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Joint GABLS-GLASS/LoCo workshop, 19-21 September 2004, De Bilt, Netherlands Interactions of the land-surface with the atmospheric boundary layer: Single.

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Presentation on theme: "Joint GABLS-GLASS/LoCo workshop, 19-21 September 2004, De Bilt, Netherlands Interactions of the land-surface with the atmospheric boundary layer: Single."— Presentation transcript:

1 Joint GABLS-GLASS/LoCo workshop, September 2004, De Bilt, Netherlands Interactions of the land-surface with the atmospheric boundary layer: Single column model experiments at Cabauw, Netherlands evaluation of land-surface and ABL schemes at Cabauw, in offline and single-column (coupled) modes examine the role of soil moisture in boundary-layer evolution and cloud development (shallow cumulus) Michael Ek NCEP/EMC, Camp Springs, Maryland USA (work with Bert Holtslag, Wageningen Univ.)

2 The interaction of the land-surface with the atmospheric boundary layer includes many processes and important feedback mechanisms. land-surface/ABL interactions

3 Coupled land- surface PBL model surface radiation simple incoming solar, longwave, albedo OSU land-surface multi- soil layers, simple canopy, Jarvis-Stewart conductance (Mahrt and Pan, 1984) ABL boundary-layer K-theory + nonlocal ABL mixing (Troen and Mahrt, 1986) surface layer M-O theory functions ABL cloud cover turbulent + mesoscale RH dist’n

4 Cabauw, Netherlands central NL 45km east of N.Sea short grass, clay soils 213m tower obs micromet site surface fluxes, soil moisture & temp, radiation radiosondes: Cabauw & DeBilt 31 May 1978 fair weather day

5 - first represent soil-vegetation system in offline model runs using land-surface-only model - drive with observed atmospheric forcing - using existing formulations without tuning model parameters land-surface-only interactions

6 temperature specific humidity wind speed incoming solar downward longwave reflected solar initial soil temperature initial soil moisture sensitivity tests drymoist ATMOSPHERIC FORCING & INITIAL SOIL CONDITIONS

7 latent heat flux sensible heat flux canopy conductance constant reference NP89 NP89 & PILPS2a roots inferred obs Beljaars and Bosveld (1997) derived for Cabauw (reference) CANOPY CONDUCTANCE TESTS infer ‘observed’ canopy conductance from observations

8 root density profiles PILPS2a root distribution yields underpredicted latent and overpredicted sensible heat fluxes due to soil moisture in upper soil layer depletion (higher root density) compared to reference case with a more uniform root density reference PILPS2a uniform soil moisture (4 model layers) latent heat flux sensible heat flux ROOT DENSITY TESTS

9 SOIL HEAT FLUX FORMULATION vegetation effect: account for vegetation cover with less soil heat flux through vegetation bare soil formulation: excessive soil heat flux through vegetation soil vegetation reference bare soil latent heat flux sensible heat flux soil heat flux due to excess soil heat flux (bare soil case) model skin and soil temps lower compared to obs  reference case surface skin temperature upper soil layer temperature

10 SENSITIVITY TO INITIAL SOIL MOISTURE (LAND-ONLY MODEL RUNS) vary initial soil moisture +/- 5% (vol.) at surface, decreasing with depth drymoist latent (sensible) heat flux increases (decreases) by about 28% (32%) surface temperature decreases  net radiation increases by <5% reduced near-soil-surface temperature gradient  soil heat flux decreases by 28%

11 ABL-only interactions - follow with ABL-only model runs (driven by observed surface fluxes) - then coupled column model runs, with prescribed (observed radiation) and modelled radiation (more fully interactive)

12 INITIAL ABL CONDITIONS specify winds  focus on ABL thermodynamics initial profiles of potential temperature and specific humidity potential temperature specific humidity saturation specific humidity profiles of wind speed (and Cabauw tower time series) wind speed

13 SENSITIVITY TO PRESCRIBED VERTICAL MOTION a nominally small vertical motion value yields ABL cloud fractions consistent with 31 May 1978 obs Cloud cover and maximum afternoon ABL depth as a function of prescribed vertical motion Cloud cover increases with increasing prescribed large-scale vertical motion (ABL-only model runs)

14 ABL DEPTH & CLOUDS ABL growth slightly too vigorous in morning, better predicted in afternoon, transition to shallow SBL afternoon cloud fractions qualitatively consistent with obs in central NL results similar for ABL- only, and coupled land-ABL model runs ABL depth afternoon ABL cloud cover

15 POTENTIAL TEMP & SPECIFIC HUMIDITY: TIME SERIES AND 12 UT PROFILIES 20-m potential temperature 20-m specific humidity 12UT potential temperature proflie 12UT specific humiidty proflie results similar for ABL-only, and coupled land- surface-ABL model runs. potential temp: slightly warmer in morning, cooler in afternoon specific humidity: less mid-morning ‘peak’ prior to late- morning rapid ABL growth, and more well-mixed.

