# midterm review. what have we discussed? importance of the atmospheric boundary layer surface energy...

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- Slide 1
- Midterm Review
- Slide 2
- What have we discussed? Importance of the atmospheric boundary layer Surface energy balance Surface water balance Vertical structure of the ABL Modeling the ABL Startocumulus-topped boundary layer Boundary layer of shallow and deep convection Land surface processes Air pollution Climate feedbacks for global warming Ocean-atmosphere interaction Abrupt climate change
- Slide 3
- Importance of the ABL The mission of meteorology is to understand and predict weather- related disasters (e.g. tornados, hurricanes, winter storms) and climate-related disasters (e.g. El Nino and global warming). The modern climatology (meteorology) was born in the 1940s (a very young science!), but has been growing very fast! Now we have a global observational network with many satellites, ships, radars and surface stations, as well as very comprehensive prediction models running on the worlds fastest supercomputers. The current status of weather and climate predictions: (1) weather prediction good to 10 days, (2) tropical cyclone prediction good in track but not in intensity, (3) climate prediction good to two seasons, (4) climate change projections have a 3-fold difference in magnitude. The main reasons of the difficulties: (1) Teleconnection problem, (2) Feedback problem, and (3) Subgrid-scale problem. Importance of the ABL: (1) interface between atmosphere and ocean/land/ice - flux transfer and feedback, (2) the human beings are living in the ABL and change the environment, (3) a basic subgrid- scale process
- Slide 4
- Surface energy balance What is energy? 3 methods of energy transfer The names of the 6 wavelength categories in the electromagnetic radiation spectrum. The wavelength range of Sun (shortwave) and Earth (longwave) radition Earths energy balance at the top of the atmosphere. Incoming shortwave = Reflected Shortwave + Emitted longwave Earths energy balance at the surface. Incoming shortwave + Incoming longwave = Reflected shortwave Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave + Latent heat flux + Sensible heat flux + Emitted longwave + Latent heat flux + Sensible heat flux + Subsurface conduction What is sensible heat flux? What is latent heat flux? Bowen ratio B= SH/LH = C p (T surface - T air ) / L(q surface - q air ) provides a simple way for estimating SH and LH when the net radiative flux Fr is available LH=Fr/(B+1), SH=Fr B/(B+1) Other heat sources: precipitation, biochemical, anthropogenic
- Slide 5
- The Electromagnetic Spectrum The limitations of the human eye!
- Slide 6
- Summary: Surface energy balance dT/dt SWdn =Scos SWup =SWdn LWdn = Tair 4 LWup = Ts 4 LH= C d LV(q surface - q air ) SH= C d C p V(T surface - T air ) Fc = - dT/dz Incoming shortwave + Incoming longwave = Reflected shortwave + Emitted longwave + Latent heat flux + Sensible heat flux + Subsurface conduction
- Slide 7
- Global water cycle Surface water balance Soil moisture: Increases with depth Palmer drought severity index (PDSI): uses temperature and rainfall Desertification Surface water balance
- Slide 8
- The global water cycle
- Slide 9
- Surface water balance dS/dt Precipitation (P) Evaportranspiration (E=Eb+Ei+Es+TR) Runoff (Rs) Irrigation (I) Infiltration (Rg) The changing rate of soil moisture S dS/dt = P - E - Rs - Rg + I (PDSI, desertification)
- Slide 10
- Vertical structure of the ABL Vertical structure of the atmosphere and definition of the boundary layer Vertical structure of the boundary layer Definition of turbulence and forcings generating turbulence Static stability and vertical profile of virtual potential temperature: 3 cases. Richardson number Boundary layer over ocean Boundary layer over land: diurnal variation
- Slide 11
- Vertical Structure of the Atmosphere Definition of the boundary layer: "that part of the troposphere that is directly influenced by the presence of the earth's surface and responds to surface forcings with a time scale of about an hour or less. Scale: variable, typically between 100 m - 3 km deep
- Slide 12
- Vertical structure of the boundary layer From bottom up: Interfacial layer (0-1 cm): molecular transport, no turbulence Surface layer (0-100 m): strong gradient, very vigorous turbulence Mixed layer (100 m - 1 km): well-mixed, vigorous turbulence Entrainment layer: inversion, intermittent turbulence
- Slide 13
