A Case Study of a Strong Lakebreeze Front in the Salt Lake Valley

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A Case Study of a Strong Lake-breezeFront in the
Salt Lake Valley

• Daniel E. Zumpfe
• 19 April 2004
• Candidate Master of Science
• Department of Meteorology
• University of Utah

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Outline

• Motivation
• Background on lake breezes
• Lake breezes and other thermally driven flows in
  the Salt Lake Valley
• Climatology of northerly wind reversals vs.
  lake-breeze fronts
• Objectives of case study
• Case study 17 October 2000
• Results and conclusions

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Motivation

• Previous work - lake breezes during Salt Lake
  Valley field programs VTMX (Vertical Transport
  and Mixing Experiment) and URBAN 2000

• Is the Great Salt Lake the most important
  control?
• Or does this reflect the interannual variability
  of large-scale weather features?

Great Salt Lake Affects Summer Weather Salt
Lake Tribune, 10 August 2003 (Debbie Hummel, AP)
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Lake Breeze

• Thermally driven mesoscale circulation arising
  from differential heating between lake and
  adjacent land surface (Defant 1951)
• Similar to
• Sea breeze (e.g., Segal 1997 et al., BAMS)
• Farm breeze (e.g., Doran et al. 1995, JAM)
• Salt breeze (e.g., Rife et al. 2002, MWR)
• Observed around the World
• Stronger horizontal and vertical wind speeds
  observed in arid climates (Shen 1998, BLM)
• Affected by
• Synoptic forcing (Segal et al. 1997, BAMS)
• Amount of cloudiness (Sun et al. 1997, JGR)
• Land surface characteristics (Shen 1998, BLM)
• Other thermally driven flows (Stivari et al.
  2003, JAM)

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Lake-breeze Front

• Discontinuity separating land-modified and
  lake-modified air
• Strengthened by an opposing flow (Segal et al.
  1997, BAMS)
• Boundary associated with relatively strong
  convergence that may contribute to clouds and
  thunderstorms (King et al. 2003, WAF)
• Lake-modified air is associated with higher
  amounts of moisture and lower temperatures than
  that over land-modified air especially in arid
  regions (Rife et al. 2002, MWR)
• Passage usually associated with distinct shift in
  wind direction and change in wind speed (Biggs
  and Graves 1962, JAM)

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Previous Lake Breeze Study (Lake Michigan)
(Laird et al. 2001, MWR)
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Thermally Driven Flows in the Salt Lake Valley

• Terrain and lake-land interface leads to
  thermally driven flows
• Diurnal variations in wind direction commonly
  observed under weak synoptic forcing (Stewart et
  al. 2002)
• Thermally driven wind types
• Slope
• Canyon
• Valley
• Lake/land breeze

C
V
C
S
C
(MODIS imagery, NASA)
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An Up-valley Wind Reversal
VPN05
HGP
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A Lake-breeze Frontal Passage
VPN05
HGP
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Thermally Driven Flows
(b)
(e)
(c)
(d)
(f)
(i)
(g)
(h)
17 October 2000
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Simulations of Thermally Driven Flows
(Rife et al. 2002, MWR)
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The Great Salt Lake

• 120 km long, 40 km wide
• Maximum depth 10 m at 1280 m asl
• No outlet
• Sustained by runoff precipitation over watershed
• Decreasing surface elevation as of late

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SLC Lake-breeze Front Climatology
Hypothesis The number of strong lake-breeze
frontal passages at Salt Lake Intl Airport (SLC)
is related to the average Great Salt Lake surface
elevation.

• Lake-breeze front criteria
• Northerly wind-shifts
• Dew point temperature increases across wind-shift
  gt 2.5ºC
• Duration at least 2 hours
• Excludes all days with precipitation and
  synoptic-scale fronts

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SLC Lake-breeze Front Climatology
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Lake-breeze Fronts vs. Lake Surface Elevation
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Results of Preliminary Study

• Characteristic differences between northerly wind
  reversals and lake-breeze fronts
• Lake level a partial predictor of lake-breeze
  fronts
• Reconsider summer 2003 and contrast with summer
  1996
• What are the characteristics of lake breeze
  fronts in the Salt Lake Valley?
• An unprecedented opportunity to investigate this
  during VTMX and URBAN 2000 field programs
  (October 2000)

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Outline

• Motivation
• Background on lake breezes
• Lake breezes and other thermally driven flows in
  the Salt Lake Valley
• Climatology of northerly wind reversals vs.
  lake-breeze fronts
• Objectives of case study
• Case study 17 October 2000
• Results and conclusions

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Case Study 17 October 2000

• Questions to be answered in this study...
• What are the characteristics of the large-scale
  and thermally driven flows in and around the Salt
  Lake Valley, upon which the lake-breeze front is
  superimposed?
• How does the 17 October 2000 lake-breeze front
  evolve as is moves southward from the Great Salt
  Lake through the Salt Lake Valley?
• What are the characteristics of the boundary
  layer in the Salt Lake Valley before and after
  the lake-breeze frontal passage?

