JP2.5a
The use of three-dimensional analyses of cloud attributes for diabatic initialization of mesoscale models (Formerly Paper J2.6)
Paul Schultz, NOAA/ERL/FSL, Boulder, CO; and S. Albers
Diabatic initialization of cloud-resolving forecast models such as MM5, RAMS, ARPS, and WRF allows these models to prognose the future of precipitating cloud systems such as snowstorms and thunderstorms that are present at t=0. This is a solution to one of the most long-standing problems in NWP: the 1-to-3-hour spin-up of cloud systems caused by the use of traditional dry initialization.
Our method for initializing clouds, their associated kinematics, and precipitation proceeds in four steps.
First, a three-dimensional analysis of cloud type and cloud fraction is performed according to Albers (Wea. Forecasting 1996), which uses data from a background model, satellites, radars, cloud reports from METARs and voice pilot reports of cloud layers. This step also produces gridded estimates of cloud liquid, cloud ice, and precipitating species such as rain and snow. Updates to the Albers methods are reported by Birkenheuer and Albers at this conference.
Second, an estimate of the vertical motions within the analyzed clouds is generated. The following rules are used:
* Cumuliform clouds are fitted with a parabolic vertical velocity profile whose magnitude is linearly dependent on cloud depth.
* Stratiform clouds are assigned a small vertical motion which is held constant through the depth of the cloud.
* Cloud vertical motion is not assigned in the presence of precipitation.
The third step is a variational balancing step, in which the three-dimensional dynamical relationship between mass and momentum is adjusted to force consistency with the diagnosed cloud vertical motions. This is described more fully in McGinley and Smart at this conference.
The adjustment in the third step often warms the cloud column so that some cloudy grid points become subsaturated. Thus, the fourth step is to re-set the relative humidity to 100% at such gridpoints. Failure to do so causes instantaneous evaportation, along with the associated cooling and subsequent false downdrafts, in the first few timesteps.
The method has only been tested using MM5, but should be equally applicable in any model with explicit representation of cloud and precipitation processes.
The presentation will describe the scheme in more detail and provide examples.
Joint Poster Session 2, Poster Session - Mesoscale Data Assimilation—with Coffee Break
Tuesday, 31 July 2001, 2:30 PM-4:00 PM
Previous paper Next paper