Overview Statistics Fog Types Ingredients for Radiation/Advection Fog Forecasting Techniques Summary
Why do we need good fog forecasts? 1981-1989, 6000 deaths nationwide due to fog Average 600 deaths nationwide per year (Source Goodwin 2002) 4 major fog related accidents in 1990-1991 involved 240 vehicles, causing 21 fatalities January 12, 2005 Dense Fog caused a 32 car pile-up on Indiana Toll road
Aviation Folks Would Appreciate It 1995-2000 NTSB reported 63% of all weather related aircraft fatalities were due to low IFR/Fog situations. (Pearson, 2002) Results in delayed operations and cost airlines millions of dollars each year. A large majority of these accidents are due to pilot error.
Types of Fog in Central Plains Radiation fog common type of fog produced over a land area when radiational cooling reduces the air temperature to its dewpoint. Advection fog- A type of fog caused by the advection of moist/warm air over a cold surface, and cools the airmass to below its dewpoint? Hybrid fog -A Hybrid fog situation involves radiation and advection processes.
Ingredients for Radiation Fog Clear Skies Ample Boundary Layer Moisture Lack of Turbulence This is all fine and good, but are these the only ingredients?
Battle of the Fluxes at Night Net Radiation Ground Heat Flux Sensible Heat FluxLatent Heat Flux Recall: Rn - G = SH + LH
Ingredients for Radiation Fog Ground Heat Flux- Flow of heat into or out of soil. Factors that affect this are porosity, moisture content and organic matter. Sensible Heat Flux- change of heat to a surface without a phase change. For example, mixing, advection, and convection. Latent Heat Flux- change of heat to a surface due to a phase change. For example, Evaporation, Condensation etc..
How latent heat affects fog development Deposition/Dewfall-Can delay onset up to 4 hours. A. Reduces Surface Humidity and increases temperature through latent heat. B. Takes place in Surface Ridges where more subsidence is present. C. Often results in patchy fog events, especially if ground is dry and warm.
Other factors involved Fog Top Cooling -Good drying above fog top inversion Stratus Build Down -Dependent on Soil Conditions and Fog Top Cooling
Forecasting Radiation Fog Tools needed 1.UPS technique - Bufkit, MR# on VB 2.Soil type - Clay, Silt, Sand etc… 3.Soil Conditions -, Cold, wet 4.Lack of vertical motion/subsidence in the Isentropic surface just above the boundary layer.
Forecasting Advection Fog Tools Needed: 1.Often Develops East of Low Pressure and along and ahead of a tight thermal/moist gradient. (Sfc. Chart) 2.Moisture Transport Vectors 3.Profilers/Signal Power 4.Model Soundings 5.Tends to Increase Sensible Heat (www.profiler.noaa.gov)
Summary How do we predict fog more accurately at WFO level? You need to know or have access to the followingHow do we predict fog more accurately at WFO level? You need to know or have access to the following: Soil types-Clay/Sand/Silt etc… Role of heat fluxes-Ground/Latent etc… UPS technique-(Bufkit) using a High vertical resolution model- for example Workstation WRF, Ruc20 Isentropic Adiabatic Omega Raw data analysis-Raobs/Sfc
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