16 SURFACE FLUXES & RADIATION surface fluxes in coupled model runs compare well with offline land-only model runs, and observations. latent heat flux sensible heat flux soil heat flux net radiation radiation terms well- represented using our simple surface radiation formulation. incoming solar downward longwave reflected solar

17 SUMMARY: LAND-SFC/ABL MODEL RUNS Model parameterization updates include modifications to land-surface formulations… …canopy conductance at Cabauw (Beljaars and Bosveld 1997) …soil heat flux formulation (account for vegetation cover) …plant root density (nearly uniform) …and a change to the boundary-layer depth formulation. For land-surface-only, ABL-only, and when coupled in land-surface-ABL column model runs… …realistic daytime surface fluxes and atmospheric profiles and ABL clouds are produced. …results compare well with observations using un-tuned parameterizations. Processes are well-represented by our column model in this coupled land-atmosphere system.

18 SENSITIVITY TO INITIAL SOIL MOISTURE IN COUPLED COLUMN MODEL RUNS as initial soil moisture decreased from observed values, ABL cloud cover  0 cloud cover ABL depth initial conditions same as in previous coupled model runs, but now vary initial soil moisture from dry to moist soil moisture increased, ABL cloud cover decreases slightly. WHY? …many land-ABL interactions

19 land-surface/ABL interactions: effect of soil moisture DRY SOIL no clouds MOIST SOIL some clouds

20 …INCREASED ABOVE-ABL STABILITY vary initial soil moisture: dry to moist, and INCREASE above-ABL stability…  surface fluxes similar to reference case  ABL depth decreased  ABL cloud cover increases with increasing soil moisture

21 …DECREASED ABOVE-ABL STABILITY vary initial soil moisture: dry to moist, and DECREASE above-ABL stability…  surface fluxes similar to reference case  ABL depth increased  ABL cloud cover decreases with increasing soil moisture

22 RH TENDENCY surface evaporative fraction   RH/  t =(Rn-G)/(  L v hq s )[e f +ne(1-e f )]  available energy term non-evaporative term ne = direct effects of non-evaporative processes on RH tendency: ABL growth  ne= Lv/cp (1+C  )[  q/  h   )+RH[(c 2 /   )-c 1 )]  dry-air entrainment ABL warming Ek and Holtslag 2004 ABL-top relative humidity (RH) expected to control cloud formation role of soil moisture involves complex surface-ABL interaction ABL-top RH tendency:

23 “Normalized” relative humidity tendency, e f +ne(1-e f ) ne<1 (surface moistening regime) RH tendency increases as e f increases, increasing probability of clouds with stronger above ABL stability or dry-air entrainment (limited) ne>1 (ABL-growth regime) RH tendency increases as e f decreases, high surface evap limits ABL growth and RH increase, so increasing probability of ABL clouds with low surface evap and weaker above-ABL stability  greatest RH tendency & ABL cloud potential: low surface evap & weak atmos stability (ne>>1) Cabauw values/times

24 CABAUW DATA ANALYSIS role of soil moisture  increase ABL-top RH (ne<1) …except during mid-day rapid ABL growth when soil moisture modestly increases ABL-top relative humidity (ne  <1) sensitivity tests

25 STRONG STABILITY CASE STRONG STABILITY, DRY SOIL, NO CLOUDS ne<1 (surface moistening regime) STRONG STABILITY, MOIST SOIL, SOME CLOUDS

26 WEAK STABILITY CASE WEAK STABILITY, DRY SOIL MORE CLOUDS WEAK STABILITY, MOIST SOIL LESS CLOUDS ne>>1 ABL-growth regime

27 Findings above qualitatively consistent with Ek and Mahrt (1994) for HAPEX-MOBILHY data (summer 1986, SW France) HAPEX-MOBILHY 13 June 1986, with strong atmospheric stability above the ABL and a larger observed evaporative fraction (ne<1) …gave a similar mid-day ABL- top relative humidity as 22 June 1986, with weaker atmospheric stability and decreased soil moisture (ne>1)

28 change in above-ABL stability affects both dry-air entrainment and ABL growth (opposing processes in RH tendency) with drier above-ABL air, ne decreases if  q > critical value (more dry, negative) …ne decreases with decreasing stability yields opposite results in our decreased stability test, so less clouds with decreasing soil moisture BOUNDARY-LAYER GROWTH vs. DRY-AIR ENTRAINMENT dry-air entrainment “wins” over boundary-layer growth

29 FUTURE examine data from other field programs, e.g. additional Cabauw, HAPEX-MOBILHY, CASES, SGP, BOREAS, etc. further land-ABL column tests to explore land-atmos interaction, RH tendency and clouds; large-scale model output near-surface RH tendency could be used to infer soil moisture given other terms in the RH tendency equation

30 LS-ABL interactions/reference s

31 Boundary-layer clouds


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