Downslope Windstorms Along the Wasatch Front

Lacey Holland

Objectives

• Create synoptic composites of conditions associated with downslope windstorms along the Wasatch Front

• Perform a case study of the 7 October 2000 downslope wind event using data from the Vertical Transport and Mixing eXperiment (VTMX)

Questions to answer ...

To what extent are strong winds on the east benches due to “canyon winds”?

To what extent are the strong winds on the east benches due to cold air flowing down and across the slope (i.e. bora winds)?

Why are the strongest winds confined to near the base of the Wasatch?

Why do the windstorms occur further away from the base of the mountains so infrequently?

Areas affected by Wasatch downslope windstorms

Western U.S. terrain

Conditions favorable for downslope windstorms along the Wasatch Front

• Strong cross-barrier flow at crest-level (700 mb closed low to SSW)

• Pool of cold air to the ENE (relatively high pressure over Wyoming)

• Wind reversal above crest-level (presence of a critical level) and elevated stable layer

Synoptic conditions favorable for downslope windstorms

A Composite of Downslope Wind Events at HIF

• Most stations in valley of limited use– Records too short– Not in proximity of affected areas

• Top 0.5% (79) events 1953-1999 used to create composite

• NCEP Reanalysis

Ten Strongest Downslope Windstorms at HIF (1947-1999)

• 4 Apr 1983 (46 m/s)• 16 May 1952 (42 m/s)• 20 Feb 1971 (38 m/s)• 22 Oct 1953 (38 m/s)• 18 Mar 1961 (37 m/s)

• 3 June 1949 (35 m/s)• 11 Nov 1978 (35 m/s)• 6 May 1949 (34 m/s)• 16 Nov 1964 (34 m/s)• 26 Jan 1957 (33 m/s)

*23 April 1999 ranks 20th

23 April 1999, 0930 UTC– example of an extreme event

Wasatch Downslope Windstorms by Month (strongest 0.5%, 1947-1999)

A Composite of 700 mb Heights in the 79 strongest downslope events

Red (blue) shading indicates positive (negative) statistical significance at the 99% level (via student t-test)

A Composite of 500 mb Heights

Vertical Transport and Mixing eXperiment (VTMX) IOP#2,

6-7 October 2000

• IOP#2: 2200 UTC 6 Oct – 1600 UTC 7 Oct

• Ranks only in the strongest 600 downslope wind events at HIF

• Balloon at Mt. Olivet Cemetery lost from its tether in strong winds

• URBAN2000 scientists report tracers stagnating downtown

Synoptic Overview

RUC2 500 mb heights RUC2 700 mb heights

Mesoscale Overview

Chronology• Prior to 0700 UTC: Developing Stage

– progression of cold air across Wyoming– drainage circulations in Salt Lake Valley (SLV)

• 0700 -1000 UTC: Initial development– Initial penetration of cold air across Wasatch– Gap flows through Parley’s Canyon– Lidar

• After 1000 UTC- Downslope wind event into SLV

Conceptual model of 7 Oct 2000

SLC

ACS/TetherVTMX9

0 10 km

050

0 m

Cross-section Across Wyoming

EVW RKSRWL

Salt Lake Valley Flows

Surface plot (0400 UTC)

ASU Tethersondes

• Located at Mt. Olivet Cemetery• 3 sondes on one balloon• Each sonde separated by 50 m• Highest sonde 10 m below

balloon

Sonde #2

Tethersondes at 0400 UTC

__ = WF

__ = T2

__ = T3

__ = T4

NOAA ETL Lidar

• Located at U42 (SLC Airport #2)

• Traverse Excitation Atmospheric pressure CO2 (TEACO2) lidar

• 10.6 m wavelength• Detection range: 1-30 km• Radial velocity accuracy: 0.3-1 ms-1

LidarLidar graphics courtesy of Lisa Darby, NOAA ETL

Description of ADAS Analyses

• Rawinsonde (PNL,NCAR,NWS), tethersonde (ASU), and surface station (PNL, Mesowest) data ingested into analyses

• 1 km resolution• Adjustment made to analysis for dense data• Further adjustments to be made

ADAS analysis, 0400 UTCContours are wind speed in m/s

Conceptual Model

SLCACS/Tether

VTMX9

0 10 km

050

0 m

Salt Lake Valley Flows

Surface plot (0600 UTC)

Tethersondes at 0530 UTC

__ = WF

__ = T2

__ = T3

__ = T4

Conceptual Model

SLCACS/Tether

VTMX9

0 10 km

050

0 m

SLC Special Sounding 0900 UTC

Tethersondes at 1000 UTC

__ = WF

__ = T2

__ = T3

__ = T4

Conceptual Model

SLC ACS/Tether

VTMX9

0 10 km

050

0 m

Salt Lake Valley Flows

UT5

VTMX9

Surface Plot (1200 UTC)

Lidar Observations

ADAS analysis, 1200 UTCContours are wind speed in m/s

ADAS E-W Cross-section near downtown (1200 UTC)

Summary• To what extent are strong winds on the east

benches due to “canyon winds”? – Lidar indicates jet out of canyon (gap flow) but spatial

extent is larger than simply the canyon opening ; Direction of flow out of the canyon determined by larger-scale flow

• To what extent are the strong winds on the east benches due to cold air flowing down and across the slope (i.e. bora winds)?– 7 Oct 2000 (and other times) are primarily bora events;

mixing out the radiational inversion can result in warming at the surface.

Summary (cont.)

• Why do the windstorms occur further away from the base of the mtns so infrequently?– East benches can stop mechanical penetration

of cold air into surface inversion in the valley– Radiatively cooled air in the valley is often

cooler than air crossing the barrier

• Why are the strongest winds confined to near the base of the Wasatch?– Need a mechanism to penetrate or to erode the

surface inversion

Acknowledgments

• John Horel

• My committee (S. Lazarus, E. Zipser)• Those who have contributed data (Sradik -

ASU, Coulter - PNL, Darby - ETL)

• Many unnamed others who have provided support, help, and motivation

THANKS!