Interactions of the land-surface with
the atmospheric boundary layer:
Single column model experiments
at 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
land-surface/ABL interactions
The interaction of the land-surface with the atmospheric boundary
layer includes many processes and important feedback mechanisms.
Coupled landsurface PBL model
• OSU land-surface multisoil 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 radiation simple
incoming solar, longwave, albedo
• 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
Cabauw, Netherlands
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
ATMOSPHERIC FORCING &
INITIAL SOIL CONDITIONS
sensitivity tests
dry
moist
temperature
specific humidity
wind speed
incoming
solar
initial soil
moisture
initial soil
temperature
downward
longwave
reflected
solar
CANOPY CONDUCTANCE TESTS
• infer ‘observed’ canopy conductance from observations
• Beljaars and Bosveld (1997) derived for Cabauw (reference)
inferred obs
NP89 &
PILPS2a
reference roots
NP89
constant
latent
heat
flux
sensible
heat flux
canopy
conductance
ROOT DENSITY TESTS
• 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
uniform
reference
PILPS2a
latent heat flux
sensible heat flux
soil moisture
(4 model layers)
root density
profiles
SOIL HEAT FLUX FORMULATION
bare soil formulation:
vegetation effect:
excessive soil heat flux account for vegetation
through vegetation cover with less soil heat
flux through
vegetation
soil heat flux
bare soil
reference
vegetation
soil
• due to excess soil
heat flux (bare soil
case) model skin
and soil temps lower
compared to obs 
reference case
latent heat flux
surface skin
temperature
upper soil
layer temperature
sensible heat flux
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
• profiles of wind speed
(and Cabauw tower time series)
• initial profiles of potential
temperature and specific
humidity
• specify winds  focus on
ABL thermodynamics
potential
temperature
wind
speed
specific
humidity
saturation
specific
humidity
SENSITIVITY TO PRESCRIBED VERTICAL MOTION
• Cloud cover increases
with increasing prescribed
large-scale vertical motion
(ABL-only model runs)
• 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
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
• results similar for ABLonly, and coupled land-ABL
model runs
afternoon ABL
cloud cover
POTENTIAL TEMP & SPECIFIC HUMIDITY:
TIME SERIES AND 12 UT PROFILIES
• potential temp:
slightly warmer in
morning, cooler in
afternoon
• specific humidity:
less mid-morning
‘peak’ prior to latemorning rapid ABL
growth, and more
well-mixed.
• results similar
for ABL-only, and
coupled landsurface-ABL
model runs.
20-m potential
temperature
12UT potential
temperature
proflie
12UT specific
humiidty
proflie
20-m specific
humidity
SURFACE FLUXES &
RADIATION
• surface fluxes in coupled model runs
compare well with offline land-only
model runs, and observations.
• radiation
terms wellrepresented
using our
simple
surface
radiation
formulation.
latent heat flux
sensible heat flux
incoming
solar
net radiation
downward
longwave
reflected
solar
soil heat flux
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 INITIAL
SOIL MOISTURE
IN COUPLED COLUMN
MODEL 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
ABL depth
cloud cover
land-surface/ABL interactions:
effect of soil moisture
DRY SOIL
no clouds
MOIST SOIL
some clouds
…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
…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
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 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
• 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)
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
• ne<1
(surface
moistening
regime)
STRONG STABILITY,
DRY STRONG STABILITY, MOIST
SOIL, NO CLOUDS
SOIL, SOME CLOUDS
WEAK STABILITY CASE
• ne>>1
ABL-growth
regime
WEAK STABILITY, DRY SOIL WEAK STABILITY, MOIST SOIL
MORE CLOUDS
LESS CLOUDS
HAPEX-MOBILHY
• Findings above qualitatively
consistent with Ek and Mahrt
(1994) for HAPEX-MOBILHY
data (summer 1986, SW France)
• 13 June 1986, with strong
atmospheric stability above the
ABL and a larger observed
evaporative fraction (ne<1)
…gave a similar mid-day ABLtop relative humidity as 22 June
1986, with weaker atmospheric
stability and decreased soil
moisture (ne>1)
BOUNDARY-LAYER GROWTH
vs. 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 decreases with
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
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/reference
Boundary-layer clouds