A Three-Dimensional Simulation of a Tropical Squall Line: Convective Organization and Thermodynamic Vertical Transport

Three-dimensional convective-scale simulations of an African squall line, observed during the French COPT 81 experiment, are presented. Three simulations with different representations of large-scale forcing are performed on a domain of 50 km (along the line) by 80 km (across the line). They exhibit a similar circulation pattern characteristic of a squall line, but differ in intensity. The first simulation supposes an unperturbed environment and produces a slow-moving squall line (7 m s−1) with weaker total precipitation rate then observed (25%). The second one includes a representation of observed thermodynamic and dynamic environment modifications, and produces a fast-moving squall line (10 m s−1) still weaker than observations (50% or the rain rate). The third simulation takes into account the forcing induced by the rear inflow jet as depicted by Smull and Houze and observed on that day. It allows the system to reach an intensity in agreement with observations. The convective region (30 km wide) appears as the superposition of several convective cells at different stages of their life cycle. New elements are formed in front of the system and are fed by the forced convergence band along the squall-line front. Mature cells produce precipitation that feeds downdrafts by loading and evaporation. Old convective cells dissipate at the simulated system rear. Between the convective updrafts, intrusions of low equivalent potential temperature (θe) are found. These are unsaturated downdraft cells feeding the gravity current. At low levels (up to 2 km), the simulated system has a two-dimensional structure, but it becomes progressively three-dimensional with height. This three-dimensional structure allows the crossing of two inflow layers of high and low θe, respectively between 2 and 6 km. This is the crossover zone whose existence was hypothesized by Zipser. A detailed description of the gravity current at small scale is given, showing an inner circulation whose intensity depends on the forcing imposed by the stratiform part.