Characteristics of Deep Cloud Systems under Weak and Strong Synoptic Forcing during the Indian Summer Monsoon Season

Abstract

Synoptic-scale weather systems are often responsible for initiating mesoscale convective systems (MCSs). Here, we explore how synoptic forcing influences MCS characteristics, such as the maximum size, lifespan, cloud-top height, propagation speed, and triggering over the Indian region. We used 30-min interval infrared (IR) data of the Indian Kalpana-1 geostationary satellite. Cloud systems (CSs) in this data are identified and tracked using an object tracking algorithm. ERA-Interim 850-hPa vorticity is taken as a proxy for the synoptic forcing. The probability of CSs being larger, longer lived, and deeper is more in the presence of a synoptic-scale vorticity field; however, the influence of synoptic forcing is not evident on the westward propagation of CSs over land. There exists a linear relationship between maximum size, lifespan, and average cloud-top height of CSs regardless of the nature of synoptic forcing. Formation of CSs peaks around 1500 LST over land, which is independent of synoptic forcing. Over the north Bay of Bengal, CSs formation is predominantly nocturnal when synoptic forcing is strong, whereas, 0300 and 1200 LST are the preferred times when synoptic forcing is weak. Long-lived CSs are preferentially triggered in the western flank of the 850-hPa vorticity gradient field of a monsoon low pressure system. Once triggered, CSs propagate westward and ahead of the synoptic system and dissipate around midnight. Formation of new CSs on the next day occurs in the afternoon hours in the wake of previous day’s CSs and where vorticity gradient is also present. Formation and westward propagations of CSs on successive days move the synoptic envelope westward.

Large‐Scale Extreme Rainfall‐Producing Synoptic Systems of the Indian Summer Monsoon

“…The gradients in vorticity and temperature help in setting up the rising motion (e.g., using QG omega equation) that results in forming a favorable region for long-lived cloud systems on the western flank of LPSs. The large long-lived clouds systems occur on the western flank of LPSs mainly in the band of 15-25 • N (Figure 11 of Phadtare & Bhat, 2019), which coincides with the location of LEREs of our study. It should also be noted that the adiabatic lifting is not sufficient to explain the deep convective systems on the western flank of LPSs (Adames & Ming, 2018b, 2018aBoos et al, 2015) and other processes that include moist thermodynamics also add ascent.…”

“…At Day‐0, the ascents merge and form a spatially large ascending region. It has been observed that the long‐lived cloud systems are preferentially triggered on the western flank of LPSs, where a steep gradient of low‐level vorticity and temperature is present (Phadtare & Bhat, 2019). The gradients in vorticity and temperature help in setting up the rising motion (e.g., using QG omega equation) that results in forming a favorable region for long‐lived cloud systems on the western flank of LPSs.…”

Unravelling the mechanism of summer monsoon rainfall modes over the west coast of India using model simulations

“…A trough present over the Indian peninsula and the Arabian Sea during the offshore mode might be promoting the westward propagation of rainfall. A similar phenomenon over the Indian region was noted by Phadtare and Bhat (2019), where deep clouds predominantly formed in the western flank of the trough and moved further westward.…”

“…A similar phenomenon over the Indian region was noted by Phadtare and Bhat (2019), where deep clouds predominantly formed in the western flank of the trough and moved further westward.…”

Multiscale interactions between monsoon intra-seasonal oscillations and low pressure systems that produce heavy rainfall events of different spatial extents

“…By using the quasi geostrophic (QG) omega equation (Hoskins et al 1978) we examine the dynamic forcing that leads to the observed differences in the distribution of vertical velocity. We use the modified definition of Q-vector (−2∇ • Q) (Kiladis et al 2006;Phadtare and Bhat 2019;Boos et al 2015) for the tropics, where the geostrophic wind is replaced by the rotational wind.…”