Surface Energy Budget, Part I COMET Planetary Boundary Layer Symposium Matt Haugland

Introduction Why is the Surface Energy Budget so important? Air is largely (80%) transparent to solar radiation. Nature of the earth’s surface determines way in which the atmosphere is heated. Differences over land surfaces result from contrasts in soil moisture, vegetation type & status, presence of snow/ice Surface energy budget is the key to understanding local climates

What is the Surface? Can be defined as almost anything Often defined as an infinitesimally thin massless plane separating the air and ground More helpful to define surface as a layer (with mass and volume), which can include air and soil

What is the Surface Energy Balance? Based on Conservation of Energy Energy is supplied by the sun Energy going into the surface equals energy leaving the surface If defined as a layer/volume, energy into the “surface” can also be stored inside the surface (as heat)

Surface Energy Balance Equation Energy IN Energy OUT Heating Rn = H + LE + G (+ P) (+DQ) where: Rn = Net Radiation H = Sensible Heat Flux LE = Latent Heat Flux G = Ground Heat Flux P = Photosynthetic Heat* DQ = Net Heating/Storage** * P term is very small, usually ignored in meteorological applications * DQ term only included if surface is defined as a volume – a massless plane can’t be heated

Surface Energy Balance Rn Physical processes… H LE Advection & Convection Evaporation & Condensation Mixing Radiation Conduction / Heat storage G

Rn = (SW↓ + SW↑ ) + (LW↓ + LW↑ ) Net Radiation Rn = (SW↓ + SW↑ ) + (LW↓ + LW↑ ) where fluxes toward (away from) the surface are positive (negative) SW↓ is the sum of the direct (from the sun) and diffuse (from the sky including clouds) shortwave radiation. SW↑ is the shortwave radiation reflected by the surface. LW↑ is the longwave radiation emitted by the surface - proportional to T4 LW↓ is the longwave radiation received by the surface from the atmosphere – proportional to T4

Net Radiation What makes net radiation high/positive? What makes net radiation low (near zero)? What makes net radiation negative?

Net Radiation What makes net radiation high/positive? Clear/Sunny conditions during the day (shortwave) Low surface albedo (shortwave) Atmosphere warmer than ground (longwave) What makes net radiation low? Clouds, low sun angle (shortwave) Atmosphere similar temperature as ground (longwave) What makes net radiation negative? Atmosphere cooler than ground (usually negative at night, most negative around sunset)

Fluxes: Positive or Negative? Positive if energy is transferred FROM the surface (surface acts as a source of energy) Negative if energy is transferred TO the surface (surface acts as a sink of energy)

Sensible Heat Flux Daytime: heating of the air (cooling of the surface) by turbulence/mixing/convection Nighttime: cooling of the air (heating of the surface) by turbulence/mixing Heating/cooling of the surface (indirectly) by advection Laminar or calm-air conduction usually ignored Determines how much heat is transferred above the surface -- important factor when predicting above-surface temperature and boundary layer depth

Sensible Heat Flux What makes sensible heat flux high? What makes sensible heat flux low? What makes sensible heat flux negative?

Sensible Heat Flux What makes sensible heat flux high? Wind/turbulence (strong vertical mixing) Less latent heat and ground heat flux Warm surface, cool air What makes sensible heat flux low? Calm/weak wind High latent and ground heat fluxes What makes sensible heat flux negative? Air warmer than surface

Sensible Heat Flux A few more notes… Sensible heat flux affects boundary layer mixing, which can affect... Dewpoint Wind speed Wind direction Air pressure Daytime evolution of near-surface parameters Areas with high sensible heat flux tend to take on characteristics of higher boundary layer top, especially toward the afternoon (stronger wind, more veering of wind, lower dewpoint, etc.)

Latent Heat Flux Results from: evaporation (↑flux; moist surface) transpiraton (↑flux; leaves) evaporation + transpiration = evapotranspiration condensation (↓flux; dew deposition) Actual energy is consumed/released by phase changes

Latent Heat Flux What makes latent heat flux high? What makes latent heat flux low? What makes latent heat flux negative?

Latent Heat Flux What makes latent heat flux high? Water on the ground (rainfall, lakes, oceans) Plants What makes latent heat flux low? Dry ground Lack of plants/vegetation What makes latent heat flux negative? Condensation (dew)

Latent Heat Flux A few more notes… More green vegetation doesn’t always mean higher LE. Transpiration is limited by water availability. Higher LE doesn’t always mean higher dewpoint. A small moist/vegetated area in a dry (mesoscale) region could have a higher LE than a moist region, but a lower dewpoint.

Ground Heat Flux Thermal energy exchange through conduction (rock, soil, concrete) Thermal energy exchange through diffusion and mechanical mixing (water) Conduction into plants can be considered part of ground heat flux Depends on surface cover and soil characteristics (including soil moisture)

Ground Heat Flux Ground Heat Flux: can consume 5-15% of energy from Rn can be used to evaluate H and LE easier/cheaper to measure than Rn, S, LE sometimes ignored if “surface” includes deep enough layer of soil annual average near zero

Ground Heat Flux What makes ground heat flux high? What makes ground heat flux low? What makes ground heat flux negative?

Ground Heat Flux What makes ground heat flux high? Surface warmer than deeper soil (conduction related to temperature gradient) High thermal conductivity & heat capacity of soil What makes ground heat flux low? Surface temperature close to soil temperature High latent/sensible heat flux What makes ground heat flux negative? Surface cooler than deeper soil (esp. at night and during fall)

Conclusions Weather and climate “start” at the ground (surface) and are driven by solar radiation. Nature of the earth’s surface determines way in which the atmosphere is heated/cooled. Differences over land surfaces result from contrasts in soil moisture, vegetation type & status, etc. Local boundary layer conditions strongly depend on local surface characteristics

Questions Any Questions??