Structure and Propagation of Indian Low-Pressure Systems in GFDL AM4.0

The mechanisms that lead to the propagation of anomalous rainfall in monsoon low pressure systems are investigated using daily output fields from GFDL's atmospheric general circulation model version 4.0 (AM4.0). On the basis of linear regression analysis of westard-propagating rainfall anomalies of timescales shorter than 15 days, it is found that these systems are organized into wavetrains of 3-4 individual events. These events amplify over the Bay of Bengal, reach a maximum amplitude over the eastern coast of India and dissipate as they approach the Arabian Sea.

It is also found that the precipitation anomalies in SMDs are highly correlated in space with column-integrated water vapor. Based on this correlation, we analyze the column moisture and frozen moist static energy (MSE) budgets as proxies for the propagation of the precipitation anomalies. Propagation of the moisture anomalies is dominated by vertical moisture advection while the MSE anomalies propagate due to horizontal advection of dry static energy by the anomalous winds. By combining the budgets, we interpret the propagation of the precipitation anomalies in terms of lifting that is forced by horizontal dry static energy advection, that is, ascent along sloping isentropes. This process moistens the lower free-troposphere, producing an environment that is conducive to deep convection. Longwave radiative heating is the main mechanism for maintaining MSE and precipitation anomalies.