The Minimum Length Scale for Evaluating QG Omega Using High-Resolution Model Data

Abstract

Abstract The minimum length scale to investigate quasigeostrophic (QG) vertical motion within a mesoscale operational model is determined using simulations of 28 baroclinic systems from the North American Mesoscale Forecast System (NAM) model. Two upscaling methods are tested to find the optimal QG characteristic length. The box method takes an average of each field before performing finite-differencing calculations. The cross method samples the data at increasing distances between finite-difference calculations. The traditional QG omega equation is evaluated with each upscaling technique and found to be reliable between 800 and 200 hPa. The minimum QG length scale is found to be L = 140 km considering correlations of QG omega back to operational model values, which are for both methods on an “extended” QG omega. The box method performs marginally better than the cross method due to a larger reduction of QG forcing in higher-order wavenumbers, but at the appropriate length scale, both methods have indistinguishable correlations.