A Phenomenological Paradigm for Midtropospheric Cyclogenesis in the Indian Summer Monsoon

Abstract

Midtropospheric cyclones (MTCs) are a distinct class of synoptic disturbances, characterized by quasi-stationary cyclonic circulation in midtropospheric levels, which often produce heavy rainfall and floods over western India during the summer monsoon. This study presents a composite and diagnostic process study of long-lived (>5 days) midtropospheric cyclonic circulation events identified by the India Meteorological Department (IMD). Reanalysis data confirm earlier studies in revealing that the MTC composite has its strongest circulation in the midtroposphere. Lagged composites show that these events co-occur with broader-scale monsoon evolution, including larger synoptic-scale low pressure systems over the Bay of Bengal (BoB) and east coast, and the active phase of regional-scale poleward-propagating intraseasonal rain belts, with associated drying ahead (north) of the convectively active area. Diabatic heating composites, in particular the TRMM latent heating and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2)-derived radiative cooling in the dry inland areas of southwest Asia north of the rain belt, are used to drive a nonlinear multilayer dynamical model in a forced-damped reconstruction of the global circulation. Results show that the midlevel circulation is largely attributable to top-heavy latent heating, indicative of the prevalence of stratiform-type precipitation in mesoscale convective systems in these moist, active larger-scale settings. Both the west coast and BoB latent heating are important, while the radiative cooling over southwest Asia plays a modest role in sharpening some of the simulated features. A conceptual model encapsulates the paradigm based on this composite and diagnostic modeling, a diabatic update of early theoretical studies that emphasized hydrodynamic flow instabilities.

Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon

“…The dynamical core of this model is based on a spectral formulation (Bourke 1974) having a rhomboidal truncation at zonal wavenumber 40 (R40), which corresponds to a Gaussian grid with an approximately horizontal resolution of 2.835° (longitude) and 1.76° (latitude) and employs sigma coordinates in the vertical direction with 25 vertical levels. The simplicity of the model makes it particularly helpful to derive dynamical insights on different aspects of monsoonal phenomena (e.g., Kasture et al 1991;Krishnan and Kasture 1995;Sundaram et al 2010;Choudhury and Krishnan 2011;Choudhury et al 2018).…”

“…Given that LH is the dominant component of diabatic heating associated with precipitation variations over the tropics and monsoonal areas (e.g., Yanai and Tomita, 1998), we have employed the TRMM-based LH fields to force the model. The details of constructing the JJAS climatological mean LH are described in Choudhury and Krishnan (2011) and Choudhury et al (2018). Figure 5 shows the column-mean JJAS climatology of LH and the simulated mid-level (in model corresponds to σ = 0.58-0.42) circulation.…”

“…Yasunari (1986) suggested that diabatic heating fluctuations over the ISM environment can induce low-frequency oscillatory behaviour of the extra-tropical upper level westerlies on intra-seasonal time-scales. A major component of diabatic heating over the tropics and monsoon dominated areas involves LH release from organised convection (e.g., Houze 1997;Yanai and Tomita 1998;Schumacher et al 2004;Houze et al 2007;Krishnamurti et al 2010;Choudhury et al 2018). Studies have noted that interactions between the LH from the Asian monsoon precipitation and the mean subtropical westerly flow environment can generate a westward extending stationary Rossby waves, leading to pronounced summer-time descent over the Mediterranean and Sahara regions (Rodwell and Hoskins 1996;Liu et al 2001).…”

Introduction to Climate Change Over the Indian Region

“…Houze 1997;Houze et al 2007;Stano et al 2002; Romatschke and Houze 2011); as well as the monsoonal circulation response to latent heating based on numerical simulation experiments (e.g. Choudhury and Krishnan 2011;Choudhury et al 2018) in the recent decades.…”

Atmospheric diabatic heating–induced wave train from the Caspian Sea to South and East Asia during the summer monsoon season

“…The variations of diabatic heating over the ISM region can lead to lowfrequency oscillations in extratropical upper-level westerlies on intraseasonal time scales (Yasunari 1986). The latent heat release from organized convection is a key component of diabatic heating over the tropics and monsoon region (Houze 1997;Yanai and Tomita 1998;Schumacher, Houze, and Kraucunas 2004;Houze, Wilton, and Smull 2007;Krishnamurti, Chakraborty, and Mishra 2010;Choudhury et al 2018). Two-thirds of the global rainfall are received over the tropical region (Tao et al 2006), and the associated latent heat contributes the dominant part of the diabatic heating spatiotemporal distribution.…”