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Chapter 6. Atmospheric Boundary layer

Chapter 6. Atmospheric Boundary layer. Central Questions: What control energy, water and momentum exchange at the interface between earth’s surface and atmosphere? Topics: Surface layer and surface fluxes Convective mixed layer and marine stratus clouds Wallace and Hobbs, Ch.9.

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Chapter 6. Atmospheric Boundary layer

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  1. Chapter 6. Atmospheric Boundary layer Central Questions: • What control energy, water and momentum exchange at the interface between earth’s surface and atmosphere? Topics: • Surface layer and surface fluxes • Convective mixed layer and marine stratus clouds Wallace and Hobbs, Ch.9

  2. 6.1 Characteristics of the ABL

  3. What is the atmospheric boundary layer? • The atmospheric boundary layer is a layer of the atmosphere directly influenced by the surface. • The wind at the surface together with and interacting with sources and sinks of thermal energy creates turbulence that mixes the near surface air over a time scale short compared to a day. • The depth to which this mixing extends under dry conditions, typically about 1-km within a factor of two, (smaller for very stable -winter, or night- conditions over land, and larger for very unstable – summer daytime – dry soil) is referred to as the atmospheric boundary layer. • Cloud condensation processes provide the most rapid mechanisms for coupling this dry atmospheric boundary layer region to the overlying atmosphere.

  4. Role of the ABL:

  5. The dynamics of ABL is different from that of the free atmosphere Laminar turbulent

  6. Motions in the ABL: • A boundary layer in a continuum system is a region with length scales that are small compared to the dominant spatial scales of the system. That is we are able to separate the system into “large scale” and “small scale”. Near surface wind Identized spectrum of winds near surface

  7. Observed differences between ABL and free atmosphere: RH q

  8. Structure of the atmospheric boundary layer:

  9. a. Surface layer is important in determining the surface energy and water fluxes: • Surface temperature change is determined by net energy balance between the incoming solar radiation, and outgoing infrared radiative flux, sensible and latent fluxes, and heat transported to deeper layer. Over Land: Over ocean: Fsolar Fsolar FLH FLH FIR FIR FSH FSH dTs/dt=Fsolar-(FIR+FLH+FSH+FG) dTs/dt=Fsolar-(FIR+FLH+FSH+FG) FG Heat diffusion is so slow that FG is much smaller than other flux terms. Also Ts reaches equilibrium quickly, so that dTs/dt~0. Thus, Fsolar-(FIR+FLH+FSH) = 0 FG

  10. What determines surface turbulent flux? • Turbulent transport, w’, & moisture gradient, dq/dz • Practically, we do not have adequate measurements to directly determine turbulent condition, instead, we use standard meteorological measurements, wind, T and q, to determine surface latent and sensible fluxes using the bulk formula. Turbulent transport for q and q.

  11. The Bulk Formulae: This form of the bulk formulae only applicable to wet saturated surface, i.e., over water and wet soil.

  12. The drag coefficient, CD and CE: • Depends on static stability of the boundary layer and surface aerodynamic conditions (roughness, wind)

  13. Although drag coefficient values are often given for neutral conditions, the atmosphere boundary layer over land is rarely neutral (stable at night and morning, unstable in afternoon). • Drag coefficient increase rapidly with instability, C |V|~ BHwB • where BH: convective transport coefficient • WB: buoyancy velocity scale, determined by static stability of the boundary layer. Zi: depth of the convection or well-mixed BL.

  14. Exercise: Compare LH in the following five cases over water or wet surface and convert into energy unit (Wm-2, LvFLH): Wind is 5 m/s, surface saturation specific humidity is 15 g/kg and specific humidity is 10 g/kg. Calculate the latent flux over a) ocean b) over short grassland c) over forests A high pressure system passes, such that wind increase to 10 m/s and specific humidity drop to 5 g/kg. d) calculate the latent flux over ocean e) calculate latent flux over forests. The latent heat of vaporization, Lv=2.5X106 J/kg CE=0.0047 for grass CE=0.018 for forests

  15. Water controls surface energy partition: • Broadly links to the surface wetness over land; • Over ocean, Ts is lower than over land for the same net radiative flux because of high LH. Annual sensible flux Annual rainfall: Annual latent flux

