Temperature Prediction ATMS 452 Temperature Measurements • 2-m • In shade • Should be over a natural vegetated surface Stevenson Screen AKA: Cotton Region Shelter The Traditional Approach ASOS Temperature/Humidity Sensor Standard NWS/FAA instrumentation now Sea Tac Example Why Not Simply Use Model Output Directly and Go Home? • Model surface elevation may be very different than actual surface elevation due to limited model resolution. – Differences can be in the hundreds of meters or more. – Example UW 36 km MM5 lowest level over Boeing field is 256 m. Actual 5 m. – Improves as resolution increases Why Not Simply Use Model Output Directly and Go Home? • Model resolution is often inadequate for properly simulating important features that influence temperature. – Examples include the Columbia River Gorge and drainage flows in valleys – Coastal temperature contrasts Near Sea Level Gap On Border of WA and OR Columbia Gorge Flow Simulation 36 km grid spacing 12 km grid spacing Portland The Dalles Portland The Dalles Pass Height = 700 m Pass Height = 600 m 4 km grid spacing Portland Cascade Locks The Dalles Pass Height = 400 m 12 km grid spacing Portland The Dalles Pass Height = 600 m 1.33 km grid spacing, Pass Height = 150 m • Portland Cascade Locks Troutdale The Dalles 444.4 m grid spacing, Pass Height = 100 m Portland Troutdale Cascade Locks • Model temperatures may be seriously in error due to poor model physical parameterizations, such as for the planetary boundary layer, radiation, surface energy fluxes, cloud and precipitation processes. – Example: overmixing in PBL can result in the inability to maintain shallow cold air masses. Result: warm bias – Example 2: improper soil moisture (too wet or dry) can greatly influence temperatures. (too wet produces too cool temps, vice versa) – Example 3: poor snow distribution (very frequent) • As a result of such errors, models can have serious systemic temperature biases. 00 UTC 12 UTC Example: Shallow Fog…Nov 19, 2005 • Held in at low levels for days • MM5 held in the deep inversion…generally without the shallow mixed layer of cold air a few hundred m deep • MM5 could not maintain the moisture at low levels Why you can’t simply take the model temps • Models can also have synoptic and timing errors that can greatly undermine temperature forecasts. Back to Basics: First Law of Thermodynamics Local Temperature Change = Horizontal Advection + Vertical Advection/adiabatic heating-cooling Diabatic Heating and Cooling Horizontal Advection • Often dominates away from terrain and coastlines. • Usually largest near the surface. Adiabatic heating/cooling • Can be VERY significant near topographic barriers • Warming with downslope flow • Cooling with upslope flow. Downslope Warming • Large warming during downslope flow • Often large over foothills (e.g., Cascades-North Bend), but apparent all over the world, including to the lee (east) of the Rockies--the Chinook Wind. • In Europe called the Foehn Wind. • Usually, air comes from mid-levels where potential temperature is higher than at the surface. A drop in dew point usually accompanies down slope flow. Brookings: The Classic Northwest Example Adiabatic cooling harder to see • Clouds and precipitation obscure the system • Adiabatic cooling leads to saturation/latent heating which results in small cooling rate than heating rate with subsidence – Saturated adiabatic lapse rate is smaller than dry adiabatic lapse rate. Diabatic Effects Diabatics • Radiation: – direct radiational heating and cooling of the air is relatively small – indirectly, very large influence on air temp through modulation of ground temperature and communication into the boundary layer by turbulence. Surface heating by the solar flux and nighttime cooling by IR flux dominate surface temperatures. – Clouds have a major effect on radiational heating/cooling. Clouds, Radiation and Surface Temps • Clouds lessen warming during day by reducing solar radiation • Clouds decrease cooling at night by intercepting IR radiation and reradiating IR back to the surface. • Thin cirrus…only minor influence (1-2F) • Thick altostratus--5-10F influence or more. One can see association between changes in clouds and temperature at short-time scales IR Opacity of Air Can Influence Surface Temps • IR opacity is proportional to humidity. • Increased water vapor content increases opacity--better absorption and emission in IR. • Less water vapor results in better IR cooling under clear conditions. That is why deserts cool very rapidly at night. Why Washington DC stays hot all night. Conduction and Turbulence Effects in the BL. Fans in orchards and vineyards Phase Changes of Water • Can have a large impact on temperature and type of precipitation. Evaporation SE Everett • Snohomish, WA • April 4, 2005 IMPACTS OF LOCAL DIURNAL CIRCULATIONS ON TEMPERATURE Hoquiam, WA: July 15, 2002 18 70 Wind Speed (kts) Temperature (ЎF) 16 65 12 60 10 55 8 6 50 4 45 2 0 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Hour (LDT) Temperature (ЎF) Wind Speed (knots) 14 Comparison of the daily temperature cycle between observations (blue dashed) and the model (red triangles) for a coastal location (Boston, MA) on 10 August 1997 impacted by the passage of a sea breeze front. 