RADIATION IN THE TROPICAL ATMOSPHERE and the SAHEL SURFACE HEAT BALANCE. Peter J. Lamb. Cooperative Institute for Mesoscale Meteorological Studies

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1 RADIATION IN THE TROPICAL ATMOSPHERE and the SAHEL SURFACE HEAT BALANCE by Peter J. Lamb Cooperative Institute for Mesoscale Meteorological Studies and School of Meteorology The University of Oklahoma NCAR/ISP Summer Colloquium African Weather and Climate: Unique Challenges and Applications of New Knowledge Boulder, Colorado August 2011

2 LECTURE OUTLINE 1. Basic Radiation Laws and Budgets 2. Global Importance of Tropical Radiation 3. Radiation Budget of Atmospheric Column over Niamey 4. Surface Heat Balance at Niamey

3 BASIC RADIATION LAWS AND EQUATIONS WIEN S DISPLACEMENT LAW λ max (μm) (1) Sun radiates at ~ 6000 o K λ max = 0.48 μm shortwave radiation (SW) Earth radiates at ~ o K λ max = μm longwave radiation (LW) STEFAN-BOLTZMAN LAW Energy Emitted = (2) EARTH-ATMOSPHERE SYSTEM Receives SW as a DISK (3) Emits LW as a SPHERE RADIATIVE EQUILIBRIUM TEMPERATURE (T p ) (4) Obtained by equating (3) and (4) and solving for T p T p 255 o K versus observed T s 288 o K Δ 33 o C is due to atmosphere s natural greenhouse effect T s increase in last 100 years 1 o C

4 TROPICAL ATMOSPHERE = NET RADIATIVE SURPLUS Now equate (3) and (4) and solve for T p for each latitude zone (e.g., 5 o wide) Resulting latitudinal profile of T p is much steeper than observed T s profile -- tropics (extratropics) are hotter (colder) than observed T ( 0 C) Solid line = modeled Dotted line = observed sin Ф Equatorward of ~ 38 o N/S SW > LW at top of atmosphere Poleward of ~ 38 o N/S SW < LW at top of atmosphere Tropical net radiative surplus "supports" extratropical net radiative deficit through MERIDIONAL HEAT TRANSPORT by atmospheric and oceanic motion TROPICAL RADIATIVE HEATING THUS DRIVES ENTIRE SYSTEM

5 RADIATIVE FLUXES AND DIVERGENCE Radiative Flux Divergence Primer Insolation Reflected SW OLR Incoming Outgoing= net radiation into column TOA Radiative Flux Divergence = net radiation into column - net radiation into surface positive values imply heating negative values imply cooling Upwelling and Downwelling SW and LW Downwelling Upwelling= net radiation into surface Surface from: Mark Miller (Rutgers University)

6 TROPICAL RADIATIVE FORCING OF GLOBAL CLIMATE SYSTEM HEAT BUDGET SCHEME a o SWLW top = Q va + Q ta + Q vo + Q to Q v = divergence of meridional transport Q t = storage change Rad = radiative flux divergence LP = condensational heating Q s = sensible heat flux = ρc D C p (T s T a )V Q e = latent heat flux = ρ C D L (q s q a )V SWLW sfc = Q s + Q e + G (land) + Q vo + Q to (ocean) ANNUAL MEAN MERIDIONAL PROFILES Apportionment of net radiative surplus (SWLW t op ) between Q va = atmospheric transport divergence and Q vo = oceanic transport divergence from: Hastenrath (1991)

7 ATMOSPHERIC COLUMN ABOVE WEST AFRICA Measure the TOA radiation using GERB & SEVIRI on Meteosat. Measure surface fluxes & the atmospheric state variables using the AMF. Niamey, Niger from: Mark Miller (Rutgers University)

8 ARM MOBILE FACILITY NIAMEY AIRPORT 2006 before rainy season

9 ARM MOBILE FACILITY NIAMEY AIRPORT after 2006 rainy season

10 INFLUENCES ON ATMOSPHERIC RADIATION Factors that may control the TOA, surface, and total column radiative divergences in SW and LW: - dust -Column Water Vapor (CWV) -clouds -vegetation -biomass burning from: Mark Miller (Rutgers University) Haywood et al., JGR, 2008

