Deep Convective Cloud Constraints on the General Circulation of Titan

The influence and implications of deep convection on Titan's global heat and moisture budgets and on the mean meridional circulation (i.e., the Hadley cell) are explored. There is a strong coupling and interdependency between deep convective clouds and their environment. The environment, through the generation of convective available potential energy (CAPE) by radiative processes and through large scale advection, controls the nature of convective clouds. While the environment constrains the cloud dynamics, it is equally true that convective cloud processes feed back to the environment. Convective clouds consume CAPE and efficiently transport heat and moisture through vertical advection within the cloud cores, detrainment of cloudy air near cloud top, and through adiabatic descent of the cloud-free environment required to offset the upward mass flux within the convection. Based on numerical simulations with the Titan Regional Atmospheric Modeling System, deep convective clouds on Titan consume CAPE on a timescale ranging from 2-10 hours, depending on the initial relative humidity. Over these timescales, the convection cools/warms, by approximately several K/earth-day, the lower/upper levels of the atmosphere. Drying/moistening of the lower/upper atmosphere is also found to be several g/kg/earth-day. If the temperature and moisture profiles are to remain quasi-constant (i.e., CAPE remains quasi-constant), then there must be a net low level meridional flux of heat and moisture to balance the convective tendencies, since radiative processes operate on substantially longer timescales. For the case of southern summer convective clouds in the polar latitudes, this flux is shown to be ~1 m/s. However, unless heat is resupplied to the lower latitudes, this poleward heat flux would result in a change in the large-scale temperature gradient. The large-scale near-surface temperature gradient on Titan is on the order of ~5K from pole to pole. If this gradient is quasi-steady, then a meridional advective velocity of 100 m/s or more would be required to balance south polar convection. We provide a more complete energy and moisture budget analysis based on cloud model results to better constrain the magnitude of Hadley cell circulation of Titan.