13.1
The over-amplification of gravity waves in numerical solutions to flow over topography
P. Alexander Reinecke, University of Washington, Seattle, WA; and D. Durran
The real-time use of high-resolution numerical weather prediction
(NWP) models is now commonplace at operational centers around the
world. However, accurate forecasts of many mesoscale phenomenon remain
elusive. One example is the tendency of models to over-predict the
amplitude of mountain waves. In this talk we will examine how
numerical errors can result in mountain-wave over-amplification in NWP
model forecasts.
Discrete analytic mountain-wave solutions are presented for the
classical problem of cross-mountain flow in an atmosphere with
constant wind speed and stability. Using second-order-accurate finite
differences on an Arakawa "C grid" to model non-hydrostatic flow
over a supposedly well resolved 8Δx-wide mountain can lead to
an over-amplification of the standing mountain wave by as much
as 30%. On the other hand, the same finite-difference model
significantly underestimates the wave amplitude in hydrostatic flow
over an 8Δx-wide mountain. Increasing the accuracy of the
advection scheme to fourth-order significantly reduces the numerical
errors associated with both the hydrostatic and non-hydrostatic
discrete solutions.
We will also consider an example of mountain-wave over-amplification
in a full NWP simulation of TREX IOP 13. It is shown that switching
from second-order advection to fourth-order advection leads to as much
as a 20 m/s decrease in vertical velocity on the lee-side of the
Sierra-Nevada, and that the weaker 4th-order solutions are more
consistent with observations.
Session 13, Waves and Rotors II
Friday, 15 August 2008, 10:00 AM-11:30 AM, Rainbow Theatre
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