Wednesday, 1 August 2001: 9:20 AM
Assimilation and Forecasting Experiments on Supercell Storms: Part I: Experiments with Simulated Data
We have performed a number of assimilation and forecasting experiments
on long lived supercellular convection. Both simulated data and observed
radar data experiments have been performed. The real data
experiments are described in Part II of this two-part presentation,
in which a tornadic supercell that occurred during the CASES experiment
near Wichita, Kansas in May of 1997 is studied. The control simulation in
the simulated data experiments uses a sounding obtained a few
hours before the tornadic supercell was observed. A warm and moist bubble
is placed at the low levels to initiate the convection. The goal of this
study is twofold: 1. to examine the sensitivity of the forecast to
initial conditions that are obtained by assimilating Doppler radar
observations using a four-dimensional variational (4DVAR) technique.
2. to investigate what are the essential features that must be retrieved in
order to produce a good forecast.
The control simulation is used to generate series of radar radial
velocity and reflectivity data in a frequency of five minutes, similar to the
WRS-88D data. These data are assimilated by the 4DVAR system to provide
initial conditions for a subsequent control forecast. The data coverage and
quality are then varied to examine the sensitivity of the forecast to the
retrieved initial conditions. We also evaluate the sensitivity of
the forecast with respect to the ambient flow and to model error associated
with the microphysical parameterization. Through these sensitivity
experiments, we attempt to determine the mechanisms that control the
motion and evolution of this supercell storm and the essential aspects in
the initial fields and in the ambient flow for a successful forecast.
Experiments are also conducted to examine the data requirement for
estimating parameters in the microphysical schemes in addition to
retrieving the initial conditions.