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

A Case Study of a Strong Lake-breeze Front in the Salt Lake Valley. Daniel E. Zumpfe 19 April 2004 Candidate: Master of Science Department of Meteorology University of Utah. Outline. Motivation Background on lake breezes

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

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  1. 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

  2. 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

  3. Motivation Above normal temperatures at SLC during summer (JJA) 2003 • 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)

  4. 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)

  5. 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)

  6. Previous Lake Breeze Study (Lake Michigan) Lake breeze frequency peaking during late summer (JJA) Large interannual variability for one month (Laird et al. 2001, MWR)

  7. 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)

  8. An Up-valley Wind Reversal VPN05 HGP

  9. A Lake-breeze Frontal Passage VPN05 HGP

  10. Thermally Driven Flows (b) (e) (c) (d) (f) (g) (h) (i) 17 October 2000

  11. Simulations of Thermally Driven Flows Simulated lake-breeze front Simulated no-lake winds Simulated down-valley, down-slope, and land breeze winds [lake] – [no-lake] = simulated lake breeze wind components (Rife et al. 2002, MWR)

  12. 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

  13. SLC Lake-breeze Front Climatology Hypothesis: The number of strong lake-breeze frontal passages at Salt Lake Int’l 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 > 2.5ºC • Duration at least 2 hours • Excludes all days with precipitation and synoptic-scale fronts

  14. SLC Lake-breeze Front Climatology Northerly Wind-shifts Lake-breeze Frontal Passages

  15. Lake-breeze Fronts vs. Lake Surface Elevation 1996 1993 1986 2003 1992 1967 1963

  16. Results of Preliminary Study 7 20 • 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)

  17. 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

  18. 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? Case Study – 17 October 2000

  19. 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

  20. Large-scale Conditions (1200 UTC 17 October) 500 hPa • Synoptic-scale ridging

  21. Early Morning Winds • Down-canyon, down-slope, down-valley, and land breeze winds present • < 5 m s-1 1100 UTC 17 October

  22. 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

  23. IOP-6 Lidar Scans (U42) away from lidar toward lidar away from lidar toward lidar

  24. Isochronal Maps Up-valley, up-slope, and up-canyon wind reversals Lake-breeze frontal movement

  25. Hourly Dewpoint Change

  26. 0600 UTC 17 October – 0600 UTC 18 October QSA SLC

  27. 0600 UTC 17 October – 0600 UTC 18 October VPN11 VPN04

  28. 0600 UTC 17 October – 0600 UTC 18 October VPN01 HGP

  29. 0600 UTC 17 October – 0600 UTC 18 October VPN12

  30. 2100 UTC SLC Sounding

  31. Wind Profiler (Raging Waters)

  32. Vertically Pointed Lidar (U42) Wind maximum Height (m asl) Time/day (UTC)

  33. IOP-7 Soundings (Wheeler Farm) Height (m asl) Adiabaticnear surface Superadiabaticnear surface Time/day (UTC)

  34. Radial Lidar Scans (U42)

  35. Range-height Lidar Scans (U42)

  36. Backscatter Lidar (Jordan Narrows)

  37. 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 Valley’s axis

  38. 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 Parley’s Canyon (not shown) • Lake breeze collapsed throughout Valley after sunset

  39. 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

  40. Conceptual Lake-breeze Front Model (17 October)

  41. 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

  42. 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

  43. Thanks to all of you for attending and not falling asleep.

  44. Sodar (Whiteman Slope)

  45. Parley’s Canyon (Mountain Dell)

  46. Preliminary Wind Climatologies October 2000 days without precipitation or frontal passages

  47. Surface Hodograph Summary (b) (e) (c) (d) (f) (g) (h) (i)

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