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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.

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Presentation Transcript
outline
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
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?
conditions favorable for downslope windstorms along the wasatch front
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
a composite of downslope wind events at hif
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)

Top 10 Downslope Windstorms at HIF (1949-1999)
vertical transport and mixing experiment vtmx iop 2 6 7 october 2000
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
Synoptic Overview

RUC2 500 mb heights

RUC2 700 mb heights

mesoscale overview
Mesoscale Overview

2300 UTC 6 October 2000

chronology
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
salt lake valley flows

WBB

U42

Salt Lake Valley Flows

VTMX9

VTMX6

UT5

VPN10

asu tethersondes
ASU Tethersondes
  • Located at Mt. Olivet Cemetery
  • 3 sondes on one balloon
  • Each sonde separated by 50 m
  • Highest sonde 10 m below balloon
tethersondes at 0400 utc
Tethersondes at 0400 UTC

__ = WF

__ = T2

__ = T3

__ = T4

noaa etl lidar
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
description of adas analyses
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
tethersondes at 0530 utc
Tethersondes at 0530 UTC

__ = WF

__ = T2

__ = T3

__ = T4

tethersondes at 1000 utc
Tethersondes at 1000 UTC

__ = WF

__ = T2

__ = T3

__ = T4

summary
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
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
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!

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