The role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during TOGA COARE

A collection of case studies is used to clucidate the influence of environment soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE). The soundings were constructed primarily from aircraft data below 5-6 km and primarily from radiosonde data at higher altitudes. The well-documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended. As noted by earlier investigators, nearly all convective bands occurring in environments with appreciable shear below a low-level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the directions of the low-level shear at a speed close to the wind maximum: when there is appreciable shear at middle levels (800-400 mb), convective bands from parallel to the shear. With appreciable shear at both levels, the lower-level shear determines the orientations of the primary convective bands. If the midlevel shear is opppsite the low-level shear, secondery bands parallel to the midlevel shear will extend rearward from the primary band in later stages of its evolution; if the midlevel shear is 90 degrees to the low-level shear, the primary band will ratain its two-dimentional mesoscale structure. Convection has no obvious mesoscale organization on days with little shear or days with widespread convection. Environmental temperatures and humidites have no obvious effect on the mesoscale convective pattern, but they affect COARE convection in other ways. The high tops of COARE convection are related to high parcel equilibrium levels, which approach 100 mb in some cases. Convective available potential energies are larger than those in the GARP (Global Atmospheric Research Program) Atlantic Troptcal Experimental (GATE) mainly because of the higher equilibrium levels. The buoyancy integreated over the lowest 500 mb is similar for the two experiments. Convective inhibitions are small, enabling convection to propagate with only weak forcing. Comparison of slow-moving shear-parallel bands in COARE and GATE suggests that lower relative humidities between the top of the mixed layer and 500 mb can shorten their lifetimes significantly. COARE mesoscale organization and evelution differs from what was observed in GATE. Less-organized convection is more common in COARE. Of the convective bands observed, a greater fraction in COARE are faster-moving, shear-perpendicular squall lines. GATE slow-moving lines tend to be longer lived than than those COARE. The differences are probably traceable to differences in environmental shear and relative humidity, respectively.