Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: P. Viterbo x
  • Refine by Access: All Content x
Clear All Modify Search
A. Hollingsworth,
,
P. Viterbo, and
, and
A. J. Simmons

Abstract

No Abstract available.

Full access
M. J. Best
,
A. Beljaars
,
J. Polcher
, and
P. Viterbo

Abstract

A generalized coupling is proposed between atmospheric models and surface schemes (land and ocean). A set of input and output variables is defined for this purpose in such a way that it can be used by many current and future models, including mosaic or tile schemes. The basic concept is to pass atmospheric variables from the lowest model level and their relation to corresponding fluxes to the surface scheme. The surface scheme returns the fluxes. In this framework, there is no need for the atmospheric model to have detailed information about the surface. Only the result of the surface computations is needed; namely, the fluxes, which are applied as a boundary condition. The equations for fully implicit coupling are derived, and the relevance for numerical stability is demonstrated. It is also shown that fully implicit coupling in a tile scheme leads to more robust results than partially implicit coupling.

Full access
M. J. Best
,
A. Beljaars
,
J. Polcher
, and
P. Viterbo
Full access
G. Seuffert
,
H. Wilker
,
P. Viterbo
,
M. Drusch
, and
J-F. Mahfouf

Abstract

This study focuses on testing two different soil moisture analysis systems based on screen-level parameters (2-m temperature T 2m , 2-m relative humidity RH2m ) and 1.4-GHz passive microwave brightness temperatures T B . First, a simplified extended Kalman filter (EKF) system is compared with an optimal interpolation (OI) method assimilating screen-level parameters in a single-column version of the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather prediction model. In the second part of this study, the EKF is applied to investigate whether the synergy of T 2m , RH2m , and additionally T B in an assimilation framework improves the simulated soil moisture and atmospheric parameters.

For a summer period (130 days) during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) 1987 it is shown that the OI and EKF analysis systems give similar results. Both systems distinguish consistently between periods of atmospheric and surface-controlled fluxes. Though the overall soil water is adjusted by the same amount, the EKF system simulates increments increasing from the first to the third layer, whereas in the OI method they are equally distributed.

The EKF system is applied for the Southern Great Plains Field Experiment 1997 (SGP97) testing the assimilation of a synergy of T 2m , RH2m , and T B . The observed root zone soil moisture is best simulated by the control run and when T B is assimilated. The assimilation of T 2m and RH2m worsens the simulated root zone soil moisture compared to observations, because during a 10-day period modeled T 2m and RH2m considerably diverge from observations and soil moisture is tuned to compensate for deficiencies in the model. But in comparison to observed net radiation, heat fluxes, and near-surface soil moisture, it is shown that the assimilation of the synergy of observation types (T 2m , RH2m , and T B ) gives more consistent results than when they are assimilated separately.

Full access
T. N. Palmer
,
Č Branković
,
P. Viterbo
, and
M. J. Miller

Abstract

Results from a set of 90-day integrations, made with a T42 version of the ECMWF model and forced with a variety of specified sea surface temperature (SST) datasets, are discussed. Most of the integrations started from data for 1 June 1987 and 1 June 1988. During the summer of 1987, both the Indian and African monsoons were weak, in contrast with the summer of 1988 when both monsoons were much stronger. With observed SSTs, the model is able to simulate the interannual variations in the global-scale velocity potential and stream-function fields on seasonal time scales. On a regional basis, rainfall over the Sahel and, to a lesser extent, India showed the correct sense of interannual variation, though in absolute terms the model appears to have an overall dry bias in these areas.

Additional integrations were made to study the impact of the observed SST anomalies in individual oceans. Much of the interannual variation in both Indian and African rainfall can be accounted for by the remote effect of the tropical Pacific SST anomalies only. By comparison with the effect of the Pacific, interannual variability in Indian Ocean, tropical Atlantic Ocean, or extratropical SSTs had a relatively modest influence on tropical large-scale flow or rainfall in the areas studied.

