The Atmospheric Boundary Layer. The Surface Energy Balance (9.2)

Transcription

1 The Atmospheric Boundary Layer Turbulence (9.1) The Surface Energy Balance (9.2) Vertical Structure (9.3) Evolution (9.4) Special Effects (9.5) The Boundary Layer in Context (9.6)

2 What processes control the depth of the boundary layer? Entrainment Large-scale vertical velocity Horizontal advection

3 1500 Entrainment a) e) Altitude (m) 1250 Altitude (m) b) c) f) g) Altitude (m) d) h) Altitude (m) Horizontal distance Horizontal distance

4 Entrainment Entrainment occurs whenever a nonturbulent volume is incorporated into a turbulent volume. Entrainment increases the depth of the boundary layer. Detrainment occurs whenever a turbulent volume becomes non-turbulent.

5 Entrainment Free Atmosphere E.Z. Capping Inversion Height, z Mixed Layer Residual Layer Stable BL Day 1 Night 1 Day 2 E.Z. = Entrainment Zone

6 Entrainment In winter, the stable nocturnal BL is much deeper, and the daytime mixed layer is so shallow that the top of the stable BL persists day and night as the capping inversion. FIG. 6. Time height plot of potential temperature during IOP 5. Isentropes are indicated by contours (bold contours every 5 K and light contours every 1 K). The 1-h change in potential temperature is indicated in shading to highlight periods of warming and cooling. The gray line at 2,500 m indicates the approximate elevation of the mountain crests that enclose the SLV. Green (blue) dots indicated the time of ISS (NWS) radiosonde launches.

7 Entrainment mixed layer top

8 Entrainment

9 Entrainment

10 Entrainment

11 Entrainment Types of CAPs daytime stable boundary layers during winter Dry Craig Clements photo Heterogeneous Jim Steenburgh photo

12 Boundary Layer Growth What processes control the depth of the boundary layer? Entrainment Large-scale vertical velocity Horizontal advection dz i dt = w e + w i or z i t = V z i + w e + w i where w e is the entrainment velocity (volume per area and time) and w i is the large-scale vertical velocity at z = z i.

13 Boundary Layer Growth Exercise 9.30 shows that w i = z i { V} where { V} is the mass-weighted horizontal divergence in the boundary layer. tropopause In regions of fair weather, w i is usually negative (downward) and tends to make the z i boundary layer z shallower. x High Low

14 Boundary Layer Growth

15 Boundary Layer Growth Entrainment w θ < 0 w θ > 0

16 Boundary Layer Growth What controls w e? Stronger turbulence (w ) increases w e. Greater stability ( θ/ z) at ABL top decreases w e. The convective velocity scale is w = g θ (w θ ) s z i 1/3

17 Boundary Layer Growth (b) Entrainment w θ < 0 z i w θ > 0 F Hzi 0 F Hs Heat Flux, F H

18 Boundary Layer Growth (a) Dq (b) z i z i (w θ ) zi = w e θ Height, z w = g θ (w θ ) s z i 1/3 0 Potential Temperature, q F Hzi 0 F Hs Heat Flux, F H (w θ ) zi = A(w θ ) s = w e θ w e = A(w θ ) s θ

19 Boundary Layer Growth The average θ in the mixed layer, θ, is affected by the surface and entrainment sensible heat fluxes: dθ dt = F Hs F Hzi z i dz i = (1 + A)F Hs z i dt = w e + w i w e = AF Hs θ

20 Boundary Layer Growth Under conditions of light winds, one can predict zi without knowing we. Use the early morning of FHs(t). θ(z) and a prediction FHs(t) can be estimated from F* (net downward radiation at the surface). The procedure is called the thermodynamic method or the encroachment method.

21 Boundary Layer Growth How much energy (heat) is needed to change the average temperature of the ABL by 1 K? Energy per unit mass is specific heat = c p T (J/kg) Energy per unit volume = ρ c p T (J/m 3 ) Energy in the ABL per unit area = z i ρ c p T = z i ρ c p θ (J/m 2 )

22 Boundary Layer Growth Energy put into the ABL per unit area and time = ρ c p F Hs (W/m 2 ) Energy put into the ABL per unit area during a short time interval t = ρ c p F Hs (t) t (J/m 2 ) = z i ρ c p θ θ(t2 ) t2 θ(t 1 ) z i (θ) dθ = t 1 F Hs (t) dt

23 Boundary Layer Growth (a) (b) F Hs Daytime Nighttime z i at t 2 Heat Flux, F H 0 t 1 t 2 Time, t θ(t2 ) t2 F_{Hs} Height, z z i at t 1 0 Sounding Early Morning z i (θ) dθ = F Hs (t) dt q q θ(t 1 ) t 1 at t 1 at t 2 Potential Temperature, q

24 Boundary Layer Growth How do we predict FHs(t)? F Hs Daytime Nighttime Heat Flux, F H 0 t 1 t 2 Time, t

25 Boundary Layer Growth Exercise 9.6 (a) At sunrise, θ/ z = γ in the stable BL and z i = 0. What is z i (t) after sunrise if F Hs is constant? θ(t) θ(0) z i (θ) dθ = t 0 z i (θ) = θ θ(0) γ F Hs (t) dt θ(t) θ(0) θ θ(0) γ dθ = F Hs t [θ(t) θ(0)] 2 = F Hs t 2γ [γz i (t)] 2 = F Hs t 2γ z i (t) = [(2F Hs /γ) t] 1/2