11 SHORTWAVE RADIATION DURING 2006 Shortwave divergence is mainly determined by the CWV and aerosol loadings. Clouds Shortwave Divergence Wm -2 TOA Divergence Surface Divergence Absorption by CWV Day of the Year, 2006 The effect Dust of Outbreaks clouds on the net column shortwave flux divergence is smaller than their effect on the component fluxes. from: Mark Miller (Rutgers University) Slingo, A., H.E White, N.A. Bharmal, G.J. Robinson (2008)

12 SHORTWAVE RADIATION HEATING The SW heating is concentrated in moist layers, as identified in soundings. January 21, 2006 from: Strong aerosol extinction between 3-4 and below 1-km Mark Above 5-km, SW heating is due to water vapor absorption Miller (Rutgers University) McFarlane, S.A., E.I. Kassianov, J. Barnard, C.Flynn, T.P. Ackerman, JGR, 2008

13 LONGWAVE RADIATION DURING 2006 Changes in the cooling at the TOA are caused by changes at the surface. from: Mark Miller (Rutgers University) Surface Divergence Longwave Flux and Longwave Divergence Wm -2 LW Divergence relatively constant Wet Season Day of the Year, 2006 OLR (Longwave Flux) As CWV increases, the atmosphere loses longwave minimum energy cooling to at the surface surface at TOA with maximum about cooling the same at surface increasing and efficiency TOA with which it traps OLR. -surface temperature is the highest, CWV lowest Slingo, A., H.E White, N.A. Bharmal, G.J. Robinson (2008)

14 SHORT WAVE + LONGWAVE RADIATION DURING 2006 The atmosphere continually loses radiative energy to space at a steady rate of approximately 75 Wm -2. from: Mark Miller (Rutgers University) Total Divergence Wm -2 Surface Divergence TOA Divergence Wet Season Day of the Year, 2006 Small positive net gain of radiation at the TOA during the course of the year region gains energy during summer and loses energy in winter Surface gains radiative energy at all times of the year Slingo, A., H.E White, N.A. Bharmal, G.J. Robinson (2008)

15 DIURNAL RADIATION CYCLE -- SEPTEMBER 21, 2006 Cloud Cover (AMF) Short Wave Red = TOA (GERB) Black = surface (AMF) Long Wave Divergence TOA - Surface from: Mark Miller (Rutgers University) Miller, M.A. and A. Slingo (2007)

16 BASIC EQUATION LAND SURFACE HEAT BALANCE SWLW = SW - LW sfc sfc = Q s + Q e + G sfc (7) or = H + LE + G COMPONENTS SWLW sfc = surface net radiation (by radiative transfer) Q s = H = sensible heat flux (by convection) = ρ C D C p (T s T a )V (5) Q e = LE = latent heat flux (by convection) = ρ C D L (q s q a )V (6) G = soil heat flux (by conduction) 0.1 x SWLW (note: oceanic counterpart, Q to, largely is due to convection) KEY CONTROL Q e NEEDS SOIL MOISTURE AND VEGETATION PRESENCE versus ABSENCE OF SOIL MOISTURE/VEGETATION DETERMINES NATURE OF HEAT BALANCE sfc

17 LAND SURFACE & ATMOSPHERIC COLUMN -- NIAMEY 2006 PRECIPITATION ANNUAL LEAF AREA INDEX CYCLES COLUMN WATER VAPOR MOIST STATIC ENERGY from: Miller et al. (2009)

18 SURFACE HEAT BALANCE -- NIAMEY 2006 ANNUAL CYCLES SW = + H = LE = LW = G G G -5.7 AVERAGE DIURNAL CYCLES from: Miller et al. (2009)

19 VEGETATION-EVAPOTRANSPIRATION LINK Evaporation anomalies following rainfall suggest a link to vegetation. Evaporation anomaly following rainfall (The anomaly is computed relative to the five days prior to the rain event.) Early in the rainy season (red), the evaporation rate increases temporarily following the occurrence of precipitation. At the height of the rainy season, the evaporation rate becomes independent of the precipitation produced by recent precipitation events (blue). from: Mark Vegetation growth in July helps evaporation to decorrelate with Miller precipitation roots tap moisture deep within the soil that was (Rutgers possibly stored during the previous rainy season. University) Miller, R. L., A. Slingo, J. C. Barnard, and E. Kassianov (2009)

20 SW SUMMARY CARTOON SW TOA LW clouds water vapor dust biomass burning SW SW H G LE vegetation LW LW