Why Can’t Use Model Output Directly for Temp Forecasts? • Model surface/2m height may be very different than real elevation due to limited horizontal/vertical resolution. (e.g., MM5 36 km surface elevation for Boeing Field is 256 m, should be 5 m)
Model resolution may be inadequate to properly simulate the temperature effects of important features such as: • Narrow gaps, such as the Fraser River Valley or the Columbia River Gorge. • Land/water contrasts, such as around Puget Sound or along the coast.
Near Sea Level Gap On Border of WA and OR
Wintry precipitation at PDX • Almost exclusively E or SE • Tendency towards E for snow and SE for freezing rain Wind distribution for days with Freezing Rain Wind distribution for days with snowfall Portland wind distribution for all days by direction and speed
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
36 km grid spacing 12 km grid spacing The Dalles Portland Portland The Dalles Pass Height = 700 m Pass Height = 600 m
4 km grid spacing Portland Cascade Locks The Dalles The Dalles Portland Pass Height = 400 m 12 km grid spacing Pass Height = 600 m
• Portland Cascade Locks The Dalles Troutdale 1.33 km grid spacing, Pass Height = 150 m
Portland Troutdale Cascade Locks 444.4 m grid spacing, Pass Height = 100 m
Portland Troutdale T on150 mSurface
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. • Example 2: improper soil moisture (too warm or dry) can greatly influence temperatures. • As a result of such errors, models can have serious systemic temperature biases.
Shallow Fog…Nov 19, 2005 • Held in at low levels for days • MM5 held in the inversion…generally without the shallow mixed layer of cold air a few hundred m deep • MM5 could not maintain the moisture at low levels
Temperature Biases • Large temperature biases can occur at certain times: • When there is a shallow layer of cold air (few hundred m deep) that is mixed out. • During transition season (particularly spring, when land surface conditions are problematic) • During summer during warm periods.
Downslope Warming • Large warming during downslope flow • Often large over Cascade foothills (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. • Uusually, air comes from mid-levels where potential temperature is higher than at the surface. A drop in dewpoint usually accompanies it.
Diabatics • Radiation: • direct radiational heating and cooling of the air is relatively small • indirectly, very large 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 lessening solar radiation • Clouds decrease cooling at night by intercepting IR radiation and reradiating IRback to the surface. • Thin cirrus…only minor influence (1-2F) • Thick altostratus--5-10F influence or more.
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.
Snohomish, WA • April 4, 2005