26 Boundary Layer Growth Exercise 9.6 (b) How does the shape of F Hs (t) relate to the shape of z i (t)? z i t 1/2 does not agree with z i t 2 which typically observed in the early Heat Flux, F H F Hs Daytime Nighttime 0 t 1 t 2 Time, t (b) morning because F Hs (t) is not constant but increases, and the initial θ(z) is not linear but exponential in shape. Height, z z i at t 2 z i at t 1 0 Sounding Early Morning q at t 1 q at t 2 Potential Temperature, q

27 Cloud-topped Boundary Layer over Land Height, z Phase 1 Nocturnal Inversion Burn-off Phase 2 Rapid Rise Phase 3 Quasisteady ~1 km clouds LCL z i sunrise mid-morning mid-afternoon

28 Cloud-topped Boundary Layer over Land Horizontal roll vortices may form under certain conditions. The roll axes are aligned with the mean wind direction. Roll diameter is comparable to the BL depth. Clouds may form in the updrafts and produce cloud streets.

29 Cloud-topped Boundary Layer over Land

30 Cloud-topped Boundary Layer over Land

31 Cloud-topped Boundary Layer over Land Vertical crosssection through laminar horizontal roll vortices Satellite view of cumulus clouds aligned into cloud streets by turbulent horizontal roll vortices

32 The Marine Boundary Layer Differs from the BL over land in several ways: The diurnal cycle is not as important. Relative humidity is greater. Cloud cover is more extensive. Radiative heating is more affected by clouds. Drizzle is significant in some regions.

33 Annual Stratus Cloud Amount Klein and Hartmann (1993), from surface observations

34 Klein

35 The Marine Boundary Layer Closed cell convection is driven by cooling at the cloud top.

36 The Marine Boundary Layer Open cell convection is driven by heating at the surface.

37 The Marine Boundary Layer closed cell convection open cell convection

38 The Marine Boundary Layer Stratocumulus-topped boundary layer

39 The Marine Boundary Layer l q t q l Figure 4: Cartoon of well mixed, non precipitating, stratocumulus layer, overlaid with data from research flight 1 of DYCOMS-II. Plotted are the full range, middle quartile and mean of θ l,q t and q l from all the data over target region binned in 30m intervals. Heights of cloud base and top are indicated as is mixed layer values, and values just above the top of the boundary layer, of various thermodynamic quantities. The adiabatic liquid water content is indicated by the dash-dot line.

40 The Marine Boundary Layer Radiative fluxes in a stratocumulus cloud layer

41 The Marine Boundary Layer Shortwave (solar) net flux and radiative heating

42 The Marine Boundary Layer Longwave net flux and radiative heating

43 The Marine Boundary Layer Figure 1. Cloud layer state as observed during RF01. From lefttoright,totalwaterspecifichumidity,q t, liquidwater static energy temperature, s l /c p, and liquid water specific humidity, q l. Lines are from soundings, darker indicating earlier, filled circles and bars denote level leg means and standard deviations, and dots denote dropsonde data from the above-cloud portion of the descent. Cloud layer maintains the capping inversion. New data, from a set of field observations collectively referred to as the second study of the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II), were

44 The Marine Boundary Layer How does the cloud layer maintain the capping inversion? Radiation cools the BL, while subsidence warms the air above the BL. The strong capping inversion reduces entrainment, which otherwise would tend to evaporate the cloud layer.

45 The Marine Boundary Layer Sc physical processes: Precipitation Drizzle: Drops > 100 µm radius, falling ~ 1 m s -1. Sedimentation (in cloud only): Cloud droplets less than 20 µm radius, falling a few cm s -1. EPIC 8-mm vertically pointing cloud radar observations of drizzling Sc hourly cloud base hourly cloud top z precip flux 1 mm/day Comstock et al Lecture 15, Slide hourly LCL

46 The Marine Boundary Layer SCBL diurnal cycle in SE Pacific sonde time series 3-hourly sondes show: 1. Mixed-layer structure with strong sharp inversion 2. Regular night-time increase in inversion height, cloud thickness. 3. Decoupling measured by cloud base - LCL increases during daytime and during periods of drizzle on 19, 21 Oct. (local noon = 18 UTC) (Bretherton et al. 2004)

47 The Marine Boundary Layer s f (z) q f (z) warm, dry, subsiding free-troposphere radiative driving 2310 q l (averaged over cloud) z i 765 entrainment cumulus mass flux transition layer surface heat and moisture fluxes q l,adiabatic sea surface Structure of the cumulus topped boundary layer as observed during the 10th research flight of the Rain in Cumulus over the Ocean Field Study. The ordinate shows the top of the cloud layer and the LCL of the mean surface layer air. The values on the x-axis give s/cp averaged over the sub-cloud layer, at 2300 m and at 3500 m for the left panel; q at 3500 m, 2300 m and averaged over the sub-cloud layer for the middle panel; and the liquidwater specific humidity over cloud passes only (where cloud coverage is typically 5-10%) in the rightmost panel.

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49 Trade-wind cumulus boundary layer

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51 Trade-wind cumulus boundary layer

52 The Marine Boundary Layer warm SST cold SST

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55 The Marine Boundary Layer As the Sc-topped BL moves over warmer SSTs, it deepens and Cu clouds appear below the Sc layer. The Sc layer becomes increasing decoupled from the surface and gradually thins due to entrainment. The Sc layer completely evaporates leaving a trade-wind Cu BL.

56 The Marine Boundary Layer Cu under Sc

57 The Marine Boundary Layer Cu under Sc

58 The Marine Boundary Layer Cold air outbreak

59 The Marine Boundary Layer

60 The Marine Boundary Layer

61 The Marine Boundary Layer