and 1800 LT (local time refers to approximate local time over central India). noon we see a continental flare up of convection and a suppression of convection over the land area between 0000 and 0600 LT. The westward progression is (in the film) more clearly evident near N during the afternoon hours whereas a reverse motion, that is, eastward, is stronger near N latitudes during the night, and this diurnal mode was more clearly evident in the modulation of upper-tropospheric winds. Features shown in Fig. 3 are often masked by synoptic-scale activity if such are present over disturbed periods. The diurnal variability of high clouds (implied by cloud-top temperatures 40 C) was typical of what we noted throughout the months of July and August. The significance of this feature becomes clearer when we see the associated diurnal divergent wind response that is described in one of the following sections. b. Diurnal modulation of the tropical easterly jet The upper-tropospheric climatology of the wind (at 200 mb) is shown in Fig. 4. The upper anticyclone (called the Tibetan high) is the most prominent feature of these circulations. We shall examine the diurnal modulation of these circulations. The high density cloud-tracked winds of the upper troposphere from Meteosat-5 were also available at intervals of 90 min. The diurnal mode was extracted for each day from these winds and a 14-day composite structure was prepared for different local times (hereafter, the term local time refers to the approximate local time over central India); these are shown in Figs. 5a d. This figure shows the diurnal winds at 0000, 0600, 1200, and 1800 LT averaged for the period July It is clearly evident that the Tibetan high circulation undergoes a diurnal modulation. The strongest winds are seen during the afternoon hours. The winds are also strong at 1200 LT. At these hours, the strength of easterly jet is above 25 m s 1. The weakest intensity of the tropical easterly jet is seen during the early morning hours (0600 LT). At this time the strength of the tropical easterly jet lies between 17 and 20 m s 1. This diurnal variation of the tropical easterly was found to reach up to 7 8 m s 1. Thus, we are seeing an important result here. The

5 466 MONTHLY WEATHER REVIEW VOLUME 128 FIG. 4. Climatology of 200-hPa winds for July. These are based on 15 yr of data from 1961 to The streamlines and isotachs (m s 1 ) are shown. The dark line emphasizes the circulation of the Tibetan high. Tibetan high undergoes a diurnal oscillation, with the tropical easterly jet on its southern flank exhibiting strong intensity fluctuations. c. Divergent circulation at the diurnal timescale The continental-scale diurnal component is best seen from the velocity potential (and divergent wind) fields composited on the timescale of the 90-min interval high density datasets of satellite-derived cloud motion vector winds. We show in Figs. 6a d the 200-hPa velocity potential anomaly fields obtained from the diurnally composited wind fields at 1200, 1800, 0000, and 0600 LT, respectively. To obtain anomalies the 14-day mean of velocity potential was subtracted from the mean of velocity potential at four local hours. The units of velocity potential anomaly are 10 6 m 2 s 1. The dominant features are seen in late afternoon (1800 LT) and early morning composites (0600 LT). The 1800 LT composite seen in Fig. 6d shows a pronounced diurnal outflow centered at 200 hpa located near 15 N over the landmass. The gradient of the velocity potential clearly shows a pronounced outflow toward the west from the central Indian landmass. This diurnal outflowing feature has a coherent geometry on the continental scale with a reasonably well-defined reach into the Arabian Sea, the equatorial Indian Ocean, and the southern part of China. It is interesting to note that such a large-scale diurnal pulsation is revealed by the data from the geostationary satellite (Meteosat-5). The other salient feature of the diurnal mode is seen at 0600 LT (Fig. 6b), where a total reversal of divergent winds is noted. This field is dominated by divergent inflow, although the intensity of inflow is not as pronounced as that of outflow during the afternoon hours. We view the fields at 1200 and 0000 LT (Figs. 6a,c) as transient hours between inflowing and outflowing features and do not allow any special preference. The importance of this finding requires further study. A four-dimensional data assimilation of the 90-min datasets using cloud-tracked and water vapor tracked winds and the OLR-based precipitation estimates (using physical initialization within the four-dimensional data assimilation) would be most useful. Such datasets can provide useful estimates of monsoon energetics at the intervals of every 90 min. Such an analysis might be very revealing on the issue of the role of diurnal transients of the kinetic energy of the monsoon and in its overall maintenance. The diurnal variation of winds and clouds over the central India were seen clearly in the time history of the zonal winds and cloud-top temperatures averaged over a 10 lat 10 long box centered at 15 N, 75 E (Figs. 7a,b). The cloud-top temperatures were derived from the net outgoing longwave radiation reaching the satellite. This figure shows that the diurnal variation is a consistent feature of circulation as well as of convection during the monsoon season. Coldest cloud-top temperatures occur in the afternoon hours (approximately 1930 LT), which may be associated with the enhancement of convection. These cloud-top temperatures of approximately 225 K denote a cloud top at roughly 12 km above central India. In order to understand the time delay, or lag, between the intensification (or weakening) of winds and the development (or suppression) of deep convection, we present here the diurnal evolution of zonal wind and cloudtop temperature fields at every 90-min interval. Figure 8 shows such patterns of wind and cloud-top temperature averaged over the box (centered at 15 N, 75 E) for a period of 14 days. It is interesting to note that whereas the weakest and strongest zonal winds occur at 0600 and 1800 LT, respectively (on some days, the minimum of zonal winds occurs at 0430 LT, and the maximum occurs at 1630 LT), the shallowest and deepest convection occurs at 0730 and 1930 LT, respectively, indicating an average lag of 1.5 h between the clouds and winds. We also observed that on individual days, this lag period may reach up to 3 h, particularly during the morning hours. These results suggest that the intensification of zonal winds leads to the moisture flux, which in turn may result in the development of deep convection. The intensification of the wind might be caused by radiative forcings, such as the heating and cooling of the Tibetan plateau, as demonstrated by Egger (1987) using a low-resolution gridpoint model. Egger (1987) also showed that the circulation in the vicinity of a plateau contributes significantly to the fluxes of mass and moisture, and an increase in humidity may follow the increase of wind speed after a lag of a few hours. However, further theoretical and observational work is required to under-

