Joint GABLS-GLASS/LoCo workshop, 19-21 September 2004, De Bilt, Netherlands

Interactions of the land-surface with the atmospheric boundary layer:

Single column model experimentsat 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 EkNCEP/EMC, Camp Springs, Maryland USA(work with Bert Holtslag, Wageningen Univ.)

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

land-surface/ABL interactions

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

Cabauw, Netherlands• central NL 45km east of N.Sea

• short grass, clay soils

• 213m tower obs

• micromet sitesurface fluxes, soil moisture & temp, radiation

• radiosondes: Cabauw & DeBilt

• 31 May 1978fair weather day

- 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

temperature

specific humidity

wind speed

incomingsolar

downwardlongwave

reflectedsolar

initial soiltemperature

initial soilmoisture

sensitivity testsdry moist

ATMOSPHERIC FORCING &INITIAL SOIL CONDITIONS

latentheatflux

sensibleheat flux

canopyconductance

constant

reference

NP89

NP89 &PILPS2aroots

inferred obs

• Beljaars and Bosveld (1997) derived for Cabauw (reference)

CANOPY CONDUCTANCE TESTS• infer ‘observed’ canopy conductance from observations

root densityprofiles

• 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

SOIL HEAT FLUX FORMULATION

vegetation effect:account for vegetation

cover with less soil heatflux throughvegetation

bare soil formulation:excessive soil heat flux

through vegetation

soilvegetation

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 skintemperature

upper soillayer temperature

SENSITIVITY TO INITIAL SOIL MOISTURE(LAND-ONLY MODEL RUNS)

• vary initial soil moisture +/- 5% (vol.) at surface, decreasing with depth

dry moist

• 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%

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)

INITIAL ABL CONDITIONS

• specify winds focus on ABL thermodynamics

• initial profiles of potential temperature and specific humidity

potentialtemperature

specifichumidity

saturationspecific

humidity

• profiles of wind speed(and Cabauw tower time series)

windspeed

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)

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 ABLcloud cover

POTENTIAL TEMP & SPECIFIC HUMIDITY:TIME SERIES AND 12 UT PROFILIES

20-m potentialtemperature

20-m specifichumidity

12UT potentialtemperature

proflie

12UT specifichumiidty

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.

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.

incomingsolar

downwardlongwave

reflectedsolar

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.

SENSITIVITY TO INITIALSOIL MOISTURE

IN COUPLED COLUMNMODEL 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

land-surface/ABL interactions:effect of soil moisture

DRY SOILno clouds

MOIST SOILsome clouds

…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

…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

RH TENDENCY

surface evaporative fraction RH/t =(Rn-G)/(Lvhqs)[ef+ne(1-ef)] 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[(c2/)-c1)] 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:

“Normalized” relative humidity tendency, ef+ne(1-ef)

• 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)

Cabauw values/times

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

STRONG STABILITY CASE

STRONG STABILITY, DRY SOIL, NO CLOUDS

• ne<1(surfacemoisteningregime)

STRONG STABILITY, MOIST SOIL, SOME CLOUDS

WEAK STABILITY CASE

WEAK STABILITY, DRY SOILMORE CLOUDS

WEAK STABILITY, MOIST SOILLESS CLOUDS

• ne>>1ABL-growthregime

• 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)

• 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 GROWTHvs. DRY-AIR ENTRAINMENT

dry-air entrainment “wins” over boundary-layer growth

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

LS-ABL

interactions/references

Boundary-layer clouds