Integrations run with identical SSTs but different initial conditions indicated that for large-scale circulation diagnostics, the impact of anomalous ocean forcing dominated the possible impact of variations in initial conditions. In terms of local rainfall amounts, on the other hand, the impact of initial conditions is comparable with that of SST anomaly over parts of India and Southeast Asia, less so over the Sahel. While this may suggest that a nonnegligible fraction of the variance of month-to-seasonal mean rainfall on the regional scale in the tropics may not be dynanamically predictable, it is also quite possible that the disparity in the apparent predictability of rainfall and circulation anomalies is a reflection of model systematic error.

Full access
G. P. Weedon
,
S. Gomes
,
P. Viterbo
,
W. J. Shuttleworth
,
E. Blyth
,
H. Österle
,
J. C. Adam
,
N. Bellouin
,
O. Boucher
, and
M. Best

Abstract

The Water and Global Change (WATCH) project evaluation of the terrestrial water cycle involves using land surface models and general hydrological models to assess hydrologically important variables including evaporation, soil moisture, and runoff. Such models require meteorological forcing data, and this paper describes the creation of the WATCH Forcing Data for 1958–2001 based on the 40-yr ECMWF Re-Analysis (ERA-40) and for 1901–57 based on reordered reanalysis data. It also discusses and analyses model-independent estimates of reference crop evaporation. Global average annual cumulative reference crop evaporation was selected as a widely adopted measure of potential evapotranspiration. It exhibits no significant trend from 1979 to 2001 although there are significant long-term increases in global average vapor pressure deficit and concurrent significant decreases in global average net radiation and wind speed. The near-constant global average of annual reference crop evaporation in the late twentieth century masks significant decreases in some regions (e.g., the Murray–Darling basin) with significant increases in others.

Full access
D. Gustafsson
,
E. Lewan
,
B. J. J. M. van den Hurk
,
P. Viterbo
,
A. Grelle
,
A. Lindroth
,
E. Cienciala
,
M. Mölder
,
S. Halldin
, and
L-C. Lundin

Abstract

The objective of the present study was to assess the performance and recent improvements of the land surface scheme used operationally in the European Centre for Medium-Range Weather Forecasts (ECMWF) in a Scandinavian boreal forest climate/ecosystem. The previous (the 1999 scheme of P. Viterbo and A. K. Betts) and the new (Tiled ECMWF Surface Scheme for Exchange Processes over Land, TESSEL) surface schemes were validated by single-column runs against data from NOPEX (Northern Hemisphere Climate-Processes Land-Surface Experiment). Driving and validation datasets were prepared for a 3-yr period (1994–96). The new surface scheme, with separate surface energy balances for subgrid fractions (tiling), improved predictions of seasonal as well as diurnal variation in surface energy fluxes in comparison with the old scheme. Simulated wintertime evaporation improved significantly as a consequence of the introduced additional aerodynamic resistance for evaporation from snow lying under high vegetation. Simulated springtime evaporation also improved because the limitation of transpiration in frozen soils was now accounted for. However, downward sensible heat flux was still underestimated during winter, especially at nighttime, whereas soil temperatures were underestimated in winter and overestimated in summer. The new scheme also underestimated evaporation during dry periods in summer, whereas soil moisture was overestimated. Sensitivity tests showed that further improvements of simulated surface heat fluxes and soil temperatures could be obtained by calibration of parameters governing the coupling between the surface and the atmosphere and the ground heat flux, and parameters governing the water uptake by the vegetation. Model performance also improved when the seasonal variation in vegetation properties was included.

