Lifecycle characteristics and organization of tropical island convection

Summer 1999.Also issued as David A. Ahijevych's thesis (M.S.) -- Colorado State University, 1999.Includes bibliographical references.The Maritime Continent Thunderstorm Experiment (MCTEX) was conducted over the Tiwi Is- lands just off the coast of northern Australia. Two MCTEX case studies are presented herein. The evolution of diurnally forced convection over the Tiwi Islands is explored, starting with the initial stages of cumulus development, to the mature thunderstorm stage, and finally to the dissipative stage. The convection is described from both a kinematic and electrical perspective. The leeward coast sea breeze front was the site of the first deep convection. Shallow precipitating cells originating from the island interior created evaporatively-cooled downdrafts that triggered the deep convection as they approached the front. Rapid development ensued as higher-order cloud mergers continued along the sea breeze front, leading to vigorous, electrically active storms. A deep, expanding cold pool at the surface cut off the main storm from its supply of buoyant air. Convection continued along the down- shear edge of the cold pool where the low-level shear tended to balance the horizontal vorticity generated by the cold pool. Polarimetric radar was used to estimate various precipitation quantities, and the relation between lightning and precipitation was investigated. Most of the results were consistent with the non-inductive charging (NIC) mechanism. No lightning was detected in predominately warm-rain cells. Only after significant quantities of millimeter-sized ice particles were produced was lightning detected. Cloud-to-ground (CG) lightning was somewhat correlated with surface rainfall, but appeared to be m re closely tied to the graupel mass in the mixed-phase region. It is postulated that the cloud merger process produces a storm environment favorable for non-inductive charging. Stronger updrafts are capable of suspending more rain- drops above the freezing level, creating an ample graupel supply. The presence of supercooled cloud water and ice crystals from the previous cells led to rapid storm electrification. CG flash rates lagged graupel mass by a few minutes, which was consistent with the formation of a lower positive charge center below the main negative charge layer. This origin of this positive charge center could be explained by non- inductive charging of graupel particles falling below the level of charge reversal.Sponsored by the National Science Foundation under grans ATM-9726464 and DGE-9616044