Understanding the vertical structure of potential vorticity in tropical depressions

Abstract

Potential vorticity (PV) has been used to understand the intensification and motion of a variety of tropical vortices. Here, atmospheric reanalyses and idealized models are used to understand how the vertical structures of moist convective heating and adiabatic advection jointly shape the vertical structures of PV in tropical depressions. Observationally based estimates reveal a top‐heavy PV structure in tropical depressions, contrasting with bottom‐heavy structures of absolute vorticity and diabatic PV generation. These distinct vertical structures are reproduced in an axisymmetric model which employs the weak temperature gradient approximation for conceptual simplicity and is forced by stratiform and deep convective heating. When applied in isolation, the stratiform and deep convective heatings produce PV maxima at 500 hPa and near the surface, respectively. When these two heatings are applied simultaneously, interactions between the stratiform and deep convective modes enhance the adiabatic advective tendencies produced by the transverse circulation, making the PV distribution more top‐heavy. In the lower and middle troposphere, radial advection also greatly reduces the radius of the PV structure relative to that of the imposed heating, consistent with structures in observed tropical depressions; the implications of these differences in radial structures for using the flux form of the relevant conservation equations (e.g. for PV substance or absolute vorticity) are discussed.

Mesoscale precipitation systems and their role in the rapid development of a monsoon depression over the Bay of Bengal

“…The low as a whole develops rapidly through such processes related to the MCS life cycle. Murthy and Boos () also demonstrated that heating from both deep convection and stratiform precipitation systems can produce the observed vertical distribution of potential vorticity in tropical depressions, indicating the development of tropical depressions through the generation of positive potential vorticity from the heating. Our study revealed that deep convection cells and stratiform precipitation are indeed observed within the low in the rapid development phase of the monsoon depression.…”

“…latent heating) in the growth of monsoon depressions. Murthy and Boos () found that both deep convective and stratiform heating produce PV maxima around 500 hPa in tropical depressions. Previous studies have proposed moist baroclinic instability as a possible cause of depression growth (e.g.…”

A Global Tropical Survey of Mid-Tropospheric Cyclones

“…Over the past decade, the emergence of high-resolution reanalysis data sets that assimilate vast amounts of satellite and ground-based observations have shown promising results in explorations of the climatology and structure of tropical lows and depressions (Hurley and Boos, 2015;Praveen et al, 2015). While there are issues with under resolution of the lower tropospheric circulation (Manning and Hart, 2007;Hodges et al, 2017), these products have been successfully used to understand rainfall distribution, dynamic & thermodynamic details, and the propagation of tropical cyclonic systems (Hunt et al, 2016a;Boos et al, 2015;Sørland and Sorteberg, 2015;Hunt et al, 2016b;Murthy and Boos, 2019). Here, our objective is to understand the structure of tropical systems with vorticity maxima in the middle or lower troposphere.…”

“…Specifically, similar to monsoon lows (Boos et al, 2015;Hunt et al, 2016a) it shows one maximum in the middle troposphere and another in the lower troposphere. The confinement of MTCs PV in the middle troposphere may be related to the high fraction of stratiform heating (Choudhury et al, 2018;, while the LTC PV is likely affected by significant deep convective heating (Murthy and Boos, 2019).…”

“…These systems and their stronger versions (known as tropical cyclones; TCs) occur throughout the tropics and have an upright vorticity structure with wind and convergence maxima in the lower troposphere. Though in comparison to TCs, which have a warm anomaly through the depth of the troposphere (Wang and Jiang, 2019), lows and depressions usually show a shallow near surface cold-core and a bimodal potential vorticity structure with one peak near 500 hPa and another near the boundary layer (Boos et al, 2015;Murthy and Boos, 2019).…”