Full access
A. Boone
,
F. Habets
,
J. Noilhan
,
D. Clark
,
P. Dirmeyer
,
S. Fox
,
Y. Gusev
,
I. Haddeland
,
R. Koster
,
D. Lohmann
,
S. Mahanama
,
K. Mitchell
,
O. Nasonova
,
G.-Y. Niu
,
A. Pitman
,
J. Polcher
,
A. B. Shmakin
,
K. Tanaka
,
B. van den Hurk
,
S. Vérant
,
D. Verseghy
,
P. Viterbo
, and
Z.-L. Yang

Abstract

The Rhône-Aggregation (Rhône-AGG) Land Surface Scheme (LSS) intercomparison project is an initiative within the Global Energy and Water Cycle Experiment (GEWEX)/Global Land–Atmosphere System Study (GLASS) panel of the World Climate Research Programme (WCRP). It is a intermediate step leading up to the next phase of the Global Soil Wetness Project (GSWP) (Phase 2), for which there will be a broader investigation of the aggregation between global scales (GSWP-1) and the river scale. This project makes use of the Rhône modeling system, which was developed in recent years by the French research community in order to study the continental water cycle on a regional scale.

The main goals of this study are to investigate how 15 LSSs simulate the water balance for several annual cycles compared to data from a dense observation network consisting of daily discharge from over 145 gauges and daily snow depth from 24 sites, and to examine the impact of changing the spatial scale on the simulations. The overall evapotranspiration, runoff, and monthly change in water storage are similarly simulated by the LSSs, however, the differing partitioning among the fluxes results in very different river discharges and soil moisture equilibrium states. Subgrid runoff is especially important for discharge at the daily timescale and for smaller-scale basins. Also, models using an explicit treatment of the snowpack compared better with the observations than simpler composite schemes.

Results from a series of scaling experiments are examined for which the spatial resolution of the computational grid is decreased to be consistent with large-scale atmospheric models. The impact of upscaling on the domain-averaged hydrological components is similar among most LSSs, with increased evaporation of water intercepted by the canopy and a decrease in surface runoff representing the most consistent inter-LSS responses. A significant finding is that the snow water equivalent is greatly reduced by upscaling in all LSSs but one that explicitly accounts for subgrid-scale orography effects on the atmospheric forcing.

Full access
Ingjerd Haddeland
,
Douglas B. Clark
,
Wietse Franssen
,
Fulco Ludwig
,
Frank Voß
,
Nigel W. Arnell
,
Nathalie Bertrand
,
Martin Best
,
Sonja Folwell
,
Dieter Gerten
,
Sandra Gomes
,
Simon N. Gosling
,
Stefan Hagemann
,
Naota Hanasaki
,
Richard Harding
,
Jens Heinke
,
Pavel Kabat
,
Sujan Koirala
,
Taikan Oki
,
Jan Polcher
,
Tobias Stacke
,
Pedro Viterbo
,
Graham P. Weedon
, and
Pat Yeh

Abstract

Six land surface models and five global hydrological models participate in a model intercomparison project [Water Model Intercomparison Project (WaterMIP)], which for the first time compares simulation results of these different classes of models in a consistent way. In this paper, the simulation setup is described and aspects of the multimodel global terrestrial water balance are presented. All models were run at 0.5° spatial resolution for the global land areas for a 15-yr period (1985–99) using a newly developed global meteorological dataset. Simulated global terrestrial evapotranspiration, excluding Greenland and Antarctica, ranges from 415 to 586 mm yr−1 (from 60 000 to 85 000 km3 yr−1), and simulated runoff ranges from 290 to 457 mm yr−1 (from 42 000 to 66 000 km3 yr−1). Both the mean and median runoff fractions for the land surface models are lower than those of the global hydrological models, although the range is wider. Significant simulation differences between land surface and global hydrological models are found to be caused by the snow scheme employed. The physically based energy balance approach used by land surface models generally results in lower snow water equivalent values than the conceptual degree-day approach used by global hydrological models. Some differences in simulated runoff and evapotranspiration are explained by model parameterizations, although the processes included and parameterizations used are not distinct to either land surface models or global hydrological models. The results show that differences between models are a major source of uncertainty. Climate change impact studies thus need to use not only multiple climate models but also some other measure of uncertainty (e.g., multiple impact models).

Full access