PowerPoint Presentation - Methods for dealing with spurious covariances arising from small samples in ensemble data assimilation

Mid-latitude geophysical flows are nearly in geostrophic balance at the scales of cyclonic storms and fronts. That is, the wind field is proportional to the gradient of the geopot. Height field. Localization can mess this up. HereÕs an example of the increment association with a single wind ob, where Pb is in exact geostrophic balance (ageostrophic wind increments are zero). Applying localization to the an ensemble sampled from that perfect Pb results in ageostropic winds. In practice, this means gravity waves will be excited during the forecast cycle. Houtekamer show that a measure of this gravity wave activity decreases as localization relaxed. In practice, the gravity waves tend not to interact very much with the balanced flow, so they can be filtered out of the forecast.