A Numerical Study of the Stratiform Region of a Fast-Moving Squall Line. Part I: General Description and Water and Heat Budgets

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Abstract
A two-dimensional nonhydrostatic cloud model is applied to the simulation of a tropical squall line that occurred on 23 June during the COPT 81 experiment. Owing to the use of an ice parameterization scheme, the simulation reproduces many interesting features of the stratiform part observed with Doppler radars. In particular, this includes the dynamical, thermodynamical, and microphysical structures of the stratiform part. Different parts are clearly identified from the simulation and observations: a leading convective zone 40-km wide with large precipitation; a developed stratiform zone stretching over 150 km with moderate precipitation; between these two regions, a transition zone 20-km wide giving only light precipitation; and a forward anvil near 12 km. The mean horizontal circulation is characterized by two mean flows: the front-to-rear flow that represents upward and rearward injection of boundary-layer air and the underlying rear-to-front flow. The simulated vertical velocity, except in the convective part, is in good agreement with observations and is characterized by a mesoscale updraft in the midtroposphere just behind the transition zone and a mesoscale downdraft under the anvil. The level of zero vertical motion, separating the mesoscale updraft from the mesoscale downdraft, has a weak slope in the horizontal as observed, and stays under the 0°C isotherm everywhere. One consequence is that the bright band is embedded in the mesoscale ascent. Detailed thermodynamical and microphysical budgets of the stratiform region are performed and lead to the following conclusions for the present simulation: 1) the warming in the poststratiform part is a consequence of the history of the storm when the subsiding low levels were dry; 2) the cooling in the mesoscale downdraft is mainly due to rainwater evaporation; 3) this net cooling is a temporal process that occurs when the stratiform anvil produced enough precipitation to counter the adiabatic warming; 4) the cooling at the base of the anvil is not due to melting but is the consequence of upward transport of low θ. The apparent heat source and moisture sink of the whole system, as well as those of the convective and stratiform parts, are also presented at different times and compared with previous numerical results and observations.

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