Calculating Moisture Budgets over the Alps Using Finite Elements

It is common practice to interpolate observation data to a regular grid using various analysis techniques before forecasts or budget calculations are performed. During this process valuable information may be lost particularly if one is interested in phenomena not much larger in scale than the spacing between observations.

We propose a method for calculating gridscale moisture budgets from quality-controlled radiosonde data without interpolation. This can be accomplished by using irregular grids and the finite element method (FEM). It has been shown that this method is superior when estimating kinematic quantities of the wind field from radiosonde data. Although quite accurate, the wind field is not suitable for budget evaluations since it does not fulfil the three-dimensional continuity equation. Thus one has to slightly modify the wind field in order to make it consistent with the continuity equation discretized on the finite element grid. This modification is weaker than the filtering that takes place during the usual analysis process at forecast centres.

Our FEM evaluations should contain less systematic errors than conventional budget evaluations since they are independent from assumptions and parameterizations in the assimilating models that affect the analysis data from forecast centres. On the other hand they may contain more noise since only consistency with the continuity equation is enforced. We therefore compare the budgets from the FEM with budgets from routine ECMWF analyses on model levels in order to study the benefits/drawbacks of the FEM method. (For the method of calculating budgets from ECMWF analyses it is referred to the paper Dorninger et. al which will also be presented at the conference.)

The FEM has been already used for calculating moisture budgets on pressure levels, thereby avoiding horizontal interpolation. We are currently trying to generalise this method in order to allow irregularity also in the third dimension, in particular at the surface. This step is necessary to cope with high-resolution MAP-SOP data as well as operational data over complex terrain.

First results will be shown at the conference.