- Static Stability Static stability refers to atmospheres susceptibility to being displaced Stability related to buoyancy function of temperature The rate of cooling of a parcel relative to its surrounds determines its stability of a parcel For dry air (with no clouds), an easy way to determine its stability is to look at the vertical profile of virtual potential temperature v = (1 + 0.61 r ) Where = T (P 0 /P) 0.286 is the potential temperature r is the water vapor mixing ratio Three cases: (1) Stable (sub-adiabatic): v increases w/ height (2) Neutral (adiabatic): v keeps constant w/ height (3) Unstable (super-adiabatic): v decreases w/ height Stable or sub-adiabatic Neutral or adiabatic Unstable or super-adiabatic
- Slide 14
- Diurnal variation of boundary layer over land Daytime convective mixed layer + clouds (sometimes) Nocturnal stable boundary layer + residual layer (leftover of daytime convective mixed layer)
- Slide 15
- Modeling the ABL Reynolds averaging: Separation of mean and turbulent components u = U + u, = 0 Intensity of turbulence: turbulent kinetic energy (TKE) Eddy fluxes Fx = - /z The turbulent closure problem: Number of unknowns > Number of equations Surface layer: related to gradient Mixed layer: Local theories (K-theory): = - Ka dA/dz always down-gradient Non-local theories: organized eddies filling the entire BL, could be counter-gradient TKE = u 2 + v 2 + w 2 /2
- Slide 16
- Reynolds averaging (1) Separate mean and turbulent components Assume you are given a time series of zonal wind speed u for a period of one hour, the zonal wind speed can be decomposed into two components: u = U + u where U = is the time average ( means time average, over one hour here) and is called the time mean component, while u is the fluctuation around U, i.e. u = u - U and is called the turbulent component. (2) Do time average = U = 0 = A = 0 Only cross terms are left. They are also called non-linear terms.
- Slide 17
- Intensity of turbulence: Turbulent kinetic energy (TKE) Mean kinetic energy MKE = (U 2 + V 2 + W 2 )/2 Turbulent kinetic energy TKE = u 2 + v 2 + w 2 /2 Time evolution (diurnal) Vertical profile represents time average
- Slide 18
- The turbulence closure problem For large-scale atmospheric circulation, we have six fundamental equations (conservation of mass, momentum, heat and water vapor) and six unknowns (p, u, v, w, T, q). So we can solve the equations to get the unknowns. When considering turbulent motions, we have five more unknowns (eddy fluxes of u, v, w, T, q) We have fewer fundamental equations than unknowns when dealing with turbulent motions. The search for additional laws to match the number of equations with the number of unknowns is commonly labeled the turbulence closure problem.
- Slide 19
- Mixed layer theory I: Local theories K-theory: In eddy-diffusivity (often called K-theory) models, the turbulent flux of an adiabatically conserved quantity a (such as in the absence of saturation, but not temperature T, which decreases when an air parcel is adiabatically lifted) is related to its gradient: = - Ka dA/dz The local effect is always down-gradient (i.e. from high value to low value) The key question is how to specify Ka in terms of known quantities. Three commonly used approaches: (1) First-order closure (2) 1.5-order closure or TKE closure (3) K-profile
- Slide 20
- Mixed layer theory II: Non-local theories Any eddy diffusivity approach will not be entirely accurate if most of the turbulent fluxes are carried by organized eddies filling the entire boundary layer. The non-local effect could be counter-gradient. Consequently, a variety of nonlocal schemes which explicitly model the effects of these boundary layer filling eddies in some way have been proposed. A difficulty with this approach is that the structure of the turbulence depends on the BL stability, baroclinicity, history, moist processes, etc., and no nonlocal parameterization proposed to date has comprehensively addressed the effects of all these processes on the large-eddy structure. Nonlocal schemes are most attractive when the vertical structure and turbulent transports in a specific type of boundary layer (i. e. neutral or convective) must be known to high accuracy.
- Slide 21
- Stratocumulus-topped Boundary Layer Definition of stratocumulus clouds Global distribution Importance for global warming Vertical structure and formation mechanism of STBL Modeling of STBL non-local forced by surface heating and cloud-top cooling
- Slide 22
- Global distribution of stratocumulus clouds Distributed over eastern part of subtropical oceans
- Slide 23
- Vertical structure and formation mechanism of stratocumulus-topped boundary layer (STBL) Intense longwave radiative cooling at cloud top drives eddies in BL Eddies pick up moisture and maintain cloud Eddies also entrain warm, dry air from above the inve

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