Numerical Simulation of Mammatus-Like Clouds in Cirrus Outflow Anvils

This study is an investigation of mammatus-like cloud formation using a three-dimensional, non-hydrostatic, compressible, numerical model with bulk parameterization microphysics. Although mammatus form in many types of clouds, mammatus associated with outflow anvil cirrus clouds associated with deep convective thunderstorms is considered in this study. A rectangular domain intended to represent a portion of a outflow cirrus anvil is used. Five simulations were performed. For four of them, observed proximity soundings from cases of observed mammatus were used to initialize the model. For the fifth simulation, a sounding from a case of an outflow anvil cirrus cloud with no visible mammatus was used. The initial potential temperature and vapor mixing ratio were horizontally homogeneous in the model. The ambient observed winds were not included in the simulations for this study. The simulations were initiated with random perturbations of the snow aggregate and ice crystal fields. The characteristics of the resulting mammatus-like clouds compared reasonably well with previously published observations of mammatus. Cloud base detrainment instability (CBDI; Emanuel 1981) was employed to explain the formation of the simulated mammatus. The condition for CBDI is that moist static energy decreases with height. The simulations of mammatus were used to evaluate the CBDI criterion by examining locations where precipitating ice crystals and snow aggregates fell into layers where the moist static energy decreased with height. In addition, three mammatus formation mechanisms were proposed by Scorer (1958): subsidence of a cloud layer, hydrometeor fallout, and evaporation of hydrometeors below cloud base. In particular, the specific cases of fallout of frozen snow aggregates from the parent cirrus anvil cloud and evaporation/sublimation were emphasized. Results show that very similar mammatus-like clouds are generated in each of the four simulations with soundings with which observed mammatus were associated. The fifth simulation, which included the sounding with which no mammatus were observed, did not produce mammatus. Sensitivity experiments showed that the CBDI criterion had to be satisfied in order for mammatus to form in the simulations. The simulated flow fields within the mammatus are consistent with the conjectures of Scorer.