6 NOTES AND CORRESPONDENCE FIG. 5. Composited diurnal averages (15 28 July 1998) of 200-mb winds; speed in m s 21 is indicated below the panels. Colored regions denotes the high magnitude wind. The four panels show the diurnal winds at 0000, 0600, 1200, and 1800 LT. FEBRUARY

7 468 MONTHLY WEATHER REVIEW VOLUME 128 FIG. 6. Composite diurnal averages (15 28 July 1998) of the 200-hPa velocity potential anomaly fields in units of 10 6 m 2 s 1. The four panels show the diurnal component at 0000, 0600, 1200, and 1800 LT. stand the exact mechanism of intensification of wind over the monsoon domain during evening hours. d. Dominant modes of variability and the phase and amplitude of the diurnal cycle We carried out spectral analysis of cloud motion wind (CMW) time series to determine the zones of influence of the diurnal-scale variability of upper-tropospheric winds during the monsoon season. Cloud motion winds are derived operationally every 90 min over the monsoon domain using the triplets of IR images (available each 30 min) from Meteosat-5. For the present spectral analysis, we used these CMW products at each 6-h interval for the period July Six-hourly CMWs at 200 hpa were gridded to a 2 lat 2 long

8 FEBRUARY 2000 NOTES AND CORRESPONDENCE 469 FIG. 7. Time series of (a) the zonal component of 200-hPa wind (m s 1 ) for a 60-day period starting from 0000 LT, 1 July The values are averaged over a box (10 20 N, E). (b) Same as (a) but for the cloud-top temperature. grid over a domain ( N, E). To remove the spatial inconsistencies and gaps in the gridded wind, we used the CMW 1.5 h before and/or after the fixed 6-h intervals. This procedure provided us with a sufficiently large region, where temporally continuous wind fields were available for a period of 14 days. The number of gridded CMW observations, at 200 hpa, over a 6-hourly period are shown in Fig. 9a. During a 15-day period most locations contain as many as observations. Over the Southern Hemisphere near 25 S, a minimum in the density of cloud-tracked winds are found; there are around five cloud-tracked wind observations over a 14-day period. Given this data density, we feel that some useful statistics can be arrived at for the region north of 10 S where an abundance of continuous cloud-tracked winds are noted. We performed Fourier analysis of the CMW time series only at those grid points where all 56 6-hourly observations were present. The 14th harmonic corresponds to the diurnal cycle, and the locations where the amplitude of this harmonic is the largest of all harmonics are considered to be the locations of the dominant diurnal mode (Fig. 9b, lightly shaded area). This region extends from India eastward across the Bay of Bengal, southeast Asia, and the south China Sea. The dark shaded regions in Fig. 9b show the region where cloud-tracked winds are seen to exhibit a longer timescale variability of the order 30 h. We have no explanation for the cause of these features. However, it is to be noted that these regions also show a significant diurnal variation of winds, but the longer time variability is stronger than the diurnal variability. Figs. 9c and 9d show the amplitude and phase for the zonal wind of the high-level cloud-tracked winds (projected to the 200-hPa level). The units for the amplitude are m s 1. The largest amplitude of diurnal oscillation ( 4 ms 1 ) resides in the accelerating part of the flow of the tropical easterly jet, between the equator and 10 N, over the Arabian Sea. Climatologically the strongest part of the tropical easterly jet is located downstream of this region of largest diurnal amplitude over the southern Arabian Sea. In general, the diurnal oscillations dominate the wind fields over a larger do-

