A modelling study of the impact of cirrus clouds on the moisture budget of the upper troposphere

[摘要] We present a modelling study of the effect of cirruscloudson the moisture budget of the layer wherein the cloud formed.Our framework simplifiesmany aspects of cloud microphysics and collapses the problemof sedimentation ontoa 0-dimensional box model, but retains essential feedbacks betweensaturation mixing ratio, particle growth, and water removalthrough particle sedimentation. The water budget isdescribed by two coupled first-order differential equations fordimensionless particle number densityand saturation point temperature,where the parameters defining the system(layer depth, reference temperature, amplitude and time scaleof temperature perturbationand inital particle number density, which may or may notbe a function of reference temperature and cooling rate)are encapsulated in a single coefficient.This allows us to scale the results to a broad range ofatmospheric conditions, and to test sensitivities.Results of the moisture budget calculationsare presented for a range ofatmospheric conditions(T: 238–205 K; p: 325–180 hPa) anda range of time scales τ_T of the temperature perturbation thatinduces the cloud formation.The cirrus clouds are found to efficientlyremove water forτ_T longer than a few hours,with longer perturbations (τ_T≳10 h)required at lower temperatures (T≲210 K).Conversely, we find that temperatureperturbations of duration order 1 h and less (a typical timescalefor e.g., gravity waves) do notefficiently dehydrate over most of the upper troposphere.A consequence is that (for particle densities typicalof current cirrus clouds) the assumption of completedehydration to the saturation mixing ratio may yieldvalid predictions for upper tropospheric moisturedistributions if it is based on the large scaletemperature field, but this assumption is not necessarilyvalid if it is based on smaller scale temperature fields.

[学科分类] 大气科学