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 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.)
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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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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.
- 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
• 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