Interactions of the land-surface with the atmospheric boundary layer: Single column model experiments at Cabauw, Nethe

1. Interactions of the land-surface with the atmospheric boundary layer:Single column model experimentsat Cabauw, Netherlands Michael Ek NCEP/EMC, Camp Springs, Maryland USA (work with Bert Holtslag, Wageningen Univ.) • 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) Joint GABLS-GLASS/LoCo workshop, 19-21 September 2004, De Bilt, Netherlands

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

3. Coupled land-surface PBL model • 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 • surface radiationsimple incoming solar, longwave, albedo

4. • 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 1978fair weather day Cabauw, Netherlands

5. land-surface-only interactions - 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

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

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

8. uniform reference •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 PILPS2a latent heat flux sensible heat flux soil moisture (4 model layers) root density profiles ROOT DENSITY TESTS

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

10. SENSITIVITY TO INITIAL SOIL MOISTURE(LAND-ONLY MODEL RUNS) dry moist •vary initial soil moisture +/- 5% (vol.) at surface, decreasing with depth •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 profiles of potential temperature and specific humidity •profiles of wind speed (and Cabauw tower time series) potential temperature wind speed saturation specific humidity specific humidity INITIAL ABL CONDITIONS •specify winds  focus on ABL thermodynamics

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

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

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

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

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. ABL depth cloud cover SENSITIVITY TO INITIALSOIL MOISTUREIN COUPLED COLUMNMODEL RUNS • initial conditions same as in previous coupled model runs, but now vary initial soil moisture from dry to moist • as initial soil moisture decreased from observed values, ABL cloud cover  0 • soil moisture increased, ABL cloud cover decreases slightly. WHY?…many land-ABL interactions

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

20. …INCREASEDABOVE-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. …DECREASEDABOVE-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. Ek and Holtslag 2004 RH TENDENCY • ABL-top relative humidity (RH) expected to control cloud formation • role of soil moisture involves complex surface-ABL interaction • ABL-top RH tendency: surface evaporative fraction   RH/t =(Rn-G)/(Lvhqs)[ef+ne(1-ef)]  available energy termnon-evaporative term ne = direct effects of non-evaporative processes on RH tendency: ABL growth  ne=Lv/cp(1+C)[q/(h)+RH[(c2/)-c1)]  dry-air entrainment ABL warming

23. Cabauw values/times •ne<1 (surface moistening regime) RH tendency increases as ef increases, increasing probability of clouds with stronger above ABL stability or dry-air entrainment (limited) •ne>1 (ABL-growth regime) RH tendency increases as ef 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) “Normalized” relative humidity tendency, ef+ne(1-ef)

24. •sensitivity tests 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)

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

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

27. • 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) HAPEX-MOBILHY • Findings above qualitatively consistent with Ek and Mahrt (1994) for HAPEX-MOBILHY data (summer 1986, SW France)

28. BOUNDARY-LAYER GROWTHvs. DRY-AIR ENTRAINMENT • 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 decreaseswith decreasing stability • yields opposite results in our decreased stability test, so less clouds with decreasing soil moisture 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/references

31. Boundary-layer clouds