Orographic drag in the Met Office Unified Model: Sensitivity to parameterisation and insights from inter‐model variability in drag partition

Research output: Book/ReportCommissioned report

Abstract

Global model experiments investigating the sensitivity of MetUM performance at both NWP and climate time-scales to individual changes in orographic drag parameterization indicate that forecast improvements may be possible via the retuning of orographic drag scheme parameters, reassuringly towards more physically realistic values. The experiments reveal considerable model sensitivity to drag parameterization configuration. The most beneficial changes are found to arrive via decreases in the low level drag associated with orographic flow blocking and an increase in the higher-altitude drag associated with gravity wave breaking. The former has the effect of reducing high latitude high pressure biases in the MetUM, whilst the latter improves in particular temperature distribution, and to a lesser extent circulation, in the stratosphere. These tendencies in stress components may be brought about via changes to each of the five tuneable orographic drag scheme parameters. For four of the five parameters, these changes yield values which are closer to those recommended in the literature (whilst the fifth parameter, being poorly constrained, is relatively flexibly tuneable).

In a month-long global model comparison between the MetUM and ECMWF IFS, considerable differences in drag partition are identified, highlighting the considerable uncertainty in the representation of orographic drag that remains. This disparity is linked to differences in the diurnal and spatial variability in surface stress over high mountain ranges. The MetUM displays marginally higher amplitude diurnal variability – arguably the opposite of that which would be intuitively expected. The beneficial tendencies in drag components found in the MetUM sensitivity experiments – an increase in gravity wave drag and a decrease in flow blocking drag (plus an ensuing compensating increase in boundary layer drag) – would bring the MetUM into closer agreement with the partition of drag components seen in the IFS.
Original languageEnglish
PublisherMet Office
Publication statusPublished - Jan 2019

Publication series

NameMet Office Technical Reports

Cite this