Abstract
Indian monsoon depressions (MDs) are synoptic-scale cyclonic systems that propagate across peninsular India three or four times per monsoon season. They are responsible for the majority of rainfall in agrarian north India, so constraining precipitation estimates is of high importance. Here, a case study from August 2014 is used to explore the relationship between varying soil moisture and the resulting track and structure of an incident MD using the Met Office Unified Model. This case study is chosen with the view to increasing understanding of the general impact of soil moisture perturbations on monsoon depressions. It is found that increasing soil moisture in the monsoon trough region results in deeper inland penetration and a more developed structure—for example, a warmer core in the midtroposphere and a stronger bimodal potential vorticity core in the mid-to-lower troposphere—with more precipitation, and a structure that in general more closely resembles that found in depressions over the ocean, indicating that soil moisture may enhance the convective mechanism that drives depressions over land. This experiment also shows that these changes are most significant when the depression is deep and negligible when it is weakening. Increasing soil moisture in the sub-Himalayan arable zone, a region with large irrigation coverage, also caused deeper inland penetration and some feature enhancement in the upper troposphere, but no significant changes were found in the track heading or lower-tropospheric structure.
Highlights
Indian monsoon depressions (MDs) are synoptic scale systems that usually originate in the Bay of Bengal and propagate northwestward across the Indian peninsula, with a mean duration of 4-6 days, and an average frequency of between two and four per summer (Boos et al 2015; Hunt et al.2016a)
The pale green area underneath is a concave hull of all points of all ensemble tracks from the control sub-experiment
A first inspection of the average tracks seems to suggest that an increase in underlying antecedent soil moisture results in deeper penetration of MDs through the monsoon trough region - this is visible both in the average termination points and the individual ones
Summary
Indian monsoon depressions (MDs) are synoptic scale systems that usually originate in the Bay of Bengal and propagate northwestward across the Indian peninsula, with a mean duration of 4-6 days, and an average frequency of between two and four per summer (Boos et al 2015; Hunt et al.2016a) Their spin-up mechanism remains uncertain (Cohen and Boos 2016), it appears likely that convective instability of the second kind (CISK; Charney and Eliassen 1964) plays at least some role (Shukla 1978); their primary propagation mechanism has been well described, albeit fairly recently (Boos et al 2015; Hunt and Parker 2016), as a coupling of horizontal nonlinear advection of the mid-tropospheric potential vorticity maximum and an image vortex interaction of the lower-tropospheric PV maximum with the no-normal flow condition imposed by the Himalayas. This is to be an important process in MDs, it is likely to be indirect (it must overcome a negative feedback at the MD centre – the associated lower-tropospheric cold core (Godbole 1977; Hunt et al 2016a) acts to cool the surface and increase stability there): the area of maximum precipitation is found to the southwest of the centre (e.g. Ramanathan and Ramakrishnan 1933) where the (adiabatic) quasigeostrophic omega equation (e.g. Holton and Hakim 2012) predicts the greatest ascent associated with the balanced
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
