Perspectives on Moist Baroclinic Instability: Implications for the Growth of Monsoon Depressions

Abstract

Little is known about the genesis and growth mechanisms of monsoon depressions, despite the great importance of these storms for the hydrological cycle of the Asian–Australian monsoon region. Of the few theoretical studies that have examined this issue, most have attributed the amplification of monsoon depressions to some form of baroclinic instability or stable baroclinic growth, highly modified by the diabatic effects of moist convection. Here, a simple criterion—namely, the upshear tilt of potential vorticity anomalies—is argued be necessary for dry or moist baroclinic growth. Reanalysis data are then used to assess whether a large ensemble of South Asian monsoon depressions has vertical structures consistent with this criterion. The evolution of these monsoon depressions is compared with that of ensembles of hurricanes and diabatic Rossby waves, the latter being prototypical examples of moist baroclinic instability. During their amplification phase, monsoon depressions do not exhibit an upshear tilt of potential vorticity anomalies. Many similarities are found between developing monsoon depressions and hurricanes but few with diabatic Rossby waves. Thus, the mechanism responsible for the intensification of monsoon depressions remains unknown, but these results indicate greater similarity with the general process of tropical depression spinup than with moist convectively coupled baroclinic instability.

The Role of Warm Conveyor Belts for the Intensification of Extratropical Cyclones in Northern Hemisphere Winter

“…Establishing a link between our results and idealized baroclinic instability studies (e.g., De Vries et al 2010;Cohen and Boos 2016) is not straightforward mainly because we focus on the period of maximum intensification when finite-amplitude anomalies interact nonlinearly. For C1 cyclones, the strongest growth during the 24 h of maximum deepening occurs for the low-level PV anomaly (Figs.…”

“…S2). This indicates that for C1 cyclones the favorable interaction between upper-level (dry) PV and low-level (diabatically generated) PV is essential, which was referred to as a ''tropopause intrusion'' by Cohen and Boos (2016) and as ''type-C baroclinic growth'' by De Vries et al (2010). The latter terminology would be consistent with Plant et al (2003) and Ahmadi-Givi et al (2004), who also emphasized the interaction of prominent upper-and low-level PV anomalies for type C cyclones while surface thermal anomalies remain weak.…”

“…In this framework diabatic PV production is parameterized in terms of vertical motion, and PV anomalies are linearly partitioned into dry and moist components, which can interact with each other by their induced meridional and vertical velocities. Four unstable interactions have been identified between moist and dry waves in different layers, ranging from the classical dry instability (Eady 1949) to fully moist instability, with two intermediate types resulting from the interaction between a moist and a dry wave [see Cohen and Boos (2016) for a detailed review]. Hence, in three out of the four categories diabatic processes are crucial, but their interaction with the rest of the dynamics is different in each case.…”

Forcings and feedbacks on convection in the 2010 Pakistan flood: Modeling extreme precipitation with interactive large‐scale ascent

“…In some theoretical studies, the effects of latent heating are taken into account by replacing the dry static stability r by an effective (or moist) static stability r e in otherwise dry models [e.g., Kiladis et al, 2009;O'Gorman, 2011;Cohen and Boos, 2016]. In the QGx equation, one may define r e as r e 5r1 R px Q;…”

“…The new parameter r e absorbs all the complexities of convection and should, in principle, depend on the state of the column. However, in studies where this device is used, it is common to further specify that r e is constant fraction of r [e.g., Emanuel et al, 1994;Cohen and Boos, 2016]:…”