  16. Bowen ratio, Bo, and evaporative fraction, l: • Bo=FSH/FLE It is an important indicator of surface wetness: wetter surface: lower Bo drier surface: higher Bo •  also represents the ratio of specific heat vs. latent heating due to change of saturation mixing ratio for an increment temperature change at the surface. Evaporational fraction: • l=FLH/Frad (Frad: net radiative flux) l≤1 wetter surface: higher l1 drier surface: lower l Often used by ecology and land surface community, l~ 0 when FLH=0, whereas Bo ~ ∞ when FLH=0. Dependence of Bo on T

  17. Surface latent flux over land: • Surface ET is controlled by multiple processes, • evaporation from bare soil and leaf, • transpiration from leaf. • Evaporation vs. transpiration: • Dry forest canopy: transpiration is ~25-60% of FR. • Wet forest canopy: evaporation is ~60-70% of FR. • Lake, river, stream: 100% evaporation

  18. Over wet surface: evaporation of intercepted leaf water: • The diffusion resistance of water on the leaf is smaller than water in leaf, thus intercepted water evaporates first, reduce energy available for transpiration. • ET is equal to the maximum or potential evaporation, Ep, and determined by the aerodynamic resistance, ra; • Intercepted leaf water: water layer on leaf ~ 0.1mm, • Deciduous tree: 20-30% of total rainfall (Jong 2001) • Rainforest: 30-50% of total rainfall (Shuttleworth 1985) • Intercepted leaf water also reduce transpiration by • 6-9% for spruce and pine • 20-40% for hardwood forest • Upto 100% for grass

  19. Evaporation over wet canopy and soil: Evaporation dominates (60-100%), similar to pen-evaporation or potential (maximum) ET, or PET. PET is determined by surface meteorological conditions and can be estimated long-term and globally using surface stations’ data with assumed rav.

  20. What controls raV? • Aerodynamic resistance is a main source of uncertainty in estimate evaporation. • It is usually estimated for neutral stable condition, a condition rarely meet over land in reality.

  21. Link of ET with surface energy budget: Ep is mainly determined by the net energy (FR-FG), (mainly radiative, FR) flux, moisture deficits, dq, and turbulence of the surface layer (rav).

  22. Equilibrium evaporation with a saturated surface: Priestley-Taylor equation: • For weak advection, the air humidity will reach an equilibrium with a saturated surface, q becomes zero. An empirical formula is sometime used to estimate Ep. LvFLH,p=PT(FR-FG) ~PTFR PT=1.26 is determined empirically, suggesting ~20% due to advection G increase with T.

  23. Corrected for thermal stability: tends to reduce dependence of Ep on surface roughness. Influence of surface aerodynamic roughness onFLH,p: Neutrally stable:

  24. Soil moisture ET over dry surface: • Turbulent flow in the surface layer: transport water vapor away from surface, ra • Water vapor gradient: qa-qs • Surface resistance: rs, determines qs. • If surface is dry, ET is controlled by both ra and rs. rsplays a more important role over a drier surface.

  25. ET from dry vegetation: • For close canopy (LAI>1) • rs-Stomatal controls the transfer of water from inside of a plant to exterior of leaf, reducing the surface humidity to below its saturation value.

  26. For close canopy (LAI>1) • rs: depends on vegetation type, soil moisture, net radiation, moisture deficit, and CO2.

  27. The Penman-Monteith equation: commonly used for ET estimate: ET for open canopy: • Combined foliage and sub-canopy ET, and evaporation from the open areas between the vegetation. The relation between ET and potential ET:

  28. What determines the surface resistance? Determined by surface type, Solar radiation, q, T, qsoil Roots depth and type are different.

  29. Exercise-2: • If the meteorological condition is the same (raV=0.01), what would be the ratio of ET/PET=(1+rs/rav) for the following vegetation types for a) tropical forest, b) savannah, c) crop, d) citrus dry, e) citrus wet?

  30. Influence of rs on ET: Eo/EL: The ratio of ET vs. evaporation with rs for different surface temperatures: 278K 303K 303K 278K

  31. Influence of vegetation type on ET for very similar radiation and climate conditions: • In situ measurements in the Amazon for a forested and adjacent pasture plots; • The difference is larger when surface soil is drier. wet season: dry season:

  32. Influences of daytime radiation and surface air moisture on surface conductance, C~1/rs Lohammer et al 2001: Influences of soil moisture on the relative humidity of surface air and ET: Pine spruce sand Sandy clay loam q q clay Rn Rn