9 470 MONTHLY WEATHER REVIEW VOLUME 128 FIG. 8. (a) 14-day average of 1.5-h zonal winds averaged over a box (10 20 N, E). (b) Same as (a), but for cloudtop temperature. main that includes the Arabian Sea, central and southern India, the Bay of Bengal, and up to the northern part of Malaysia, where the amplitude of diurnal mode exceeds 2 m s 1. The strong diurnal pulsation of the wind over the large area between 60 and 110 E must have some important dynamic implication for the maintenance of the tropical easterly jet maximum over the southern Arabian Sea. The spatial pattern of the diurnal phase suggests that the diurnal variability of winds propagates southward from the location of Tibet (30 N, 95 E) and then toward the west, in a clockwise rotation. Animation of the 200-mb winds also indicates that there is a downstream amplification of wind intensity, between 0 and 120 E. e. Viewing the diurnal modulation of convection from the TRMM satellite An example of high-resolution rainfall (mm day 1 ) derived from the TRMM s most recent rainfall algorithm is shown in Figs. 10a d, showing the diurnal fields for 0000, 0600, 1200, and 1800 LT, respectively. Figure 10 is based on 14 days of TRMM observations between 15 and 28 July 1998, and rainfall at each local hour is the mean rainfall rate during 3 h, with center at the local hour of interest (e.g., rainfall rate at 0600 is assumed to be the mean rain rate between 0430 and 0730 LT). Unlike geostationary satellites such as Meteosat-5, TRMM may require a number of days (depending upon the latitude of the area) to cover the entire diurnal cycle, due to its orbital characteristics. Thus, Fig. 10 represents a composite picture of events at different local hours, and it does not have a strict correspondence with Fig. 3, which shows the diurnal behavior of cloudiness during a single day. Also, the definition of local time differs slightly between the Meteosat-5 and TRMM products used in this study. For Meteosat-5 images and wind products, the local time refers to the approximate local time over the central Indian region, while in the case of TRMM, the local time of each point of the observation is its exact local time, which is a function of its longitude. This difference occurs due to the fact that the Meteosat-5 satellite observes the entire domain in its field of view, at the same time, while TRMM observes each segment at a different time, and thus TRMM observations can be easily sorted by their exact local times. Figure 10 clearly indicates the contrast of the rainfall distribution patterns between morning (0600 LT) and evening (1800 LT), over the regions surrounding the Indian subcontinent. During the morning hours, the precipitation is confined to the Bay of Bengal and the west coast of India, and the Indian landmass experiences a dry spell. During the evening hours, the location of high precipitation shifts over the land area. A comparison between Figs. 10a and 10d suggests that the oceanic convection, over the Bay of Bengal, decays between 1800 and 0000 LT, and builds rapidly between 0000 and 0600 LT. As indicated earlier, these observations belong to a period when no weather disturbances (lows or depressions) were present over the monsoon domain, and thus the observed features in Fig. 10 can largely be attributed to the diurnal-scale variations during the monsoon season. 4. Concluding remarks The continental-scale diurnal cycle of the monsoon circulation is best evidenced from mapping of the divergent circulations on this timescale. That became possible because of the availability of cloud-tracked winds over 90-min intervals from Meteosat-5. The lateral spread of this diurnal divergent circulation is seen to extend to central China and to the southern part of the China Sea, to the west to the western Arabian Sea, and to the south toward the equatorial latitudes. The amplitude of this divergent wind is comparable to those of the Hadley Walker overturnings, that is, 1 2 m s 1. Hence this diurnal system must have an important role in the overall maintenance of the monsoon circulation. Following Krishnamurti et al. (1998) an estimate of the magnitude of the energy exchange from the divergent to the rotational component was carried out that shows that this exchange for the diurnal component has a value of around m 2 s 3, which is approximately 20% of the average value of this exchange over the

