# Presentation: 452567

## Land-atmosphere interaction:impact of soil moisture on boundary-layer cloud development

**15-19 July 2002, Wageningen, The Netherlands**

*Land-**atmosphere interaction:**impact of soil moisture on boundary-layer cloud development*

- • evaluation of a land-surface scheme and coupled land-surface/atmospheric boundary-layer column model runs at Cabauw (Netherlands) show reasonable results
**• ****investigate role ****of soil moisture in ABL evolution and ABL cloud development**

**Michael Ek**
- National Centers for Environmental Prediction
- Camp Springs, Maryland USA
**A. A. M. Holtslag**
- Meteorology and Air Quality, Wageningen University, NL

## land-surface/ABL interactions

- solid arrows=positive feedback, dashed arrows=negative feedback; two consecutive negative feedbacks make a positive feedback

##  Coupled Atmosphericboundary-layer –Plant - Soil model

** ****Coupled Atmospheric****boundary-layer –****Plant - Soil model**

**• ****land-surface**
- multi-soil layers, simple canopy,
- Jarvis-Stewart conductance
- MRF model LS scheme, pre-NOAH
**• ****ABL boundary-layer**
- K-theory + nonlocal ABL mixing
- MRF model ABL scheme
**• ****surface layer**
- M-O theory functions
**• ****ABL cloud cover**
- turbulent + mesoscale RH dist’n
**• ****surface radiation**
- simple incoming solar, longwave, albedo

## Cabauw, Netherlands

**• ****central NL**
**45km east of N.Sea**
**• ****short grass site, clay soils**
**• ****213m tower**
**multiple levels of wind, temperature, moisture**
**• ****micromet site**
**surface fluxes, soil moisture & temp, radiation**
**• ****radiosondes**
** ****Cabauw & DeBilt**
**• ****31 May 1978 ****synoptically ‘quiet’**

## land-surface-only interactions

-       using land-surface-only model
-  drive with observed atmospheric forcing
-  using existing formulations *without tuning* model parameters

## ATMOSPHERIC FORCING &INITIAL SOIL CONDITIONS

**temperature**

**specific humidity**

**wind speed**

**incoming**
**solar**

**downward**
**longwave**

**reflected solar**

**initial soil**
**temperature**

**initial soil**
**moisture**

**sensitivity tests**

**dry**

**moist**

**ATMOSPHERIC FORCING &****INITIAL SOIL CONDITIONS**

## CANOPY CONDUCTANCE TESTS

**latent**
**heat**
**flux**

**sensible**
**heat flux**

**canopy**
**conductance**

***constant***

***reference***

***NP89***

***NP89 &***
***PILPS2a***
***roots***

***inferred obs***

**•**** **infer ‘observed’ canopy conductance from observations
**•**** **Beljaars and Bosveld (1997) derived for Cabauw (reference)

**CANOPY CONDUCTANCE TESTS**

## ROOT DENSITY TESTS

**•**** **PILPS2a root distribution yields latent (sensible) heat fluxes under- (over-) predicted due to soil moisture in upper soil  layer (with higher root density) being more quickly depleted, compared to reference (quarter-rule) or uniform root density.

***reference***

***PILPS2a***

***uniform***

**root density**
**profiles**

**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 heat
- flux through
- vegetation

- bare soil formulation:
- excessive soil heat flux
- through vegetation

**•**** **due to excess soil heat flux (bare soil case) model surface skin and soil temperatures lower compared to observed  reference case

***reference***

***bare soil***

**latent heat flux**

**sensible heat flux**

**soil heat flux**

**surface skin**
**temperature**

**upper soil**
**layer 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)

## land-surface/ABL interactionswithout radiation feedbacks

-  coupled LS-ABL column model runs, with prescribed (observed) radiation, so ‘proper’ transmission/emission & clouds only diagnosed.

## land-surface/ABL interactionswith radiation feedbacks

-  finally coupled LS-ABL column model runs, with modelled radiation (more fully interactive), so clouds interactive.

## INITIAL ABL CONDITIONS

**•**** **initial profiles of potential temperature and specific humidity

**•**** **profiles of wind speed
- (and Cabauw tower time series)
**•**** **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 (+0.5 cm/s at 2 km, decreasing linearly to zero at the surface) yields ABL cloud fractions consistent with observations (31 May 1978)

- Cloud cover and maximum afternoon ABL depth as a function of prescribed vertical motion** **

## ABL DEPTH & CLOUDS

**•**** **ABL growth slightly too vigorous in morning hours; better predicted in afternoon, and during late-afternoon transition to shallow stable boundary layer.

**•**** **afternoon ABL cloud fractions qualitatively consistent with observed conditions in central Netherlands.

**•**** **results similar for ABL-only, coupled land-surface-ABL model runs (using prescribed=observed or modelled radiation)

**ABL depth**

**afternoon ABL**
**cloud cover**

## 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 (cooler) in morning (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 (using either observed, or modelled radiation and cloud interaction) compare well with offline land-only model runs, and observations.

**•**** **radiation terms well-represented using our simple surface radiation formulation.

**latent heat flux**

**sensible heat flux**

**soil heat flux**

**net radiation**

**incoming**
**solar**

**downward**
**longwave**

**reflected**
**solar**

## 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
- LS-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 LS-ABL model in
- this coupled land-atmosphere system.

