Assessing the Influence of Background State on Monsoon Vortex Intensification
Monsoon depressions are atmospheric vortices that are about 1,000 km wide; they produce extreme rainfall and catastrophic floods, yet the conditions that foster their intensification are poorly understood. UC Berkeley researchers in the RGMA Monsoon Extremes project used a high-resolution, cloud-resolving model to simulate a large number of monsoon depressions in different environments, then analyzed how energy was transferred between these environments and the vortices.
This work showed that two features of the background environment are highly important for the intensification of monsoon depressions. First, there must be large changes in the horizontal wind speed with latitude, which is referred to as horizontal wind shear; this shear allows the vortex to intensify through a hydrodynamic process known as barotropic instability. Second, the amount of water vapor in the atmosphere must be larger to the north of the vortex than to the south, so that the rotational winds around the vortex wrap humid air around the storm and thus feed precipitation. From the many simulations conducted for this work, the researchers created a multi-dimensional array characterizing the sensitivity of intensification rates to vortex environments. This advances understanding of intensification mechanisms and will be useful in constructing and assessing projections of future monsoon depression behavior.
UC Berkeley researchers examined processes fundamental to the intensification of monsoon depressions using an array of simulations of an idealized high-resolution atmospheric model. In each simulation, a vortex of initially small amplitude is subjected to a different background environment. Based on the evolution of the initial vortex, two features of the background environment emerged as important: the low-level gradient of water vapor and the low-level gradient in eastward wind speed. As the low-level water vapor gradient steepens, the vortex becomes stronger and produces more rain. This strengthening occurs when vortex winds transport water vapor counter-clockwise and upward, causing it to condense and release heat as it forms rain. The vortex also becomes stronger with increasing gradients of eastward wind speed, which makes the environment more barotropically unstable. The absence of either of these two features of the background environment prevents the growth of the vortex.