S-‐190: Weather GEOG301: Fire Weather 4 Keys to Fire Weather On

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GEOG301: Fire Weather S-­‐190: Weather Nick Nauslar Grand Junc9on Air Center Desert Research Ins9tute 4 Keys to Fire Weather •  Air Temperature and Rela9ve Humidity (RH) •  Precipita9on •  Atmospheric Instability •  Wind On a calm clear night… (1) Longwave radia9on emiTed back to space, surface cools rapidly (2) Air “close” to ground cools rapidly as well (3) Cold dense air sinks into valleys (4) Temperature “Inversion”: cold air below warm air Stable Atmosphere •  Resists upward mo9on in the atmosphere •  Fire generates ver9cal mo9on near the surface (instability) but the upward mo9on decreases •  Stable air limits indraZs into the fire at low levels and lowers fire intensity •  Inversions trap smoke, reduce visibility Thermal Belt: Nigh\me Inversion Thermal Belt: Nigh\me Inversion hTp://weather.uwyo.edu/cgi-­‐bin/sounding?region=naconf&TYPE=GIF%3ASTUVE&YEAR=2013&MONTH=08&FROM=0800&TO=1312&STNM=72786 Top of nigh\me inversion Eleva9on varies from night-­‐to-­‐night NIGHTTIME DAYTIME Ac9ve nigh\me fire 1
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Mixing Height Calcula9ng Convec9ve Mixing Height •  Defined: depth of the surface based mixed layer which is well mixed due either to mechanical turbulence or convec9ve turbulence. — 
•  Height at which smoke will stop rising — 
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•  Pollutants entering this layer should be ver9cally dispersed in about an hour — 
Obtain data from a radiosonde launch Es9mate maximum day9me temperature, or add 5C to morning surface temperature Draw dry adiaba9c line star9ng from maximum surface temperature Where this line intersects the observed temperature you have your mixing height •  Key determinant for smoke related concerns and prescribed fire Idealized Diurnal Mixing Height Curve The idealized mixed layer height cartoon below highlights the rela9onship of the mixing height with the solar cycle. 5kZ Maximum mixing height occurs between 1-­‐2 hours aZer solar noon. Height (Z) 4kZ 3kZ Mixing height decreases very slightly aZer peaking Maximum mixing height increase is dependent on 9me of year, atmospheric profile, as well as synop9c and mesoscale condi9ons. 2kZ New stable boundary layer develops 1-­‐2 hours before sunset. Mixing heights increase 1-­‐2 hours aZer sunrise. 1kZ 3AM 6AM sunrise 9AM 12PM 3PM Time (hours) solar noon 6PM 9PM 12AM Subsidence Inversion & Mixing Heights •  Large scale sinking of air associated with high pressure aloZ (Ridge) •  Air warms as it sinks causing lower RH and higher temperatures •  Accompanied by clear skies •  Can cause severe burning condi9ons and poor smoke dispersal 3AM sunset Height of Column (Mixing Layer) Sfc warms, upper-­‐air cool 2
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Unstable condi9ons •  Promotes upward mo9on & gusty sfc winds •  Column heights increase causing stronger indraZs and updraZs •  Spot fires from firebrands being carried upward Haines Index Metric of poten9al fire growth (Plume-­‐dominated fire) UpdraZ strength, column height, convec9ve sfc winds, firebrands and whirls Hot air rises Creates inflow to the fire Creates inflow to the fire Haines Index = Instability + Moisture Depression T1 – T2 + T1 -­‐ D1 Can cause wind shiZ, increase of wind speed as fire grows Diurnal Terrain Driven Winds (In the absence of large-­‐scale forcing) •  Differences in hea9ng and cooling occur (sun hi\ng certain slopes first) •  Upslope in the day9me and downslope during the night •  Typical wind speeds 3-­‐8mph but can be much higher •  Timing depends on terrain set up and current weather Mountain-­‐Valley Circula9on Summary •  Timing of transi9on from upslope/downslope to downslope/upslope very important •  Depth of inversion and loca9on thermal belt determines fire ac9vity overnight •  Strong upslope flow even with minimal moisture produces thunderstorms (igni9ons and gusty, erra9c winds) Terrain Induced Thunderstorms •  Upslope flow produces cumulus clouds and some9mes thunderstorms (with enough moisture, upslope flow, instability) •  Dissipate by evening hours with lack of upper level support •  Concerns: lightning (new igni9ons, firefighter safety), changes in winds, temperature, and moisture Cri9cal Fire Weather Condi9ons •  Strong and shiZing winds •  Low RH •  High temperatures •  Unstable atmosphere •  Dry lightning •  Cri9cal fire wx condi9ons: –  Dry Cold fronts –  Foehn/severe downslope winds –  Dry Thunderstorms 3
