Case Study of a Convective Cluster Over the Rain Shadow Region of Western Ghats Using Multi-platform Observations and WRF Model

Abstract

Unique observational features of a convective cluster, occurred on 12-Sept-2015, over the rain shadow region in the leeward side of Western Ghats have been presented in this study. The synoptic environment had preconditioned the formation of a convergence zone over the study area. Moisture transport from the Arabian Sea was responsible for the moistening in the lower layers. Boundary layer convective thermals contributed to middle level moistening and subsequent onset of the cloud cluster was accompanied by a sudden surge of moist and warm air into the middle troposphere, and subsequent lifting of freezing level (FL) and wet bulb temperature zero (WBT₀) levels. Sudden changes in the FL and WBT₀ levels in association with the gust front prior to the initiation of the cloud system has been documented with high-resolution measurements using microwave radiometer and wind profiler. Thermodynamical parameters from radiometer illustrate the percussive conditions for formation of the cloud system. The cloud cluster had resulted in 25.50 mm rainfall, attributing to ~ 91% of convective rain. Intense fall velocity (10–12 ms⁻¹) was noted up to ~ 7 km during the convective rain and the fall velocity was reduced to ~ 7 ms⁻¹ (below the melting layer) during the stratiform counterpart. The cloud system was forecasted using WRF model (version 3.6.1), which was reproduced reasonably well as in the observations and the model output has been analyzed to understand the morphology of the system. The features such as formation of a cold pool, initiation of convective rainfall from the system were well forecasted by the model. Microphysical characteristics of the cloud cluster have also been examined. Riming was the dominant microphysical process within the convective regime. A major contribution to precipitation was from melting of ice hydrometeors especially graupel and snow was noted. Deep warm layer and associated production of supercooled liquid by the lifting of liquid water above the freezing level in updrafts exceeding 15 ms⁻¹ was important for the production of a mixed-phase cloud system. Vapor deposition and aggregation process was noted in the stratiform/anvil counterpart, which also contained mixed phase hydrometeors, primarily of snow.

Appendix

1.1 CAPE

Convective available potential energy (CAPE) indicates the maximum amount of energy, associated with an ascending air parcel, available for convection. It is considered as the integrated positive area on a Skew-T sounding and expresses the atmospheric condition between the level of free convection (LFC) and level of neutral buoyancy (LNB). Mathematically it is defined as:

$$CAPE = \mathop \int \nolimits_{LFC}^{LNB} g \frac{Tv, parcel - Tv,env}{Tv,env} dz$$

where T_{v, parcel} denotes the virtual temperature of the ascending parcel, T_v,env denotes the virtual temperature of the environment and ‘g’ denotes the gravitational acceleration.

CAPE is measured in Joules per kilogram (J kg⁻¹). It is a very useful indicator for the formation of the convective system. A higher value of CAPE indicates a high probability of convection with intense updraft speed.

1.2 CINE

Convective inhibition (CINE) measures the required amount of energy to overcome the negatively buoyant area of a Skew-T diagram for convection to occur. A triggering mechanism is required to overcome the inhibition. Surface heating, frontal lifting, lower level convergence, and upper-level divergence can act as good triggering mechanism. Mathematically CINE is defined as:

$$CINE = \int \nolimits_{Sfc}^{LFC} g \frac{Tv, parcel - Tv,env}{Tv,env} dz$$

where T_{v, parcel} denotes the virtual temperature of the ascending parcel, T_v,env denotes the virtual temperature of the environment and ‘g’ denotes the gravitational acceleration.

The unit of CINE is Joules per kilogram (J kg⁻¹). It measures the stability of the atmosphere. Higher (negative) value of CINE makes an unfavorable condition for convection to occur. Convection is likely to occur with a high CAPE, low CINE in presence of triggering mechanism.