10 FEBRUARY 2000 NOTES AND CORRESPONDENCE 471 FIG. 9. (a) Shaded region shows the area where complete time series of 6-hourly wind observations (200 mb) were available for the period Jul (b) Periods of dominant modes of variability of the 200-hPa winds. Lightly shaded area shows the region of dominant diurnal mode. Darker regions show a periodicity of longer period ( 30 h). Very light regions at a few places denote a periodicity smaller than 24 h ( 20 h). (c) Amplitudes of the diurnal mode of the 200-hPa winds (m s 1 ). (d) Phase (in degrees) of the diurnal mode of 200- hpa zonal winds.

11 472 MONTHLY WEATHER REVIEW VOLUME 128 FIG. 10. Composited surface rainfall rates derived from the TRMM TMI based on data for 14 days of July Units are mm day 1. The four panels show the diurnal components of precipitation rates for 0000, 0600, 1200, and 1800 LT.

12 FEBRUARY 2000 NOTES AND CORRESPONDENCE 473 monsoon domain, obtained from daily data (Krishnamurti et al. 1998). Further studies are needed to elucidate the role of this diurnal pulsation of winds on the overall maintenance of the monsoon. Numerical weather prediction and climate modeling results need to be examined in detail to determine the phase and amplitude of the diurnal-scale pulsation. The other important result of this study was on the diurnal amplification and weakening in the Tibetan high circulation. The tropical easterly jet on the southern flank of this anticyclone exhibits a strong diurnal fluctuation in its intensity. That evidently is related to the diurnal response to surface heating, convection, and the buildup and weakening of the thermal winds. Modeling of the continental-scale diurnal oscillation may require a high-resolution model that resolves diurnal convection and the associated divergent wind components. Furthermore, the land surface processes, the simulation of the diurnal cycle (its phase and amplitude), and the disposition along the vertical of the surface heat sources are important components. Simulation of the Tibetan high and the tropical easterly jet is a prerequisite for providing the proper setting for the steering of mesoconvective elements. The motion of these cloud clusters from the Bay of Bengal inland in the daytime hours and a reverse motion in the early morning hours appears to be related to the formation of a low pressure system that forms in the afternoon hours as the cloud systems are steered to the west. During the reverse phase the shallow low is steered to the east by the monsoon westerlies. More than several days are necessary to derive a composite global tropical structure of the diurnal mode of rainfall. Acknowledgments. The authors are thankful to the anonymous referees whose valuable suggestions contributed a great deal to improving the quality of this paper. This research was supported by the following grants to The Florida State University: NASA Grant NAG8-1199, NASA Grant NAG5-4729, NOAA Grant NA86GP0031, and NSF Grant ATM We are thankful to NASA and the European Space Agency for making the valuable data from TRMM and Meteosat-5 available to us. REFERENCES Ananthakrishnan, R., 1977: Some aspects of the monsoon circulation and monsoon rainfall. Pure Appl. Geophys., 115, Egger, J., 1987: Valley winds and the diurnal circulation over plateaus. Mon. Wea. Rev., 115, Haldar, G. C., A. M. Sud, and S. D. Marathe, 1991: Diurnal variation of monsoon rainfall in central India. Mausam, 42, IMD, 1998: Indian daily weather report. 15 pp. [Available from India Meteorological Department, Lodhi Road, New Delhi, , India.] Krishnamurti, T. N., M. C. Sinha, B. Jha, and U. C. Mohanty, 1998: A study of south Asian monsoon energetics. J. Atmos. Sci., 55, Kummerow, C., W. S. Olson, and L. Giglio, 1996: A simplified scheme for obtaining precipitation and vertical hydrometeor profiles from passive microwave sensors. IEEE Trans. Geosci. Remote Sens., 34, Murakami, M., 1983: Analysis of the deep convective activity over the western Pacific and Southeast Asia. Part I: Diurnal variation. J. Meteor. Soc. Japan, 61, Nitta, T., and S. Sekine, 1994: Diurnal variation of convective activity over the tropical western Pacific. J. Meteor. Soc. Japan, 72, Ramage, C. S., 1964: Diurnal variation of summer rainfall of Malaysia. J. Trop. Geogr., 19, Riehl, H., and A. H. Miller, 1978: Differences between morning and evening temperatures of cloud tops over tropical continents and oceans. Quart. J. Roy. Meteor. Soc., 104,