A Method for the Initialization of the Anelastic Equations: Implications for Matching Models with Observations

An algorithm is proposed, whereby the combined use of the equations of cloud dynamics and the observed wind will permit a unique determination of the density and pressure fluctuations. The algorithm has several unique features: 1) it involves the use of only the momentum equations, without resorting to any thermodynamical (microphysical) parameterizations; 2) there is no need to make any artificial assumptions about the boundary conditions; instead, the algorithm determines (from wind observations) its own optimal boundary conditions. This latter feature is crucial for severe storm observations where very often the boundary conditions are not those of the larger-scale environment. The viability of the method is tested with data generated by a numerical model. From these data, the wind and its time derivative are estimated and used to calculate the density and pressure fluctuations. These calculations are then compared to the originally given density and pressure. The sensitivity of the method to various observational errors is assessed by inserting realistically simulated errors into the observed kinematics. The principal findings are as follows: 1) good results can be obtained if the filtering (noise removal) technique takes into account subgrid-scale processes; 2) at least for some applications, doubt is cast on the validity of the local, steady-state hypothesis, which is now commonly used. It is argued, nevertheless, that the nonsteadiness can be readily evaluated from observations.