MAP IOP 10 South Foehn Event in the Wipp Valley:

Verification of High-Resolution Numerical

Simulations with Observations

A. Gohm*, G. Zängl**, G. J. Mayr*

* University of Innsbruck, Austria

** University of Munich, Germany

ICAM/MAP 2003 Brig, Switzerland, May 19 to 23, 2003

Gohm, A., G. Zängl, G. J. Mayr, 2003, submitted to Mon. Wea. Rev.

The scientific objective:

To assess to what extent the temporal evolution and spatial structure of small-scale orographic flows can be simulatedwith a state-of-the-art mesoscale model run in a very high-resolution mode.

The phenomenon:

Deep south foehn in the Wipp Valley on 24-25 October 1999

The scientific tools:

• Penn State/NCAR mesoscale model MM5• NOAA/ETL ground-based scanning Doppler lidar (TEACO2)• NCAR airborne aerosol backscatter lidar (SABL)• ZAMG Doppler sodar• radiosoundings and weather stations

MM5 basic setup:

• 6 domains with x = 64.8 – 0.267 km

• 39 full-sigma levels

• initialized with operational ECMWF analysis at 23 Oct 18 UTC and 24 Oct 00 UTC

Wipp Valley Innsbruck

Brenner Pass

Backscatter intensityversus potentialtemperature,15 UTC 24 Oct 1999:

P1

P2

MM5 00 UTC run

AML top heights &isentropes indicateregions of flow descentand jump-like features

indications forunderestimation offlow descent

SABL lidar on NCAR Electra

InnsbruckBrenner

flow

Wind profile at Brenner Pass, 24 Oct 1999:

ZAMG PA2 sodar MM5 00 UTC run, D6 simulated windspeed 15 % too high

lower gap area30% too large in D6

simulated massflux 50% too high

00 (18) UTC run: RMSE = 5.6 (6.2) m/sME = +1.5 (+1.6) m/s

Alpine crest line

lower gap

upper gap

Wind speed, 24 Oct 1999:

too strong winds

Potential temperature, 24 Oct 1999:

-2 K bias

Radial windvelocity,24 Oct 1999:

lidar Doppler lidar

09 UTC

MM5 (00 UTC run)

-1.6-3.1ME

5.36.9RMSE

15Z09ZTime

abs(radial velocity)00 UTC run – lidar

15 UTC

Doppler lidar

MM5 (00 UTC run)

Inversion upstreamof the pass,09 UTC 24 Oct 1999:

upstream inversion1 km too low early inthe event (~09 UTC)

southerly flow tooshallow

MM5 (00 UTC run): domain 4

Conclusions:

The model captured several striking features:

• foehn break-through in the Inn Valley

• magnitude of surface wind speed at several weather stations

• regions of strong descent

• jump-like features related to deep amplified gravity waves

Discrepancies were found between simulations and observations:

• overestimation of the mass flux through lower Brenner gap (+50%)

• underestimation of the descent of potentially warm air through upper gap

• bias of surface pot. temperature in the northern Wipp Valley (-2 K)

• wrong inversion height upstream of the pass early in the event

ECMWF analysis12 UTC 24 Oct 1999:

500 hPa geopot. height:

Sea level pressure:

L

H

MM5 flow structure,15 UTC 24 Oct 1999,00 UTC run:

surface wind speed:wind speed &

pot. temperature:

Brenner Innsbruck

Pressure gradient, 24 Oct 1999:

as function of time

as function of wind speed

Doppler lidar rad. vel.

09 UTCRadial windvelocity,24 Oct 1999:

15 UTC

lidar

MM5 (00 UTC run) rad.vel.

MM5 (00 UTC run) + wspd-1.6-3.1ME

5.36.9RMSE

15Z09ZTime

abs(radial velocity)00 UTC run – lidar