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Foehn Downslope Winds •  Dry wind (some9mes warm) that develops when air descending the leeward slopes of a mountain range is adiaba9cally compressed –  Strong winds cause stable air in the lower troposphere to ascend on the windward side –  Wind velocity at mountain pass level perpendicular to topography –  Regional “names” of winds Downslope momentum flux Offshore Wind Events •  Typically follow frontal passages and defined by strong subsidence •  Easterly wind component implies something about the pressure gradient… Cold Front Passage Dry cold fronts bring –  Gusty ourlow due to convec9on and limited moisture –  Strong winds –  Wind shiZs –  Rapid decrease in both temperature and dewpoint temperature –  In summer months, temperature may not decrease as much –  “Dry slot” In stable air, lee flow over mountain can mix strong winds to surface Breakdown of the Upper Ridge 1. 
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Upper-­‐level ridge (ULR) builds over the west, thermal trough forms over OR/WA ULR breaks down, shiZs thermal trough + ULR east to Idaho Weak shortwave cools to upper levels, not lower levels Dry Lightning (<0.1”) Basic Ingredients -­‐ Instability -­‐ Mid level moisture (helps promote instability and liZ) -­‐ Low low-­‐level moisture (discriminates wet from dry) -­‐ “Inverted V” Sounding 4
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Fire Weather Forecasts and Outlooks Polarity Gradient •  Na9onal Weather Service (NWS) Products and Services: –  Fire Weather Planning Forecasts –  Spot Forecats –  Fire Weather Watches –  Red Flag Warnings •  Upper cloud becomes + charged as ice crystals gain charge while graupel becomes -­‐ charged, can lead to cloud-­‐cloud or sheet lightning •  Cloud-­‐ground lightning (25%) of strikes, more favorable when freezing level is lower NWS: Red Flag Warning •  Issued issued when BOTH dry fuels and weather condi9ons suppor9ng fire igni9on and/or significant growth is occurring or is expected to occur. •  Subjec9ve and vary from WFO to WFO Spot Weather Forecasts Provide detail the General forecast lacks Can be tailored to the most cri9cal 9me periods Include the effects of topography: •  Local winds, and windshiZs •  Inversions, and 9me of dissipa9on •  Slope and aspect considera9ons Example: hTp://spot.nws.noaa.gov/cgi-­‐bin/spot/sporcst?
site=mso&file=20140908.JOHNS.01 What determines local winds? •  Dissipa9on of wind by terrain features •  Accelera9on of wind by terrain features •  Diversion of wind around terrain features Terrain Correc9on Factors Suggested General Wind correc9on factors: •  Assuming: –  Gently sloped terrain. –  Neutral or unstable (or above inversion). –  Windward slope exposed to general winds. Upper 1/3 of slope: 0.4 to 0.6 of mountain top wind Middle 1/3 of slope: 0.3 to 0.4 of mountain top wind Lower 1/3 of slope: 0.2 to 0.3 of mountain top wind Sheltered Areas: near zero 5
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Wind Velocity A power law profile is used to describe the varia9on of wind speed with height in the surface boundary layer U = U1 (Z/Z1)p Where, U1 is the velocity at Z1 (usually 10 m) U is the velocity at height Z. The values of p are given in the following table. Stability Class Rural p Urban p Very Unstable 0.07 0.15 Neutral 0.15 0.25 Very Stable 0.55 0.30 6