The two‐dimensional dynamics of West African squall lines

The two‐dimensional dynamics of multicell and unicell squall lines are examined using a numerical model. Both types feature a convective jump updraught with extensive, deep, rearward‐extending anvils. the multicell case contains travelling convection cells and is unsteady but persistent, whereas the unicell case is quasi‐steady. In the multicell case, an extensive density current is produced by the evaporation and drag of rain falling from a succession of convection cells which travel rearwards relative to the density current head. the circulation within the density current is complex and consists of three mutually interactive branches: a rotor, a cold downdraught, and a subsidence region. In the unicell case, a density current is not produced and the updraught is forced by a propagating wave of elevation, generated as an interaction between the updraught, water loading and evaporation in a very localized area. The squall line structure is compared with other models and it is shown that certain propagating two‐dimensional tropical and mid‐latitude squall lines are dynamically similar, their detailed structure being controlled by the inflow wind profile. The multicell case is compared with a quasi‐two‐dimensional West African squall line which occurred on 22 June 1981 during the Convection Profonde Tropicale (COPT 81) experiment performed in the northern part of the Ivory Coast. Good agreement is found between the model and the observations, and hence the dynamics of the observed squall line is quantified. the unicell case, in which a density current and conventional downdraughts are absent, has yet to be identified in atmospheric observations.