A numerical study of rotating convection during tropical cyclogenesis

Abstract

We present idealized numerical model experiments to investigate the convective generation of vertical vorticity in a tropical depression. The ambient vertical vorticity is represented by a uniform solid-body rotation. The calculations are motivated by observations made during the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment. A specific aim is to isolate and quantify the effects of low-to mid-level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. The results do not support a common perception that dry air aloft produces stronger convective downdraughts and more intense, cold-air outflows therefrom. Indeed, we find that dry air aloft weakens both updraughts and downdraughts, corroborating the recent results of James and Markowski.As in the recent calculations of Wissmeier and Smith, the growing convective cells locally amplify the ambient rotation at low levels by more than an order of magnitude and this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Moreover, significant amplification of vorticity occurs even for clouds of only moderate vertical extent. The maximum amplification of vorticity is relatively insensitive to the maximum updraught strength, or the height at which it occurs, and it is not unduly affected by the presence of dry air aloft. Thus the presence of dry air is not detrimental to the amplification of low-level vorticity, although it reduces the depth through which ambient vorticity is enhanced.Results for a limited number of different environmental soundings indicate that the maximum amplification of vorticity increases monotonically with the strength of the thermal perturbation that initiates the convection, but the amount of increase depends also on the thermodynamic structure of the sounding.

An investigation of how radiation may cause accelerated rates of tropical cyclogenesis and diurnal cycles of convective activity

“…This present study makes a similar argument for the effect of enhanced relative humidity in the tropical disturbance due to radiative forcing. Support for the hypothesis is provided by recent cloud model experiments that indicate that for non-supercell environments a major effect of dry air aloft is to reduce the intensity of convection, including total condensation and rainfall (James and Markowski, 2009;Kilroy and Smith, 2013).…”

Tropical low formation during the Australian monsoon: the events of January 2013 (paper updated July 2016)

“…10). This drying would have suppressed deep convection in that region, thereby reducing the ability of the system to concentrate vorticity (Kilroy and Smith 2013) and increase its circulation (see section 3). …”

Examining the Roles of the Easterly Wave Critical Layer and Vorticity Accretion During the Tropical Cyclogenesis of Hurricane Sandy

“…As the disturbance translates westward and evolves from 18 to 20 October, it approaches a region in the Caribbean Sea with u e values on the 850-hPa surface that are above 345 K (not shown). These values represent a favorable thermodynamic environment for deep convection and vorticity amplification by vortex tube stretching in the low and middle troposphere (Kilroy and Smith 2012). The open pathway from the south allows the very moist air from the south and southeast to become entrained into the pre-Sandy wave, while excluding the drier air to the north.…”