Observations and modelling of severe windstormsduring T-REX: importance of the upstream profile

Ralph Burton1, Simon Vosper2, Peter Sheridan2, Stephen Mobbs1

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

School of Earth and EnvironmentICAS

Leeds T-REX team: Barbara Brooks, Ian Brooks, Rosey Grant, James Groves, Martin Hill, Matt Hobby, Felicity Perry, Victoria Smith, Will Thurston

Introduction: T-REX

Comparing models and observations:Relating the winds in the valley to the upstream flow

Summary

School of Earth and EnvironmentICAS

The biggest field campaign ever mounted to studyrotors/gravity waves

T-REX: March – April 2006

ARL White Sands Missile RangeScripps Institute of Oceanography

Colorado Research AssociatesCooperative Research in Environmental

ScienceDesert Research Institute

DLRLawrence Livermore National Laboratory

UK Met OfficeNASANCAR

Naval Research LaboratoryNOAA

Arizona State UniversityColorado State University

Harvard UniversityUniversity of Houston

University of InnsbruckUniversity of Leeds

University of New HampshireNorth Carolina State

UniversityStanford UniversityUniversity of Utah

Yale University

From “Hazardous

Mountain Windsand their Visual

Indicators”,1988

Accident rate less than 3 per 100,000Accident rate greater than 3 per 100,000

Accident rate 40% higher in the 11 mountainstates

DEM of the U.S. showing regionsof elevated terrain

Will “Thirsty” Thurston

Inyo Register,March 2006

Photo: Carlyle Calvin, UCAR

Schematic of wave-rotor system

Photo: A. Doernbrack

T-REX IOP6: 25th – 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

Hei

ght

(m)

w (m/s)

26th March 2006 6:39PM LT

What we want to do, in a nutshell:

Take an upstream profile, and attach a number N1 to it

Hei

ght

(km

)

5

10

15

Θ (K)

What we want to do, in a nutshell:

Take an upstream profile at time T, and attach a number N1 to it

Look at the downstream winds at time T: attach a number N2 to them

Look at lots of cases (different T).Are N1 and N2 related? - Predictability

Hei

ght

(km

)

5

10

15

Θ (K)

For the downstream winds.

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

Produce a time series of σ |U|

Locations of the AWS: DRI Leeds

Produce a time series of σ |U|

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 topography: influence of the wind profile”,Tellus A, 57, 117-135

For the upstream profiles.

UM configuration

UM model simulations for IOP6

Principal Components Analysis (PCA) of the upstream profiles

Take time series of upstream profiles; UM output every 10 seconds

Perform PCA: extracts two features:

EOFs (Empirical Orthogonal Functions)Describe the dominant structures in the profiles (inversions etc)

PC Scores (Principal Component Scores)A numerical value: a measure of howsimilar the profile is to the dominant structuresat each point in time;

Produce a time series of the principal component scores

PCA is an objective method

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

N dU/dz

Time series of principal components; wind effect parameter σ |U| : UM

Time series of principal components; wind effect parameter σ |U| : AWS

Correlations: UM wind effect parameter σ |U| with upstream profile structure

UM winds

Correlations: AWS wind effect parameter σ |U| with upstream profile structure

AWS winds

Insert pic here

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 upstream N and dU/dz profiles for all of IOP6 (19 hours);

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.

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Correlations between first PC score for N and dU/dz