Control of Tropospheric Static Stability on the Organization of Mesoscale Convective Systems and Tropical Cyclones

The development of deep convection and the organization into mesoscale convective systems (MCSs) depend on the environmental conditions such as the vertical structures of temperature, humidity, and winds. Vertical shears are known to determine the organization mode of MCSs, while temperature and humidity critically affect the development and intensity of MCSs. The vertical structures of tropospheric temperature significantly vary depending on climatological conditions, large-scale atmospheric circulation, and weather settings, and have impacts on the generation of MCSs. This study numerically investigated the effects of tropospheric static stability on the organization and intensity of MCSs and examined the commonality with the case of tropical cyclones (TCs).

For the MCS case, we conducted a number of numerical experiments by changing the tropospheric temperature lapse rate (i.e., temperature at the tropopause and the tropopause height) as well as moisture profile with the use of the Weather Research and Forecasting (WRF) model at a convection-resolving resolution. It was shown that the strength and areal extent of updrafts within the simulated squall lines are significantly regulated by environmental temperature lapse rate. A condition with a larger lapse rate leads to the development of widespread, strong updrafts. The difference in the vertical profile of buoyancy for lifted air parcels is a key to delineate the difference in the updraft statistics under different thermodynamic conditions. In contrast, it was found that the precipitation amount is controlled by the vertical distribution of convective available potential energy (CAPE). The precipitation amount increases with the increase in the depth of the layer having a significant amount of CAPE. In this way, tropospheric lapse rate is useful in comparing the characteristics of precipitation by MCSs.

For the TC case, we investigated how the tropospheric stability change affects the intensification of TCs by conducting a large number of sensitivity experiments with the axisymmetric version of Cloud Model Version 1 (CM1). As in the MCS case, the evolution and intensity of the simulated TCs are strongly affected by the change in temperature lapse rate. Compared to the lapse rate, it was found that the increase in tropopause height and the decrease in tropopause-level temperature play a secondary role, although under a unchanged lapse rate condition a higher tropopause and a colder tropopause-temperature would add a positive impact on the intensification of TCs.