Organization of tropical convection in low vertical wind shears: The role of water vapor

A modeling study is conducted to gain insight into the factors that control the intensity and organization of tropical convection, and in particular t o examine if organization occurs in the absence of factors such as vertical wind shear or underlying sea surface temperature (SST) gradient. The contr ol experiment integrates a cloud-resolving model for 15 days using a 3D dom ain exceeding 1000 km in length, with no imposed winds, and horizontally un iform SST and forcing for convection. After 2 days of random activity, the convection organizes into clusters with dimensions of approximately 200 km. Convective systems propagate through the clusters at speeds of 2-3 m s(-1) , while the clusters themselves propagate at minimal speeds of around 0.5 m s(-1). Examining the thermodynamic structure of the model domain, it is found that the convective free bands separating the clusters are very dry throughout the troposphere, and due to virtual temperature effects, are correspondingl y warmer in the lower troposphere and boundary layer. This suggests a posit ive feedback between convection and water vapor, where convective moistenin g of the local atmosphere renders it more favorable to future convection. T he existence of this feedback is demonstrated by experiments in which the f ree-tropospheric water vapor is perturbed in convective regions, and it is found that the lower-atmospheric water vapor is most critical in controllin g convection, most likely through the role of downdrafts. Examination of th e boundary layer in the control experiment also indicated that convectively generated cold pools also play a key role in the organization of convectio n, possibly by their influence on the boundary layer water vapor field. In order to see how the water vapor feedback modifies established convectiv e organization, a further experiment was conducted with an SST gradient imp osed, which established a mock Walker cell type circulation, with ascending motion over the warmest SSTs. After 5 days, the SST gradient is reversed t o see how the convection would establish itself over the new SST maximum. T his highly idealized experiment therefore represents a surrogate for the at mospheric response to SST "hotspots,'' that observations have shown to form under the descending branch of large-scale tropical circulations such as t he Madden-Jullian oscillation, due to increased incident solar radiation an d decreased latent heat fluxes at the surface. It is found that the convect ion does not spontaneously initiate over the new SST maximum, but instead m ust propagate toward it. After a further 5 days, much longer than the bound ary layer adjustment timescale, the warmest SSTs are still completely free from convection. This is directly due to the dryness of the atmosphere caus ed by the initial period of subsidence. A further set of experiments examines the robustness of the feedback in cas es of imposed vertical wind shear. It is found that strong wind shears prev ent the feedback by effectively mixing water vapor. However, the feedback i s still very important in cases of weak wind shears.