Influence of Clouds on Surface Heat Fluxes in an Energy Deficient Regime

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Influence of Clouds on Surface Heat Fluxes in an Energy Deficient Regime. Dea Doklestic G&G 570 Class Project Heat Budget Group Presentation, June 16, 2011. Questions. How do clouds influence surface heat budget? How are turbulent heat fluxes influenced during overcast conditions?
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Influence of Clouds on Surface Heat Fluxes in an Energy Deficient RegimeDea Doklestic G&G 570 Class ProjectHeat Budget Group Presentation, June 16, 2011Questions
  • How do clouds influence surface heat budget?
  • How are turbulent heat fluxes influenced during overcast conditions?
  • Is latent heat flux mainly controlled by RH or plant transpiration?
  • How is longwave radiation influenced by clouds
  • Surface Heat BudgetLW ↓H LESWLW ↑GRnet = S(1 – a) + LW ↓ - LW ↑Rnet – G = H + LERegion of InterestIvotuk, Alaska
  • Flux Tower equipped with pyranometers to measure incoming and outgoing SW radiation and infrared thermometers to measure outgoing and incoming LW radiation
  • Turbulent fluxes estimated using the eddy covariance method (sonic anemometer)
  • Location: 68N, 155W
  • Available data: 2004 - 2006
  • Approach
  • 2 different states
  • Snow on the ground (DOY 132 – 145)
  • No snow on the ground (DOY 195 – 208)
  • Albedo (Visual)Image obtained from: http://daac.ornl.gov1. Penman-MonteithEquation for evaporation from a wet surface(Potential Evaporation)
  • Evaporation from a wet surface given the available energy and vapor pressure deficit
  • Assumed to be maximum possible evaporation for the given available energy. (Water supply not limited)
  • 1. PE and Latent Heat FluxBefore melting of snowPE and Latent heat FluxSnow – free conditionsPE and Latent Heat Flux ConclusionsSpring SummerLatent heat flux exhibits a diurnal cycleMaximum occurs during midday, minimum over nightPE grossly overestimates latent heat fluxCloudiness does not seem to influence LH much
  • When the ground is still covered with snow, LH is zero
  • Different from modeled PE
  • PE over snow shows a diurnal cycle which is completely absent in measured data.
  • 2. Influence of Clouds on Longwave RadiationBefore melting of snow2. Influence of Clouds on Longwave RadiationSnow-free conditions2. Clouds and Longwave RadiationConclusionsSpring SummerOn clear days – OLR exhibits a diurnal cycle. During summer, even on cloudy days there is some diurnal variation in OLRLikely due to more SW radiation being absorbed at the ground because of a much lower albedo. )That drives a diurnal cycle in surface temperature)
  • On clear days – OLR exhibits a diurnal cycle.
  • Maximum radiation emitted during daytime, minimum during night
  • On cloudy days – diurnal cycle in OLR vanishes
  • Incoming LW radiation higher on cloudy days – both in spring and summer
  • Shows no diurnal variations
  • Do the clouds inhibit snow melting or do they accelerate it?
  • During winter conditions when SW radiation is zero, the presence of clouds will increase RNET
  • In spring – solar radiation gets higher – clouds have a double effect:
  • Decrease incoming shortwave radiation
  • Decrease outgoing longwave radiation & increase incoming LW
  • These two effects work in opposite directions, so which one is dominant?
  • Approach
  • Model incoming SW radiation:
  • Incoming LW – assumed to be constant throughout the day. Set to 200Wm-2
  • Outgoing LW – has a diurnal cycle which is modeled as:
  • i goes from 1 – 48 and corresponds to a 30-minute period
  • This equation gives a minimum OLW value of 250 Wm-2 and a maximum value of 320 Wm-2 at noon
  • Results Comparison of Modeled vs. Measured Net RadiationInfluence of clouds on melting of snowConclusions
  • Time period DOY 132 – 145
  • Modeled perfectly clear days
  • Transmisivity 0.9
  • Albedo 0.75
  • Net radiation modeled this way was lower than observed net radiation on cloudy days
  • In this particular case, cloudiness seems to accelerate melting of snow
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