Modeling of a Tropical Squall Line in Two Dimensions: Sensitivity to Radiation and Comparison with a Midlatitude Case

Abstract

A two-dimensional cloud model is used to study a tropical oceanic squall-line system. The dynamical and microphysical structures of the simulated squall-line system and the impact of environmental wind profiles on these structures are presented. The influence of the microphysics treatment on cloud radiative properties and the sensitivity of this simulated system to radiation is also investigated. In addition, partitioned heat, moisture and water budgets, and two radiative transfer schemes are used to assess the role of anvil clouds on the simulated system and on the assumption used in a bulk parameterization for cloud radiative properties. The comparison with a midlatitude study is also made to show its climatic implication. The major conclusions are as follows. The simulated tropical squall-line system replicates many observed features. A transition zone in the simulated multicellular storm is primarily caused by the jetlike wind profile, while it is due to longwave radiation in the midlatitude system. The effect of a jetlike wind profile is to weaken/strengthen the convective/anvil portion of the simulated system, which leads to an overall decrease of total surface precipitation by 17%. The moisture budgets indicate that tropical deep convection serves as a more efficient engine, pumping low-level moisture upward to form the upper-level anvil cloud, than its midlatitude counterpart although the convective instability is lower in the tropical environment. Microphysical production is the primary source of the water budget (?3/5) in the simulated tropical anvil, and rest (?2/5) is contributed by horizontal transport of hydrometeors from deep convection. This is just the reverse of the midlatitude case. The simulated tropical oceanic anvil has a stronger shortwave radiative forcing than the midlatitude continental anvil, although they have comparable longwave forcings. The small difference in total precipitation of the simulated system caused by different radiation transfer schemes appears to justify the assumption of using a bulk parameterization for cloud radiative properties. Comparisons of water budges and cloud radiative properties between simulated tropical and midlatitude anvils suggest the need to parameterize the tilting structure of mesoscale convective systems for improving the representation of cloud processes in general circulation models.