## SENSITIVITY TO INITIALSOIL MOISTURE

- • vary initial soil moisture:
- dry  moist in coupled land-surface/ABL model runs
- (REFERENCE CASE)
- • soil moisture decreased (from observed)**, **ABL depth increased, cloud cover  zero
- • soil moisture increased
- (from observed), ABL depth decreased, cloud cover decreased slightly.
**WHY?**** ...land-atmosphere interactions!**

***cloud cover***

***ABL depth***

***Cabauw obs***

## ...AND INCREASEDABOVE-ABL STABILITY

- • vary initial soil moisture:
- dry  moist ...AND *increased* above-ABL stability
- • surface fluxes similar to reference case
- • ABL depth decreased
- • as initial soil moisture increased from dry to moist
-   ABL cloud cover increases

## ...AND DECREASEDABOVE-ABL STABILITY

- • vary initial soil moisture:
- dry  moist ...AND *decreased* above-ABL stability
- • surface fluxes similar to reference case
- • ABL depth increased
- • as initial soil moisture decreased from moist to dry
-   ABL cloud cover increases

## RELATIVE HUMIDITY TENDENCY AT ABL TOP

**• **ABL-top relative humidity (RH) expected to control cloud formation
**• **role of soil moisture involves a complex interaction of surface and atmospheric processes:
-   Surface evaporation
-   Surface heating
-   ABL-top dry & warm air entrainment
-   ABL growth

**extends work by Ek & Mahrt (1994)**
**and Chang & Ek (1996)**

## ABL-TOP RH TENDENCY

**• ****nep>1 (ABL-growth regime)**
**RH tendency increases as *****ef***** decreases**
**• ****nep<1 (surface moistening regime) RH tendency increases as *****ef***** increases**

**Relative humidity tendency, *****e******f ******+ ******nep******(1-******e******f******)***
**(normalized by available energy term)**

-                      surface evaporative fraction
-                                          
***RH/****t  = **(Rn-G)/(****L**v**hq**s**) **[****e******f****** ****+ ****nep****(1-****ef****)]*
*                     *                    
- available energy term     non-evap. term
*nep *= effects of non-evaporative processes on RH tendency:
-                                               ABL growth
-                                      
*nep**=L**v**/c**p**(1+C******)[********q/****h************)**+RH[**(c2/************)**-**c1**)]*
*                          *                      
-          dry-air entrainment       ABL warming

## REFERENCE CASE

**• **nep<1
- surface
- moistening
- regime

**REFERENCE CASE, DRY SOIL**
**nep <******** 1, no clouds**

**REFERENCE CASE, MOIST SOIL**
**nep <******** 1, some clouds**

**REFERENCE CASE**

## STRONG STABILITY CASE

**STRONG STABILITY, DRY SOIL**
**nep < 1, no clouds**

**STRONG STABILITY, MOIST SOIL**
**nep < 1, some clouds**

**• **nep < 1
- surface
- moistening
- regime

## WEAK STABILITY CASE

- ABL-growth
- regime

**WEAK STABILITY CASE**

**WEAK STABILITY, DRY SOIL**
**nep >> 1, more clouds**

**WEAK STABILITY, MOIST SOIL**
**nep >> 1, less clouds**

## CABAUW DATA ANALYSIS

**•** soil moisture role  increase ABL-top RH (nep<1)
- ...except during the mid-day rapid ABL growth  soil moisture modestly increases ABL-top relative humidity (nep<1)

**•** sensitivity
- tests

## HAPEX-MOBILHY data analysis

- • 13 June 1986, with strong atmospheric stability above the ABL and a large observed evaporative fraction (via higher soil moisture) gave a similar mid-day ABL-top relative humidity as 22 June 1986, with weaker atmospheric stability and soil moisture that had decreased by 20%.

- 22 June 1986 – ABL growth regime (nep>1)
- 13 June 1986 – surface moistening regime (nep<1)

**HAPEX-MOBILHY data analysis**

## ABOVE-ABL STABILITY VS. DRY-AIR:EFFECT ON NON-EVAP RH TENDENCY TERMS

**•**** **drier above-ABL air, *nep* decreases (more negative)
**•**** **change in above-ABL stability affects both dry-air entrainment and ABL growth (opposing processes)
**•**** **if ***q > -RH h c**2** *(less negative)  *nep* increases with decreasing stability
**•**** **this threshold q value decreases (more negative) for increasing RH, h, and *c**2* (decreasing T)
-  this is the case at Cabauw from morning to mid-day

**ABOVE-ABL STABILITY VS. DRY-AIR:****EFFECT ON NON-EVAP RH TENDENCY TERMS**

## CONCLUSIONS

- - characterize the role of soil moisture and atmospheric processes in land-surface/ABL interaction
- - coupled land-surface/ABL column model runs and data analysis for Cabauw consistent with analytical development
- • Consistent with Ek and Mahrt (1994), HAPEX-MOBILHY (summer 1986, southwest France), similar ABL-top RH & cloud cover for two fair-weather days with contrasting regimes
**•** Future:
- - examine other data sets with different soil moisture and atmospheric regimes.

## LS-ABL interactions/references

- Ek, M. and A. A. M. Holtslag, 2002:  Interactions of the land-surface with the atmospheric boundary layer.
-   Part 1:  Evaluation of a land-surface scheme at Cabauw.
- Ek, M. and A. A. M. Holtslag, 2002: Interactions of the land-surface with the atmospheric boundary layer.
-   Part 2:  Coupled model evaluation and impact of soil moisture on cumulus initiation.
*J. Hydrometeorol.* (under revision).