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Data

• Comprised of VTMX and URBAN 2000 data (Doran et
  al. 2002 Allwine et al. 2002)
• Surface, radar, lidar, profiler, sodar, and
  rawinsonde data
• Data meant for investigating stable nocturnal
  processes and tracer experiments
• Data used mostly from between IOP-6 and IOP-7

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Large-scale Conditions (1200 UTC 17 October)
500 hPa

• Synoptic-scale ridging

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Early Morning Winds

• Down-canyon, down-slope, down-valley, and land
  breeze winds present
• lt 5 m s-1

1100 UTC 17 October
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Hat Island (Great Salt Lake)

• Diurnal Lake temperature range 12.4 15.0º C
• Diurnal air temperature range over the Lake 10.6
  14.4º C
• Diurnal air temperature range over the Valley 1.8
  24.6º C

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IOP-6 Lidar Scans (U42)
away from lidar
toward lidar
toward lidar
away from lidar
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Isochronal Maps
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Hourly Dewpoint Change
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0600 UTC 17 October 0600 UTC 18 October
SLC
QSA
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0600 UTC 17 October 0600 UTC 18 October
VPN04
VPN11
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0600 UTC 17 October 0600 UTC 18 October
HGP
VPN01
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0600 UTC 17 October 0600 UTC 18 October
VPN12
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2100 UTC SLC Sounding
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Wind Profiler (Raging Waters)
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Vertically Pointed Lidar (U42)
Height (m asl)
Time/day (UTC)
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IOP-7 Soundings (Wheeler Farm)
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Radial Lidar Scans (U42)
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Range-height Lidar Scans (U42)
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Backscatter Lidar (Jordan Narrows)
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Answers to Question 1

• What are the characteristics of the large-scale
  and thermally driven flows in and around the Salt
  Lake Valley, upon which the lake-breeze front is
  superimposed?
• Opposing winds appeared to strengthen lake-breeze
  front
• Southerly winds most evident in the west/central
  Valley
• Up-slope and up-canyon winds precede lake-breeze
  front
• Apparent propagation of up-valley wind reversal
  prior to lake-breeze frontal passage along the
  Valleys axis

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Answers to Question 2

• How does the 17 October 2000 lake-breeze front
  evolve as it moves southward from the Great Salt
  Lake through the Salt Lake Valley?
• Lake-breeze frontogenesis accompanied by a strong
  moisture gradient within a 3-4 km band
• Frontal passage evident with sharp increase in
  moisture and wind speed
• Front moved up-valley at roughly 3 m s-1
• Front became superimposed on the up-valley and
  up-slope with an indication of the front
  extending up Parleys Canyon (not shown)
• Lake breeze collapsed throughout Valley after
  sunset

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Answers to Question 3

• What are the characteristics of the boundary
  layer in the Salt Lake Valley before and after
  the lake-breeze frontal passage?
• Front characterized by increasing wind speed 3-5
  m s-1 in lowest 200-300 m agl
• Increased mixing 600-800 m agl
• Removal of near-surface superadiabatic layer to
  nearly adiabatic
• Gravity wave-like structures evident in southern
  end of Valley

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Conceptual Lake-breeze Front Model (17 October)
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Future Work

• NCEP Regional Reanalysis data
• Preliminary investigation little or no evidence
  of lake breezes or lake-breeze fronts
• Use analyses to determine mean synoptic patterns
  during occurences
• Investigate reasons for consecutive days with
  lake-breeze fronts following periods of
  precipitation and/or strong synoptic-scale
  forcing
• Stable vs. near-neutral boundary layer below
  crest level (around 700 hPa)
• Possibly more frequent in transition seasons?
• Expand lake-breeze front climatology to include
  all four seasons

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Acknowledgments

• Thanks goes to
• My advisor John Horel
• Thesis Committee members Kevin Perry and Jim
  Steenburgh
• Dave Myrick and Ken Hart (FrameMaker)
• Jay Shafer (GEMPAK and figures)
• MesoWest and U of U Meteorology (time-series,
  hodographs, soundings)
• VTMX principle investigators and data collection
  groups
• DOE Chemical and Biological National Security
  Program (URBAN 2000 data, images)
• NCAR-ATD (backscatter lidar images, surface data)
• NOAA-ETL (U42 lidar images/data)
• Dave Whiteman at PNNL (literature search)
• The entire INSCC crew and Student AMS
  (distractions)
• Friends in Utah and Family in Nebraska

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Thanks to all of you for attending and not
falling asleep.
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Sodar (Whiteman Slope)
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Parleys Canyon (Mountain Dell)
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Preliminary Wind Climatologies
October 2000 days without precipitation or
frontal passages
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Surface Hodograph Summary
(b)
(e)
(c)
(d)
(f)
(i)
(g)
(h)