  33. Main sources of uncertainty in estimate ET: • Control of stomata: the biophysical processes that control stomatal function is incomplete, e.g., • whether respond to transpiration or directly to q; • Dependence of stomatal resistance on soil moisture deficit is not adequately understood • Influence of large eddy and advection on the aerodynamic resistance • Evaporation from sparse forest: estimate of intercepted rainfall and transpiration is uncertain • Effect of climate change on ET, e.g., increase of CO2 on transpiration of water vapor • ET estimate on regional and catchman scales: the effects of entire mixing layer, instead of just surface layer, and mesocale circulation and advection, are not well understood;

  34. LE LW  SR Es/t The surface layer Imbalance of surface energy flux: • ~30% imbalance in part due to effect of advection Negrón Juárez et al. 2007

  35. On catchman and regional scales: • Need to consider the entire PBL. Brutsaert 1982

  36. 6. 2 Cloud Topped Boundary Layer • Importance of cloud topped BL: • Characteristics of the cloud-topped BL (CTBL): how are they different from cloud free BL • What control the structure of the cloud-topped ABL? • Source and sinks of TKE • Local process, influence of entrainment and vertical mixing • Clouds • Changes near the top of the BL-radiation and entrainment become central • Influences of waves and advection • What control clouds and entrainment? • Cloud formation and distribution • What controls entrainment and inversion?

  37. Importance: • Strong impact on global and regional radiation budget: • Albedo of typical stratocumulus ~ 0.6-0.7, of the remainder, ~25% is absorbed in the cloud layer. Under a clear sky, the albedo of ocean surface is usually 0.1. • 4% increase in the area of ABL clouds could offset greenhouse effect due to 2CO2. • Stabilizes the free atmosphere and suppresses deep convection. • Transport moisture from the surface to cloud layer: e.g., cumuli transport moisture to the base of stratcumulus.

  38. CTBL coupled to surface:

  39. BLCu can influence diffused radiation and photosynethesis • Normalized downward solar radiation: • K/ KTOA=a + bC • Where • K : downward solar at surface, • KTOA : downward solar at top of atmosphere • C: cloud cover • Normalized std K/ KTOA: are often caused by diffused radiation from side of BLCu Most of the samples come from BLCu cases North central Massachusetts: 54N, 72.18W, Freedman et al. 2001

  40. Marine ABL with Stratocumulus:

  41. Types of CTBL: h Coupled to surface ABL • Fully coupled CTBL: Classic • Multi-layer clouds, ABL only extends to the top of the lowest cloud layer • A radiative driven cloud layer decoupled from surface: nighttime marine BL 0 hc, decoupled from surface, Maybe lifted from ABL h ABL 0 h hc, radiative driven ABL 0

  42. Shear driven stratus-topped BL: • Weak buoyancy in cloud layer and strong wind shear. Decoupled single-layer stratocumulus: e unstable within cloud layer, weak wind shear: turbulence is driven by radiative cooling at cloud top. Upside down convection. Mixing extends down below cloud base, as seen in winds, e and q profiles.

  43. Surface heating drive cloud toped boundary layer (over land): Positive sensible and latent fluxes and strong upward moisture flux

  44. What controls the diurnal growth of the boundary layer-A BL Slab model • Ball 1960, Randall 2003:

  45. Cloud radiative effects: Solar radiation: • A simple parameterization: ∂K*(z)/∂z KT* KB* • Absorb solar radiation and heating cloudy layer throughout the cloud layer.

  46. Cloud radiative effects: • Longwave: • Liquid water droplets act like black body with the decay length of 30m. Thus, the net IR absorption can be approximately estimated as • Depends on cloud liquid water path. For thin clouds, cooling occurs through a deep cloud layer. For thick stratocumulus, cooling occurs in top 30-50 m.

  47. Cloud radiative effects: Longwave: • Absorb infrared radiation, strong cooling at the cloud top, and warming the cloud base. The cooling rate: • Strong cooling at cloud top. One advantage for the LEM is their ability to resolve thin layer of longwave cooling.

  48. solar longwave Influence of cloud liquid water path on solar and longwave radiation: A: 0.004 kgm-2, B: 0.008 kgm-2, C: 0.016 kgm-2, D: 0.032 kgm-2; E: 0.064 kgm-2.

  49. Dynamic and microphysical structure of clouds in CTBL: Convective Plume and Mixing: Mixing line: • In a non-precipitating convective plume, rtot, and qe would be conserved if there were no mixing. • The changes of rtot, and qe are due to mixing between air from different sources. The mixing determines the buoyancy within clouds, thus the depth, development and dissipation of the clouds. • Mixing line allows us to trace the origins of air that is mixed in clouds. e & rtot without lateral and downward mixing

  50. What cause breaking of Stratocumulus deck?

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