2-m air temperature and 10-m wind velocity in the afternoon of a sea breeze event on April 18, 2005. The land-sea temperature contrast drives the onshore sea breeze. Local spatial variations in sea surface temperature (SST) can leave a signature on the sea breeze, especially when complex coastlines create multiple sea breeze fronts (e.g., the sea breeze penetrating from Long Island Sound in the image above). In our multiply-nested COAMPS simulations, nest 4 (1.33 km resolution) and nest 5 (0.44 km resolution) were enhanced to account for urban effects and to include time-varying (hourly) high-resolution SST's from Alan Blumberg's New York Harbor Observing and Prediction system (NYHOPS). The diurnal heating of the ocean surface is thus represented in our simulations. We are in the process of quantifying the impact of these effects on the sea breeze circulation for several distinct events, and evaluating the forecast skill of the system. Cold Pools Cold air can collected in mesoscale basins…such as eastern Washington, Willamette Valley, Central Valley of CA, and many more Near Sea Level Gap On Border of WA and OR Eastern Washington Cold Pool Climatology Cold Pool Forecasting Issues • Models often mix out cold air too readily…resulting in warm temperature biases • They are often capped by inversion, which is very persistent. • Mixing out (in reality) of cold pools is a difficult forecasting problem…often takes a strong synoptic system capable of mixing out the inversion overhead. Gap Winds and Temperatures • Often cooler air in and immediately downstream of gap. • Usually lower dew point as well. Near Sea Level Gap On Border of WA and OR Portland wind distribution for all days by direction and speed • Wintry precipitation at PDX – Almost exclusively E or SE – Tendency towards E for snow and SE for freezing rain Wind distribution for days with snowfall Wind distribution for days with Freezing Rain December 11-15, 2000 Case Study • Strong winds for nearly four days – Gale force a times • Wintry mix in Western Gorge and exit area. – Little snow accumulation but significant icing. • Selected because: – Data availability was good (esp. ACARS) – Model Initialization ok Domain Definition 36 km grid spacing 12 km grid spacing Portland The Dalles Portland The Dalles Pass Height = 700 m Pass Height = 600 m 4 km grid spacing Portland Cascade Locks The Dalles Pass Height = 400 m 12 km grid spacing Portland The Dalles Pass Height = 600 m 1.33 km grid spacing, Pass Height = 150 m • Portland Cascade Locks Troutdale The Dalles 444.4 m grid spacing, Pass Height = 100 m Portland Troutdale Cascade Locks T on 150 m Surface Portland Troutdale Coastal Circulations • Onshore (cool) versus offshore (warm) flow • Here in the NW: onshore or marine pushes • Found all over the world Southerly Buster…SE Australia…just like the push Convergence Zones MOS Compared to the NWS: Temperature Cool Season Mi. Temp – 12 UTC Cycle Average Over 80 US stations 7 6 5 4 3 2 '66-67 '71-72 '76-77 '81-82 '86-87 '91-92 '96-97 '01-02 Cool Season 24-h GUID. 24-h LCL 48-h GUID. 48-h LCL GFS MOS BEST Temperature MAE (F) for the seven forecast types for all stations, all time periods, 1 August 2003 – 1 August 2004. Large one-day temp changes MAE for each forecast type during periods of large temperature change (10F over 24-hr), 1 August 2003 – 1 August 2004. Includes data for all stations. MAE for each forecast type during periods of large departure (20F) from daily climatological values, 1 August 2003 – 1 August 2004. Number of days each forecast is the most accurate, all stations. In (a), tie situations are counted only when the most accurate temperatures are exactly equivalent. In (b), tie situations are cases when the most accurate temperatures are within 2F of each other. Looser Tie Definition Number of days each forecast is the least accurate, all stations. In (a), tie situations are counted only when the least accurate temperatures are exactly equivalent. In (b), tie situations are cases when the least accurate temperatures are within 2F of each other. Looser Tie Definition Highly correlated time series Time series of MAE of MAX-T for period one for all stations, 1 August 2003 – 1 August 2004. The mean temperature over all stations is shown with a dotted line. 3-day smoothing is performed on the data. Cold spell Time series of bias in MAX-T for period one for all stations, 1 August 2003 – 1 August 2004. Mean temperature over all stations is shown with a dotted line. 3-day smoothing is performed on the data. MAE for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region. MOS Seems to have the most problems at high elevation stations. Bias for all stations, 1 August 2003 – 1 August 2004, sorted by geographic region.