Bruin, H.A.R., de

Dolman, A.J.

Feddes, R.A.

Kabat, P.

Kok, M.J.T.

Ronda, R.

Soet, M.

Verweij, W.

English

Wageningen Yield 

Representation of the seasonal hydrological cycle in climate and weather prediction 
models in west Europe. 
A.J. Dolman', M. Soet', R. Ronda2, B. van de Hurk", P. Kabat1, H.A.R. de Bruin2, A.A.M. 
Holtslag\&R.A. Feddes4. 
1. DLO Winand Staring Centre, PO Box 125, Wageningen Netherlands 
2. Department of Meteorology, Agricultural University, Wageningen, the Netherlands 
3. Royal Dutch Meteorological Institute, de Bilt, the Netherlands 
4. Department of Water resources, Agricultural University, Wageningen, the Netherlands 
Introduction 
The hydrological cycle over land is governed by complex interactions between surface 
evaporation, hydrology, precipitation, cloud formation and large scale circulation. The most 
important of these interactions are shown in Figure 1. For the purpose of this paper it is 
sufficient to identify the two major feedback loops: a diurnal one and a seasonal one. The 
diurnal loop represents feedback processes between the vegetation, planetary boundary layer 
and the atmosphere. The seasonal loop represents the longer term memory of the system 
through the soil water reservoir. Since many of these interactions are non-linear, accurate 
Solar Input Large scale dynamics 
Figure I. Schematic of the feedbacks and relations in the land surface atmosphere system 
I. 
predictions of present and future weather and climate depend critically on the 
parameterizations of the hydrological cycle which affect both the diurnal and seasonal 
interactive feedback loops. (Milly and Dunne, 1994; Garratt, 1993). From this figure it is also 
clear that ultimately the effect of a possible change in a parameterization can only be assessed 
in a full 3-D climate of Numerical Weather Prediction models (NWPM's), as only here all 
feedback loops are present, from and surface to clouds, precipitation and large scale forcing. 
Several studies with numerical weather prediction models (Betts et al., 1996) and climate 
models (Milly and Dunne, 1994) have shown that the initialization of soil moisture fields in 
NWPMs and General Circulation Models (GCM's) can have a large impact on the land 
surface hydrology and climate of the model. Recently, the need to realistically simulate 
anomalously dry or wet seasons has led to the replacement of climatological soil moisture 
fields by more sophisticated physical and prognostic treatments of soil hydrological 
parameters A new land surface parameterization scheme (Viterbo and Beljaars, 1995) was 
recently introduced in the global NWPM of the European Centre for Medium-range Weather 
Forecasts (ECMWF). The replacement of old soil moisture schemes takes place in parallel to 
the replacement of "old" vegetation models with increasingly complex descriptions of 
vegetation atmosphere interactions. However, the incorporation of more physically based soil 
parameterizations had also led models to drift towards very dry climates (Viterbo and 
Courtier, 1995). 
Furthermore, studies in the context of the Project for Intercomparison of Land Surface 
Parameterization Schemes (PILPS), (Henderson-Sellers, 1996, Shao et al., 1996) have shown 
that a great variability exist in the performance of land surface models with respect to the 
parameterization of the soil hydrological processes. It thus would appear that we are faced 
with the situation where on the one hand, a great sensitivity of our current climate and 
weather forecasts on soil moisture is known to exist, and on the other hand, we know that the 
parameterizations of soil moisture and soil hydrology are probably the weaker part of the 
parameterizations of land surface atmosphere interactions in current weather forecast and 
climate models. 
It is therefore timely to address the problems of soil moisture parameterization and excessive 
drying in models in a project combining the expertise from the areas of soil hydrology and 
numerical weather prediction. Thus the project's main aim is to elucidate the coupling 
mechanisms between soil moisture, evaporation and precipitation over the European 
continental area by improving the representation of hydrological and land surface processes in 
a climate model at time scales ranging from one hour to a full season. 
Methodology 
The research strategy consists of off-line evaluation of land surface parameterization schemes 
(0-D simulations), 1-D simulations using a complete physical package of a state-of-the-art 
climate model in a single column, and 3-D simulations for an entire growing season using a 
regional climate model. This strategy allows the study of the effects of the different time 
scales on the soil moisture/evaporation-precipitation feedback. It also allows the identification 
of the relative importance of land surface processes versus other physical parameterizations in 
