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Observations of Great Salt Lake Breezes During Salt Lake Valley Persistent Cold Air Pools Erik Crosman, John Horel, Neil Lareau, and Xia Dong ` University of Utah Department of Atmospheric Sciences 15th AMS Conference on Mountain Meteorology Steamboat Springs, CO Paper 8.3 Tuesday, 21 August 2012 Part of Larger Effort to Improve Understanding of Summertime and Wintertime Sea and Lake Breezes Summer 2009 Great Salt Lake Breeze Undergraduate Field Study Winter 2010-2011 Persistent Cold Air Pool Study •Great Salt Lake relatively ideal environment for observing lake breezes •Large variability in atmospheric, land and lake surface state •Lake breezes for small and medium sized lakes less well-understood than sea breezes •Very few observations of mid-latitude wintertime lake breezes •No previous studies on lake breezes occurring during wintertime cold air pools The Persistent Cold Air Pool Study (PCAPS) Lareau,N., E.Crosman, C.D.Whiteman, J.D.Horel, S.W.Hoch, W.O.J.Brown, and T.W.Horst,2012:The Persistent Cold Air Pool Study Bulletin of the American Meteorological Society In press Unprecedented wintertime observations! Great Salt Lake 4,400 km2 Salt Lake Valley ~1,000 km2 1 December 2010-5 February 2011 Research Questions 1. What are the characteristics of and processes controlling winter lake breezes during persistent cold air pools? Great Salt Lake Lake Breeze Front PCAPS IOP1 5 December 2010 Research Questions Salt Lake Valley Lake 2. How do the lake breezes impact persistent cold air pools? Photo Dave Bowling PCAPS IOP5 7 January 2011 MODIS July 17 2009 Great Salt Lake Sensitivity of Summer Lake Breezes to Land Surface Sensible Heat Flux May 17 Salt Lake City July 17 High heat flux MODIS May 17 2009 Great Salt Lake Medium heat flux Salt Lake City Land Surface Skin Temperature (°C) Low Flux LES Simulation ‘Winter’ 100 W m-2 z (m) 0 20 Lake 40 60 km 80 Land High Flux LES Simulation ‘Summer’ 100 120 400 W m-2 z (m) 0 20 Lake 40 60 km u ms-1 80 Land 100 120 Crosman and Horel 2010 Sea and Lake Breezes: A Review of Numerical Studies BLM 137:1-29 Crosman and Horel 2012 Idealized Large-Eddy Simulations of Sea and Lake Breezes: Sensitivity to Lake Diameter, Heat Flux and Stability BLM 144(3):309-328 Observations of Typical Summer vs. Winter Great Salt Lake Breezes 3 Feb 2011 ‘PCAPS Winter’ 17 July 2009 ‘Summer’ 10 5 0 Summer Winter Cross-Coast Breeze Wind Speed (m s-1) 5-8 2-3 Depth (m) 600 m 200 m Duration (hr) 10 hr 6 hr Not Expecting ‘Enhanced’ Lake Breezes During PCAPS! •Wind speed, duration, and depth comparable to summer in several cases •Lake breeze surface frontal contrast and inland movement higher than summer IOP3 13 December 2010 ⁰C ⁰C •Lake/land ΔT of 7 ⁰C •Lake breeze wind speeds ~4-5 m s-1 •Lake breeze depth near shoreline >500 m Inland movement ~10 km h-1 ⁰C ⁰C IOP3 13 December 2010 Processes Contributing to Great Salt Lake Summertime Breezes • Heat Flux (insolation, soil moisture) • Background wind (occurrence or not and inland movement) Great Salt Lake Salt Lake Valley Processes Contributing to Great Salt Lake Wintertime Breezes During CAPs Winter lake breezes during PCAPS complex due to: 1. Weak solar forcing allows other processes (e.g., lake heat flux, cold air drainage) to become significant 2. Interactions between solar heating, boundary-layer stable layers and background winds result in mixing and horizontal temperature gradients 3. When these processes favorably combine the lake breeze is enhanced mixing Insolation Ɵ + ΔƟ Valley wind channeling stability Ɵ Snow cover Clouds and fog Great Salt Lake Lake heat flux Cold air drainage pooling