Tropical low formation and over land and sea as seen in the ECMWF analyses

This talk presents the results of several case studies of tropical low formation over both land and sea during the Australian monsoon as seen in the European Centre for Medium Range Weather Forecast (ECMWF) analyses. One particular case involved a system that formed over the sea near Darwin and subsequently intensified over land. Another disturbance intensified as it made a loop over land. Other cases show formation occurring over land with intensification taking place when the system moves over the sea.

In all cases, interpretations of formation and intensification are given in terms of vorticity dynamics. The analyses indicate that the intensification of a low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure.

The thermodynamic support for intensification and maintenance over land is investigated also. It is found that the horizontal transport of moisture into a column centred on the low is approximately equal to the moisture lost by precipitation. The contribution to the moisture budget by surface fluxes is small in comparison. Provided that the surrounding environment of the storm has high enough values of total precipitable water, then convective bursts can continue to occur even when the system is located far inland.

The findings indicate that the processes of intensification are the same over land as those that operate over the ocean. There appears to be no need to invoke complex soil processes to explain intensification over land.

References:

Smith RK, Montgomery MT, Kilroy G, Tang D, and Müller S, 2015: Tropical low formation during the Australian monsoon: the events of January 2013. Submitted to Aust. Meteor. Ocean. Journl. (in press).

G. Kilroy, R. K. Smith, M. T. Montgomery, B. Lynch and C. Earl-Spurr, 2015: A case study of a monsoon low that intensified over land as seen in the ECMWF analyses. Submitted to Quart. J. Roy. Meteor. Soc. (Oct).