Transcript Mesoscale Circulations during VTMX

Downslope Winds Along the
Wasatch Front
Lacey Holland
Outline
• Objectives
• A Climatology of Downslope Events at Hill AFB
(HIF)
• Synoptic Overview of 7 Oct 2000
• Mesoscale Overview
• Valley flows
• Tethersonde and lidar data
• ADAS analyses
Objectives
• 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
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% events used to create composite
• NCEP Reanalysis
Top 10 Downslope Windstorms at
HIF (1949-1999)
•
•
•
•
•
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)
Wasatch Downslope Windstorms by
Month
A Composite of 700 mb Heights in
strongest downslope events
Vertical Transport and Mixing
eXperiment (VTMX) IOP#2,
6-7 October 2000
• IOP#2: 2200 UTC 6 Oct – 1600 UTC 7 Oct
• Tethered 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
2300 UTC 6 October 2000
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
(0400 UTC)
VTMX9
SLC
ACS
Cross-section Across Wyoming
RWL
EVW
RKS
Surface plot
Salt Lake Valley Flows
VTMX9
VTMX6
WBB
UT5
VPN10
U42
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 mm wavelength
• Detection range: 1-30 km
• Radial velocity accuracy: 0.3-1 ms-1
lidar
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 evening
Conceptual Model at 0700 UTC
VTMX9
SLC
ACS
sfc
Tethersondes at 0530 UTC
__ = WF
__ = T2
__ = T3
__ = T4
Conceptual Model at 0830 UTC
VTMX9
SLC
ACS
SLC Special
Sounding
0900 UTC
Tethersondes at 1000 UTC
__ = WF
__ = T2
__ = T3
__ = T4
Conceptual Model at 1030 UTC
VTMX9
SLC
ACS
Lidar Observations
Lidar graphics
courtesy of Lisa
Darby, NOAA ETL
ADAS Analyses
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;
radiational inversions can erode and warm the surface,
however.
Summary (con’t)
• 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!