Understanding Controls on Monsoon Depression Intensity
Propagating atmospheric vortices in the South Asian and Australian monsoon regions produce a large fraction of seasonal mean rainfall in these regions. The more intense instances of these systems are known as monsoon depressions and are responsible for the majority of extreme precipitation events. However, the exact mechanism through which these disturbances intensify remains a topic of active research. UC Berkeley researchers found that the horizontal wind shear and gradients in environmental humidity can both contribute to the growth of monsoon depressions but with some degree of decoupling of the rotational winds and the precipitating convection.
Many attempts have been made to understand the mechanisms governing the evolution of monsoon depressions, invoking hydrodynamic instabilities that amplify the vortex at the expense of the background winds or through its coupling with moist convection. By using an idealized moist model with parameterized precipitation, this study finds that both barotropic and moisture-advection mechanisms of instability can contribute to monsoon depression amplification. The existence of differing sensitivities of the vortex intensification rate and the precipitation intensification rate to the environmental gradients is a novel finding. This work has implications for the forecasts of monsoon depressions, suggesting that high-resolution numerical models might better represent the intensification of monsoon depressions as model bias in the background low-level wind and humidity is reduced. These issues also have great importance for disaster preparedness, given the large fraction of South Asian hydrological disasters that are associated with monsoon depressions.
UC Berkeley researchers studied the growth of monsoon depressions in an idealized moist model. They found that both the horizontal wind shear and gradients in environmental humidity can contribute to the growth of monsoon depressions, with the rotational winds being amplified most strongly by horizontal wind shear and the precipitation rates being most sensitive to environmental moisture gradients. These results highlight the possibility that rotational winds in a lower-tropospheric vortex can intensify at different rates than the precipitation in such a vortex.