Evaluating modifications of the soil module terra
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Evaluating modifications of the soil module TERRA. Felix Ament, MeteoSwiss. COSMO General Meeting, September 2007. Dry soil moisture bias. OPRerational COSMO, two-layer version. Testsuite, multi-layer version. Soil moisture. T2m. Strong dry out bias!. Negative effect on T2m forecast.

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Evaluating modifications of the soil module TERRA

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Evaluating modifications of the soil module TERRA

Felix Ament,

MeteoSwiss

COSMO General Meeting, September 2007


Dry soil moisture bias

OPRerational COSMO, two-layer version

Testsuite,multi-layer version

Soil moisture

T2m

  • Strong dry out bias!

  • Negative effect on T2m forecast.


Handling the dry out problem

ECMWF.


Design of TERRA standalone experiments

  • Atmospheric Forcing: COSMO analysis data

  • Domain: see left; 64x61 gridpoints at 7km resolution

  • Period: year 2006 plus December 2005 for spin up

  • Initialization: Operational COSMO analysis

Meteorological Forcing: T, p, u, q, Qdown

COSMO analysis

Precipitation RR

SVAT „TERRA“

  • Simulation of

  • Energy balance

  • Soil processes

  • Annual cycle of vegetation

Working in the dark – nearly no or insufficient observations!

time


Rain

Evaporation

Surface Runfoff

Snow

Intermediate Runfoff

SM

Ground Runfoff

Nudged mulitlayer versus two layerAnalysis of the water budget

Features of “Nudged Multilayer”:

  • Despite Nudging, LE is reduced in July/August and Tmax is higher.

  • Most of the nudged water (=residuum) is converted into runoff.

  • Remarkable: Less precipitation.

Nudged multilayerOperational 2-layer


CTL standalone versus OPR 2-layer

Features of “CTL standalone”:

  • Again, reduced LE in July / August (no response in T_2m due to external forcing)

  • Dry out in summer, but recovers until the end of the year.

  • Higher runoff.

  • Do we really have a dry-out problem?

  • Probably, the T_2m diagnosis is misleading?

Doubts


Sensitivity experiments

Lower boundary

Drainage &diffusion

Vegetation

Exchange


Lower Boundary Condition I- concepts

RIGIDGWATER

dry

wet

medium

rigid lid

Free drainage

ground water


Lower Boundary Condition IIGround water condition

GWATER

Problem: Definition of soil moisture gradient at top of water

Solution: Solve Darcy equation with these simplifications:

  • F is constant below centre of lowest layer

  • D is constant there, too

  • K varies only linearly with Q :


Drainage and capillary rise I

BROOKS1BROOKS2

  • CTL: Rijtema (1969), e.g. for drainage K:

  • Brooks and Corey (1964) – much more popular

  • However, Brooks and Corey formulation requires three parameters to derive drainage and capillary rise (depending on soil moisture) – they are not well defined.

    • BROOKS1: 6 type DWD soil classification; lookup table adopted from R. Grasselt (UBonn)

    • BROOKS2: 6 type DWD soil classification; lookup table from J. Helmert (DWD) adopted from Shao and Irannejad (1999)


Drainage and capillary rise II

Ecoclimap

  • PEDO

  • fields of soil pro-perties (e.g. pore volume)

Rawls and Brakensiek, 1989

DWD classification

USDA classification

  • BROOKS3

  • 11 classes

  • Lookup by Shao

  • not fully done!

  • ECOSOIL

  • 6 classes

  • Lookup table by DWD


Runoff_g

Drainage and capillary rise III

MACROPOR

Marcopores

  • help to infiltrate water rapidly during rainfall

  • might avoid runoff generation of saturated top layer

Parameterization (adopted from VEG3d, e.g. Braun 2002)

mit Fmax=10 und Qmin=0.5.


Vegetation I

VEGPARA

  • Minimal / maximal stomatal resistance as well as plant albedo have constant value in TERRA CTL

  • VEGPARA uses spatially varying values depending on land-use

CTL

CTL


Vegetation II

ECOVEG

External vegetation parameters prescribed by ECOCLIMAP dataset (Mason et al., 2002):

  • Exhibits more variabilty

  • Systematic higher root depth

  • More detailed seasonal cycle (not shown)

(all maps are valid for July)


Vegetation III

ROOTDIST

ROOTDIST

  • Linear root depth distribution

CTL

  • Uniform root depth

Recipe

  • Diagnose soil moisture stress function fSM,loc for each layer separately

  • Determine mean SM stress by average weighted by layer thickness Dz and root density rroot

  • Extract transpired water proportional to fSM,loc Dz rroot


Atmospheric exchange I

ZOLOC

Local roughness length z0,local

  • CTL roughness depends not only on local conditions, but also on variance of orography to account for gravity wave drag.

     Very high roughness length over mountainous areas.


Atmospheric exchange II

NP89

Top Layer SM at Lindenberg

Dickinson, 1984: BATS scheme

Designed for a two layer soil module!

Noilhan and Platon, 1989 (NP89): ISBA scheme, Meso-NH


Rain

Evaporation

Surface Runfoff

Snow

Intermediate Runfoff

SM

Ground Runfoff

Result I - bare soil evaporation NP89

  • Significant reduction of Evaporation during spring and fall, …

  • … but no effect during summer!


Rain

Evaporation

Surface Runfoff

Snow

Intermediate Runfoff

SM

Ground Runfoff

Result II – Budget Summary

Deviations in mm


Conclusions

  • COSMO TERRA-ML is very robust; modifications have in general surprisingly small impact

  • TERRA-ML standalone has proven to be useful tool to asses the midterm effect of model modification.

  • However, objective decisions about implementation of modification is difficult, due to lack of observational data.

  • Scientifically the following modification can reasonably be recommended:

    • NP89 (removes high evaporation in spring & fall)

    • VEGPARA (better representation of forest)

    • (GWATER (counteracting dry-out))

    • (BROOKSX (being state-of-the-art))

  • Outlook:

    • Cross studies (e.g. BROOKS and GWATER)

    • Long term integration to reach model balance.

    • Combination with improved T_2m diagnosis.


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