ABSTRACT
Bhuwan Chandra Bhatt
| The precipitation variability and circulation characteristics around the
Himalayas are examined using the Tropical Rainfall Measuring Mission (TRMM)
data, the GEWEX Asian Monsoon Experiment (GAME) reanalysis, NCEP-NCAR reanalysis
and some of the Indian radio sounding datasets. Though sampling errors
inherent to TRMM measurements around the Himalayas could influence our
results, PR-observed precipitation features show agreement with earlier
rainfall climatological studies that have utilized high resolution ground-based
datasets. By surveying the precipitation characteristics, one outstanding feature is the difference among the diurnal cycle over the Tibetan Plateau, northern Indian subcontinent and southern slopes of the Himalayas: there is a peak of precipitation in the afternoon to evening over the Tibetan Plateau and the northern Indian subcontinent which does not show significant variation on a seasonal scale; while the peak of precipitation has a pronounced seasonal and diurnal cycle over the southern slopes of the Himalayas. There is pronounced diurnal variation of precipitation with afternoon maximum in March-April-May (MAM) over the southern slopes of the Himalayas. Other seasons which exhibit daytime maximums are September-October-November (SON) and December-January-February (DJF). A unique feature of the summer monsoon precipitation is the midnight-early morning peak which is evident only over the southern slopes of the Himalayas. Another principal feature of the summer monsoon precipitation is the dichotomy between the diurnal cycle over the southern slopes and the adjacent foothill region: precipitation appears over the southern slopes at after midnight hours; while it is more frequent in the adjacent foothill region at early to late morning. It is anticipated that morning precipitation moves southward in the mature monsoon season. The secondary peak of precipitation over the southern slopes of the Himalayas appears during daytime in the summer monsoon season. From the analysis of near-surface rainfall over the finescale topography, it is observed that daytime precipitation is concentrated over the ridges and strong ridge-valley gradients with rain appearing over the southern slopes of the Himalayas in the summer monsoon season. During midnight-early morning, intense rainfall concentrates over the ridges as well as in river valleys. There is a relatively more intense rainfall at daytime over the ridges on the southern slopes in the premonsoon season. The results based on the two rain-rate thresholds (light rain rate, [\textless =5 mm h$^{-1}]$ and moderate to heavy rain rate, [\textgreater 5 mm h$^{-1}]$) suggest embedded convection within a large region of light rain during afternoon to evening over the southern slopes of the Himalayas in the summer monsoon season. The moderate to heavy conditional rain rate exhibits a relatively stronger diurnal cycle of precipitation over the southern slopes of the Himalayas. However, it may be biased because of sampling. Unlike the horizontal distribution of precipitation, the vertical distribution of precipitation shows almost similar geographical variability over the southern slopes of the Himalayas. There is trailing stratiform precipitation regime over the southern slopes of the Himalayas in the summer monsoon season. However, premonsoon precipitation is noticed as isolated deep precipitation cells with no brightband. It is anticipated that premonsoon precipitation cells can develop vertically deep with lightning. The diagnostic analysis of the atmospheric system is also performed. The GAME reanalysis reveals a robust diurnal cycle of the atmospheric system that is coherent with the diurnal cycle of precipitation around the southern slopes of the Himalayas. The premonsoon precipitation seems to be associated with deep convections. The significant increase of the moisture due to the southeasterly wind in the mature monsoon season seems to produce a favorable condition for the midnight-early morning rain. The nighttime cooling at high mountains could enhances the down-valley wind. The collision of the up-valley wind by general monsoon flow and the down-valley wind forces precipitation systems during midnight-early morning. The radiative cooling at the top of clouds may enhance the convection. The precipitation associated with the moist convection seems to generate a cold pool which results in a density current. The downward movement of the density current causes the southward movement of the precipitation system in the morning. |
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