Wednesday, 1 August 2001: 11:14 AM
Tests of an ensemble Kalman filter at convective scales (formerly paper P6.1 in 18WAF/14NWP)
Routine assimilation of radial velocity and reflectivity
from Doppler radars and very short-term prediction at
convective scales remains an outstanding problem in NWP.
Application of four-dimensional variational schemes (4DVar)
at these scale has shown promise, but there are a number of
difficulties still to overcome, such as need for a covariance
estimate for the background forecast, inherent nonlinearities
in the dynamics and microphysics, and the need to maintain
linearized and adjoint versions of the forecast model.
An alternative to 4DVar are filtering techniques that use
ensembles of nonlinear forecasts to provide the necessary
estimates of the background uncertainty, such as the ensemble
Kalman filter and its variants. Although they are unproven,
these techniques address many of the difficulties encountered
with 4DVar. We will report results from preliminary tests of
such techniques at the convective scales using simulated data
(for example, from a numerical simulation warm thermal or
spreading cold pool). These experiments also contain a wealth
of information related to the dynamics and statistics of forecast
errors at the convective scales.