Quasigeostrophic Controls on Precipitating Ascent in Monsoon Depressions

Abstract

South Asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind. Observations of maximum precipitation downshear of the cyclonic center have led to prior theories of quasigeostrophic (QG) control of moist convection in these storms. This study examines the interaction between adiabatic QG lifting and moist convection in monsoon depressions using an atmospheric reanalysis and idealized model. Inversion of the QG omega equation in the reanalysis shows that in the downshear, heavily precipitating region, adiabatic QG ascent, due to advection of vorticity and temperature, is comparable to diabatic ascent in the lower troposphere, while diabatic ascent dominates in the middle and upper troposphere. The causal influence of adiabatic QG lifting on precipitating ascent in monsoon depressions is then examined in the column QG modeling framework, where moist convection evolves in the presence of vorticity and temperature advection. The heavy observed precipitation rates are only simulated when moist convective heating amplifies QG ascent, with this interaction accounting for roughly 40% of the increase in precipitation relative to the basic state. Another 40% of this increase is produced by enhanced surface wind speed in the surface enthalpy flux parameterization, which represents the effect of cyclonic winds in the monsoon depression. Horizontal advection of the mean-state poleward moisture gradient accounts for the remaining 20% of the precipitation increase. In the upshear region, adiabatic QG subsidence and horizontal moisture advection both suppress precipitation, and are opposed by wind-enhanced surface enthalpy fluxes.

The structure of strong Indian monsoon low‐pressure systems in subseasonal‐to‐seasonal prediction models

“…In contrast, it is located to the relative south of the centre in the BoM, CMA, ECCC, HMCR, and ISAC-CNR models and the MMM. The region of maximum precipitation is located to the westsouthwest of the LPS centre, which is attributed to quasigeostrophic dynamical lifting (e.g., Sanders, 1984;Boos et al 2015) and horizontal moisture advection (e.g., Adames and Ming 2018), with adiabatic quasigeostrophic ascent dominating in the lower troposphere over the maximum precipitation region (Rajamani and Rao, 1981;Murthy and Boos, 2020). We investigate moisture-flux convergence at 850 hPa (Figure S3), since other fields, such as vertical velocity, are not commonly available in S2S models.…”

Interactions between Water Vapor, Potential Vorticity, and Vertical Wind Shear in Quasi-Geostrophic Motions: Implications for Rotational Tropical Motion Systems

“…Both v 0 a and v 0 Q can be diagnosed from the QG omega equation, as was done by Nie and Sobel (2016) and Murthy and Boos (2020). However, we will focus on the thermodynamic equation [Eq.…”

Upper‐Tropospheric Troughs and North American Monsoon Rainfall in a Long‐Term Track Dataset

“…The pattern and magnitude of the pressure velocity are similar. This implies that the upper‐level ascent and descent in the vicinity of a TUTT center is largely the result of dry adiabatic processes rather than some interaction with moist convection, a notable finding given the great importance of diabatic effects for ascent in lower‐tropospheric vortices in some monsoon regions (e.g., Murthy & Boos, 2019). Figure 7c shows vertical profiles of pressure velocity.…”