Observations and modelling of IOP6: response of the valley windsto the upstream profile

R. Burton1, S. Vosper2, P. Sheridan2, S. Mobbs1

1 Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, U.K.2 Met Office, Exeter, U.K.

School of Earth and EnvironmentINSTITUTE FOR ATMOSPHERIC SCIENCE

Introduction

IOP6: severe winds in the valley

Relating the winds in the valley to the upstream flow

Further work

School of Earth and EnvironmentINSTITUTE FOR ATMOSPHERIC SCIENCE

Photo: A. Doernbrack

T-REX IOP6: 24th – 26th March 2006

“A full temporal evolution of a trapped-lee-wave rotor eventwas captured in this IOP. There was a strong and well definedwave/rotor event with wave clouds, roll clouds, cap cloud over the Sierra, and a dust storm in Owens Valley.”

(Mission Summary)

Kuettner 1959

Locations of the AWS: DRI Leeds

Comparing winds from UM and AWS is difficult:instead we seek a statistical parameter to describethe effect of severe winds.

From Mobbs et al. 2005 (“Observations of downslope winds and rotorsin the Falkland Islands”, QJRMS, 131, 2839-2860

σ2 = σu2 ++ σv

2 |U| = average wind speed

It is thought that the stratification and the shearof the upstream profile are important parametersin determining the nature of the downstream response.

From Hertenstein and Kuettner, “Rotor types associatedwith steep lee topogreaphy: influence of the wind profile”,Tellus A, 57, 117-135

It is known that the stratification and the shearof the upstream profile are important parametersin determining the nature of the downstream response.

Can the wind effect parameter be linked to theupstream profile?

From Hertenstein and Kuettner, “Rotor types associatedwith steep lee topogreaphy: influence of the wind profile”,Tellus A, 57, 117-135

UM configuration

UM: Principal components analysis of the upstream profilebetween 3 and 5 km

These are the dominant structures in the N and dU/dz profilesfor all upstream profiles (sample size: 6840)Maximum in both shear and stability at 4250m

Time series of principal components; wind effect parameter: UM

Time series of principal components; wind effect parameter: UM

Wind starts to veer to the south:another mechanism?

Time series of principal components; wind effect parameter: AWS

Correlations: UM wind effect parameter with upstreamprofile structure

UM winds

Correlations: AWS wind effect parameter with upstreamprofile structure

AWS winds

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Summary

The dominant structure in the UM profiles between 3km and 5km is a peak in shear and a peak in stability at z = 4250m

There is a very strong ( r = 0.95; r = 0.87) correlation betweenthe UM winds in the valley and the shape of the upstream N and dU/dz profiles for all of IOP6;

The correlation is not as strong for the observed winds. But thenthe model is reacting to the model profiles, which may differ slightly to the observed profiles.

Is this behaviour unique? Or does it apply to other IOPs?Ongoing.

School of Earth and EnvironmentINSTITUTE FOR ATMOSPHERIC SCIENCE

Correlations between first PC score for N and dU/dz