Title: Quantifying dynamical inconsistencies in EnKF data assimilation of convective storms

Author: Heiner Lange (Meteorological Institute Munich, Hans-Ertel-Centre for Weather Research)
George C. Craig (Meteorological Institute Munich)
Tijana Janjic (Deutscher Wetterdienst, Hans-Ertel-Centre for Weather Research)

In an LETKF OSSE for Doppler radar assimilation (Lange and Craig, 2014), we tested different combinations of horizontal localization radii, observation averaging resolutions and assimilation intervals. We found that a forecast from a well-converged analysis ensemble with 5 minute update rate, 8 km covariance localization and an observation resolution of 2 km (L8) was superior to a forecast from an analysis ensemble with 20 minute update, 32 km localization and 8 km observation resolution (L32SOCG20). This advantage of L8 held within the first forecast hour. After 3 hours, the quality of the two forecasts was equal. We concluded that the error growth of the smallest scales had saturated within that interval.

We observed an aggregation of gravity wave noise in the L8 assimilation cycle which led to the arise of strong spurious updrafts in the forecasts. In L32SOCG20 however, the forecast updrafts were confined to the cold pool egdes, as in the model nature run, in a dynamically consistent way.

As a measure of the dynamical inconsistencies in convective DA, we are retrieving a pressure perturbation field from the LETKF analysis wind field by the means of the model momentum equations (as in Sun and Crook 1996). By comparing this retrieved pressure with the pressure field of the LETKF analysis, we can see how far the analysis is off-balance and dynamically inconsistent.

We will investigate the degree of this dynamical inconsistency for different combinations of horizontal and vertical localization radii, assimilation intervals, spatial smoothing and the usage of the retrieved pressure field as a substitute for the analyzed LETKF pressure field (Potvin and Wicker, 2013). Using space-time correlations of analysis and forecast precipitation and temperature fields, we will quantify the amount of spurious convection for the different settings to establish an independent measure for the dynamical consistency of convective DA.

GMAO Head: Steven Pawson Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: Feb 9 2015