1-D single atmospheric column tests and against larger scale synoptic (advective) processes in 
the 3-D simulations. In order to evaluate the skill of current land surface parameterization 
schemes, use will be made of existing data sets, notably the Hydrologie Atmospheric Pilot 
Experiment (HAPEX-MOBILHY) (Andre et al, 1988) and HAPEX-Sahel (Goutorbe et al., 
1997). 
M. 
The approach is aimed at upgrading the soil moisture parameterization in a state of the art 
regional climate model, (Regional Atmospheric Climate Model, RACMO) and testing the 
effect of new implementations on the continental West-European Climate. RACMO is a 
regional weather prediction model with a physical package from the ECHAM4 climate model 
from the Max Planck Institute in Hamburg (Christensen and van Meijgaard,1992). We use the 
routinely used surface parameterization of the ECMWF model as a starting point for analysis 
and improvement. The research approach follows the complete trajectory from 0-D to full 
3-D simulations; this allows assessment of various aspects of soil moisture parameterizations 
in NWPMs and GCMs. 
1-Dimensional studies 
The current land surface parameterization scheme of the ECMWF/RACMO models (Viterbo 
and Beljaars, 1995) contains a layered soil transport scheme with respect to both water and 
heat. The representation of the hydrological processes in the scheme is based on physical 
hydrological transport laws. It is designed to represent accurately the time scales involved 
with soil hydrological transport, varying from the model time step (typically 5 minutes) to the 
seasonal time scale. It treats soil moisture in a prognostic mode. As such, it is capable of 
adding predictability to the weather forecast owing to the intrinsic memory captured in the 
simulated development of the soil moisture reservoir. The scheme may be regarded as a 
state-of-the-art implementation of soil hydrology in weather prediction and climate models. 
However, the scheme is necessarily confined to a relatively coarse vertical resolution (4 
layers), and does not include any horizontal differentiation of soil types or layer depths. Also, 
the minimum time step is determined by considerations of the numerical scheme for the 
atmospheric model, and is thus not necessary adequate to simulate rapid soil hydrological 
processes. Vegetation characteristics are represented in the land surface parameterization 
scheme by means of a simple canopy resistance formulation, which is assumed to be uniform 
for all possible vegetation types and conditions. 
Detailed knowledge of the dependence of soil moisture transport on soil type, stratification, 
moisture content etc. is available and parameterized in a multi-layer soil model SWAPS (Soil 
Water Atmosphere Plant Svat-model), (Ashby et al., 1996). SWAPS is a land surface model 
developed from the SWAP model ( describing the full seasonal dynamics of transient soil 
water and heat flow. It also incorporates a state of the art vegetation model (Dolman, 1993). 
Evaluation of the skill of both the climate and field based land surface schemes is carried out 
by comparing model results to observations using forced atmospheric boundary conditions. 
For this, field data from HAPEX-Sahel and HAPEX-MOBILHY are used. 
HAPEX-MOBILHY represents an agricultural crops (irrigated soy bean) and HAPEX-Sahel a 
fallow savannah. 
Figure 2 shows a comparison of simulated and observed evaporation for the two models used 
in this study for fallow savannah. The data is split into two periods: a period when the soil 
was well watered and a period of dry down, at the end of the 1992 rainy season. It is clear 
from this picture that both models represent the well watered situation reasonably well, with 
the full physical model SWAPS performing somewhat better. However, in the dry down the 
current land surface scheme of RACMO underestimates the evaporation dramatically 
compared to both the measurements and the simulations of SWAPS. The main cause for this 
is the parameterization of soil moisture stress in the RACMO land surface scheme. In a 
comparison with HAPEX-Mobilhy data similar results were obtained: in cases of a fully 
developed crop with no water limitation present, the performance of the two models is good 
and very similar. When bare soil evaporation plays an important role, the RACMO model 
performs not so well and also underestimates percolation or drainage to the lower soil layers. 
On the basis of these results the description of the soil hydraulic parameters was changed 
5. 
from the original Clapp and Hornberger to the more physically realistic Mualem-van 
Genuchten description (van Genuchten, 1980). 
2 3 4 
measured 
2 3 4 
measured 
Figure 2. Comparison of estimated evaporation ofRACMO and SWAPS with measured evaporation for 
a fallow savannah for well watered (closed circles) and dry conditions (asterisks). 
3-Dimensional studies 
A series of 48-hour runs has been executed with RACMO for the period of 1 March till 15 
