Vertical Advection Term Research Articles

A Modeling Study of Heating and Drying Effects of Convective Clouds in an Extratropical Mesoscale Convective System

The two-dimensional version of the cumulus ensemble model developed by Soong and Ogura is applied both to a prestorm situation and to the mature stage of the extratropical mesoscale convective system (MCS) that developed on 10–11 April 1979 (AVE-SESAME-79 I) over the central United States. The objective is to investigate the statistical properties of convection, developing in response to an imposed large-scale forcing, and the thermodynamic feedback effect of clouds on the large-scale environment in midlatitudes. The result is compared to that recently obtained by Tao for a tropical rainband. The outstanding result of the model integration for 17 h of physical time is that statistical properties of clouds averaged horizontally over 128 km of the model domain undergo temporal variations for a given time-independent large-scale forcing, rather than settling down into a steady state. When applied to a prestorm situation, the model predicts heavy precipitation that continues to fall for the first 5 h, followed by a 4 h period without precipitation. A second burst of deep convection then occurs. An analysis of the result reveals that the pause of precipitation occurs when the subcloud layer is dried up primarily due to the net vertical transport of moisture associated with clouds. Convection again starts developing when the moisture in the subcloud layer is replenished by the imposed large-scale forcing. The precipitation rate averaged over the precipitation period is found to exceed the supply of moisture by the large-scale forcing. The result implies that the fraction of moisture convergence in a vertical air column that contributes to moisten the environmental atmosphere in Kuo's cumulus parameterization scheme can be negative. Further, the result indicates the following: 1) The updraft mass flux increases with height until it reaches the local maximum at 350 mb, indicating that the cloud population is dominated by deep clouds, in contrast to the bimodal or broad spectral distribution of clouds observed in the tropics. 2) The cloud heating effect does not balance the large-scale cooling effect, reflecting the fact that the storage and horizontal advection terms are not negligibly small compared to the vertical advection term in the large-scale heat budget; and 3) The net vertical fluxes of heat and moisture are not negligibly small cormpared to condensation and evaporation processes at upper levels in the heat and moisture budgets, reflecting the fact that the atmosphere considered here is more unstably stratified and updrafts are stronger than the tropical counterparts.

Moisture analysis of the A‐scale phase means during GATE

AbstractA moisture analysis, based on GATE A‐scale phase means, is presented for western Africa (land) and the eastern Atlantic (water) area. Vertically‐integrated moisture budget terms are calculated for the whole area, as well as for land, water and A/B ship array areas. Precipitation minus evaporation (P̄−Ē) is solved as the residual. Upward vertical motion, associated with organised convective disturbances and westward‐propagating waves, causes the vertical advection term to dominate. Over land P̄−Ē decreases slightly from Phase I to Phase III, whereas over water it increases greatly during the same time span. When the evaporation rate of 3.8mmd−1 from the Phase III B‐scale results of Thompson et al. (1979) is applied to our A/B scale results, it yields a precipitation rate of 10.8mmd−1. This compares favourably with the remotely‐sensed estimates of 12.7mmd−1 for the B‐scale array obtained by radar and 14.0mmd−1 for the A/B ship array derived from satellite observations. Furthermore, P̄−Ē in the GATE area is found to be considerably greater than the global estimate for approximately the same latitude band.

THE ROLE OF LARGE-SCALE ASYMMETRIES AND INTERNAL MIXING IN COMPUTING MERIDIONAL CIRCULATIONS ASSOCIATED WITH THE STEADY-STATE HURRICANE

Abstract The role of asymmetries (large-scale horizontal eddies) in satisfying the mean angular momentum budget for the steady-state hurricane is studied by computing transverse circulations for a prescribed tangential vortex on the scale of 1000 km. For realistic diabatic heating rates at large distances from the hurricane center, the correlation between radial velocity and absolute vorticity must be negative in the upper troposphere and positive in the lower troposphere. The transverse circulations show small-scale oscillations that increase as internal mixing is increased. This paradox results from balancing “noise” in the prescribed tangential wind profiles by oscillations in the radial and vertical advection terms.