Salt Lake Valley (⁰C) 12 Dec 13 Dec IOP3 12-13 December 2010 Temperature Air over Valley Lake breeze front Air over Lake 5 11 17 23 5 5 m s-1 mixing Insolation Ɵ + ΔƟ COLD 17 Time (MST) 11 Valley wind channeling stability Ɵ COLD Snow cover Clouds WARM and fog Great Salt Lake Lake heat flux Cold air drainage pooling Salt Lake Valley Low-Budget Research Aircraft: Motorized Paraglider Valley Glider shoreline 5 m s-1 IOP3 13 December 2010 Research Questions Salt Lake Valley Lake 2. How do the lake breezes impact persistent cold air pools? Photo Dave Bowling Difference in BL temperature (⁰C) between Valley and Near Lake Large Horizontal Variations in Salt Lake Valley Persistent Cold Air Pools Occur During Lake Breezes Lake Breezes Day of January 2011 Lake Breeze ‘Recharge’ •8 documented cases •Lake a ‘reservoir’ of cold air 26 Jan 1100 Height (m) IOP9 26-27 Jan 2011 •Influences CAP intensity and Ɵ (K) duration 27 Jan 292 287 500 0 17 26 Jan 1100 Height (m) 23 05 11 Time (MST) 17 27 Jan 282 23 Ɵ (K) 292 287 500 0 05 11 Time (MST) Estimating mixed-layer heights (Horel et al. previous talk 8.2) 17 23 17 23 282 Lake Breezes ‘Recharge’ Persistent Cold Air Pool Boundary Layer U (ms 26 Jan Height (m) 1100 27 Jan 6 Lake breeze winds 3 500 less daytime heating near lake 0 0 17 1100 Height (m) IOP9 26-27 Jan 2011 -1) 23 26 Jan 05 11 Time (MST) 17 27 Jan 23 U (ms-1) 6 ‘Pulsing’ nocturnal drainage flows 3 500 More nocturnal cooling over valley 0 17 23 05 11 Time (MST) 17 23 0 Great Salt Lake Lake Breezes Modulate Clouds, Fog MODIS Terra ~IOP9 18 UTC 28 Jan 2011 Great Salt Lake PCAPS IOP5 Jan 6 2011 fog Salt Lake Valley Lake Breezes Modulate Valley Pollutants IOP9 27 Jan 2011 Noon 2 pm Wind Speed (m s-1) Convergence and Advection Noon 2 pm Summary • Winter lake breezes typically ~1/3 speed, depth of summer case • Winter lake breeze forcing mechanisms complex and variable • Several ‘enhanced’ wintertime lake breezes unexpectedly observed during PCAPS • Afternoon lake breezes were found to ‘recharge’ cold air in the lowest 200-800 m of Salt Lake Valley boundary layer • Lake breezes play role in cold air pool fog and low cloud occurence • Lake breeze passage lowered pollutant concentrations within SLV Questions? Photo Sebastian Hoch Observed Spectrum of Winter GSL Breezes Less complex Weakest surface heat fluxes (land surface) More complex Strongest surface heat fluxes background winds Generally discussed spectrum surface heat fluxes -land surface -lake surface background winds -terrain channeling and blocking -differential mixing stability cold air pooling clouds 10 Factors That Impact Sea and Lake Breezes mixing Lake surface sensible heat flux Nocturnal drainage cold pool Crosman and Horel 2010 Sea and Lake Breezes: A Review of Numerical Studies BLM 137:1-29 Summer Great Salt Lake Breeze Great Salt Lake Salt Lake Valley Tooele Valley ~2 pm 17 July 2009 snow Salt Lake Valley Land Surface Skin Temperature (°C) High land surface sensible heat flux drives lake breeze circulations Background synoptic wind modulates occurrence, intensity and inland extent Lake Breezes Modulate SLV Cloud Cover Photo: Sebastian Hoch 18 UTC IOP9 28 Jan 2011 High RH Great Salt Lake MODIS Terra ~18 UTC 28 Jan 2011 Salt Lake Valley Low RH Utah Valley Understanding The Effects of The Great Salt Lake Lake breeze fronts strengthen the lowlevel stability during CAPs Cold lake air Warm Salt Lake Valley air Lake modulates clouds and fog during CAPs Large-eddy sensitivity simulations currently being run to assess the role of GSL in •Strengthening and prolonging cold air pools •Impacting fog occurrence and formation •Serving as a reservoir for cold air drainage