November 1995 for Europe. Figure 3 shows the temperature bias (modelled-observed) of 
three runs with differing boundary conditions. The daily restart run is a simulation where the 
boundary conditions of the model are updated daily from the large scale weather analysis of 
ECMWF, sea surface temperatures are also updated daily in this run. The climate run is a run 
where the model is let free with boundary conditions updated only at the ten most exterior 
grid points of the domain and the sea surface temperatures follow a climatological field. The 
run called analysed sst is similar to the climate run, except that the seas surface temperatures 
are now also updated daily. 
The figure shows that in the climate run the model predicts too low temperatures, by almost 
3.5 in October. The two other cases show a positive bias, indicating that the model is too 
warm. In these two cases positive biases start around spring and point to the drying 
phenomena as observed by Viterbo and Coutier (1995). Soil moisture dries out too quickly in 
spring and leads to a change in energy partitioning and increases in surface air temperatures. 
There are similar changes in specific humidity with the model underestimating the surface 
humidity. 
In Europe the weather is dominated strongly by advection from the Ocean and the results 
obtained by changing the forcing of the sea surface temperature underline this point 
dramatically. However, the sign and strength of the bias in the climate run with climatological 
sst's suggests that a feedback may be occurring in the model which tends to enhance the 
cooling trend initiated in the spring of 1995 in this simulation. The physical processes 
responsible for this need to be investigated more closely. 
b 
Average temperature bias at synops stations 
daily restart 
analysed sst 
climate 
95$30112 95043012 95062912 
Date 
95082812 95102712 
Figure 3. Temperature bias (simulated-observed) for all land point in RACMO simulation of Europe in 
1995. Detail of the runs are explained in the text 
Discussion and conclusions 
The single soil and surface type currently used in the ECMWF model probably does not 
adequately represent the spatial variability in both soil and land use parameters in Europe. 
There is thus a need to introduce horizontal variability in soil and vegetation properties in the 
scheme. Information on the horizontal variability of soil types on the resolution of the 
RACMO model (50 km) is available for the European area (van Dam et al, 1995). This 
information will be used to represent horizontal variability of land cover in Europe, by 
extracting relevant parameters for the land surface scheme for appropriate land use and soil 
types. This will be done for the Mualem-van Genuchten parameter set and will involve 
scaling high resolution soil type information to the size of the RACMO gridboxes with 
existing or newly developed aggregation rules (Kabat et al., 1997). 
Similar studies to investigate the effect of two contrasting land surface types in a single grid 
box are underway and use a slab model for the atmospheric boundary layer. Such situations 
where existing aggregation rules (Dolman and Blyth, 1997) fail, are lakes in a vegetated 
environment or melting snow in patches of rock and vegetation. In these cases effective 
parameters cannot be defined because they would yield negative diffusion coefficients as a 
result of the apparent counter gradient fluxes occurring at the scale of the boundary layer. The 
use of a tile or mosaic approach is currently investigated to solve this problem. 
Furthermore, the impact of these modifications on the simulated interaction between the land 
surface and the overlying atmosphere is evaluated by incorporating and testing the newly 
derived parameterizations in a 1-D model atmospheric model. The emphasis in this study is 
on the diurnal time scale. An improved understanding of this interaction for different land 
7 
surface types is an essential link between the implementation of sophisticated 
parameterizations and the operational use of these in a full 3D climate model context. For 
this, a one-dimensional version of the regional climate model RACMO is used (Christensen 
and van Meijgaard, 1995). 
The results obtained in the RACMO modelling suggest that further analysis is needed to 
determine the relative strength of oceanic versus land surface forcing. However with 
improvements in the surface scheme and the incorporation of spatial heterogeneity, some of 
the dry bias may be removed. 
References 
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Representation of the seasonal hydrological cycle in climate and weather prediction models in west Europe

de Bruin, H.A.R.

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Representation of the seasonal hydrological cycle in climate and weather prediction models in west Europe

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