Presentation abstracts

Jan Polcher (LMD Paris)
Welcome and introduction

Bert Holtslag (Wageningen University)
Report of GABLS workshop, ECMWF, 25-27 March 2002

GABLS was initiated in 2000 and endorsed as a GEWEX activity in 2001. The main goal of GABLS is to improve the representation of the Atmospheric Boundary Layer in Climate models. As such GABLS will provide a platform in which scientists working on boundary layers at different scales will interact. Such activity is important in itself and also very relevant for other activities in GEWEX and for activities within WCRP and IGBP. The first GABLS workshop was held at ECMWF, 25-27 March 2002, with a focus on Stratified Boundary Layers (SBLs) over Land. In that meeting about 30 persons attended, including process-oriented and large-scale modellers. A summary of the outcome of this meeting will be presented, with particular emphasis on the connection of this activity with GLASS.

Luis Bastidas (University of Arizona)
Land-atmosphere feedback in SVAT parameter estimation

Christa Peters-Lidard (Hydrological Sciences Branch NASA)
Coupled PBL-SVAT Modeling for Land Data Assimilation Systems

Local coupled PBL-SVAT modeling has been identified as a priority for Land Data Assimilation Systems (LDAS) to adequately simulate surface- (and potentially boundary-) layer feedbacks to assimilated water and energy states. It is recognized that adjusting surface state variables while retaining "fixed" near-surface thermodynamic profiles may result in potentially unrealistic fluxes from the LDAS. However, the influence of clouds, low-level jets, sea breezes and other nonlocal phenomena must be considered in developing adequate local PBL-SVAT models. This talk will present several illustrative case studies, including observations and 1- and 3-d models of weakly and strongly forced convective boundary layers with and without clouds.

Michael Ek (NCEP)
Local coupled land-atmosphere interaction: impact of soil moisture on boundary-layer cloud development

We study daytime land-atmosphere interaction using a column model consisting of an atmospheric boundary-layer (ABL) scheme coupled to a land-surface (LS) scheme and validate against observations taken at Cabauw, Netherlands. In column model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds. We examine the soil moisture role in ABL cloud development via model runs, analytical development, and observations in terms of ABL-top relative humidity (RH) tendency. Soil moisture may increase or decrease ABL-top RH and thus ABL clouds depending on initial conditions and many LS-ABL interactions. Land-atmosphere interaction in the NCEP NWP models will also be addressed.

Adam Schlosser (NASA Goddard Space Flight Center)
Implication of land-atmosphere feedback on global simulations

Martin Best (Met Office Hadley Centre)
The PILPS4C land-atmosphere coupling interface

In practise, the way in which land surface schemes (LSS) are coupled to atmospheric models varies considerably between the various LSS that are available in the community. This has restricted the comparison of these land surface schemes to either surface only testing, or testing each LSS within its own atmospheric model. However, it would be desirable to be able to connect any land surface scheme to any atmospheric model so that we can have more control over these experiments to enable a cleaner comparison.

This talk sets out some of the basics which need to be considered when designing an universal coupling between LSS and atmospheric models. Whilst considering this design, future developments of both LSS and atmospheric model must be taken into account, so that the coupling mechanism does not become obsolete in a short space of time.

Aaron Boone (CNRM)
Results on the RhoneAgg project

The Rhone-AGGregation (Rhone-AGG) Land Surface Scheme intercomparison project is an initiative within the Global Energy and Water Cycle Experiment (GEWEX) Global Land-Atmosphere System Study (GLASS)/Global Soil Wetness Project (GSWP). This project makes use of the Rhone 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. Three distinct components comprise this system: an analysis system to determine the near-surface atmospheric forcing, a Land Surface Scheme (LSS) interface and a distributed hydrological model. The main issues addressed by the Rhone-AGG project are how various state of the art LSSs are able to simulate the river discharge over several annual cycles when inserted into the Rhone modeling system, the impact of scale on the modeled runoff and discharge, the determination of whether any differences in simulations can be attributed to the disparate level of complexity of the surface representation and the varying aggregation methods employed, how simulated soil moisture is impacted by changing the spatial scale, and the evaluation of the impact of grid resolution on the simulated snowpack. A series of experiments encompassing three annual cycles were performed by 15 LSSs to address these issues, and a total of twenty sets of results were analyzed.

Paul Houser (NASA)
Properties of land-atmosphere interaction in land surface data assimilation

Andrea Molod (NASA Goddard Space Flight Center)
The Vertical Influence of Land Surface Heterogeneities in the GEOS GCM

Many studies have shown the impact on GCM simulated climate of the model used to handle sub-grid scale vegetation and soil heterogeneities. An `Extended Mosaic' (EM) technique for the land-atmosphere coupling was developed to allow the direct impact of surface heterogeneities to extend upwards into the overlying turbulent boundary layer. EM allows the GCM to simulate a Model Blending Height (MBH), the level at which the distinct character of the turbulence over different land scene types is `blended'. Simulations with EM show that the MBH is generally one third to one half of the Planetary Boundary Layer (PBL) height, and is generally higher when the surface temperature and surface sensible heat flux is higher. High extremes in surface temperature, however, are shown to result in a lower MBH. The behavior of the MBH is an indicator of the mechanism by which the surface heterogeneities extend their direct influence upwards into the boundary layer, and exert their influence on the climate system.

Rolf Reichle (NASA Goddard Space Flight Center)
Lessons learned from data assimilation into uncoupled land models

In the past few years there have been numerous studies about the assimilation of land surface observations, primarily of soil moisture and soil temperature, into land surface models that are not coupled to the atmosphere. I will present a few examples of these results, with a focus (1) on the choice of assimilation method and (2) on the ability to use data assimilation for downscaling from coarse scale satellite observations to finer scale hydrologic estimates, and (3) on the implications of the lessons learned for coupled land data assimilation.

Gisela Seuffert (ECMWF)
The ELDAS single column set-up for land data assimilation

Within the framework of ELDAS a data assimilation system for the optimal estimate of soil moisture will be built. The aim is to combine the use of 2m temperature and relative humidity observations with information of IR heating rates and microwave brightness temperatures as well as precipitation and solar radiation observations. The system will be developed and tested with a single column model (SCM) before it will be implemented operationally into the full 3D NWP model.

I will present results of soil moisture analysis (nudging and OI scheme) using the SCM for the FIFE site. These results include a comparison between former results of Douville et al. (1999) and results with the new analysis data (ERA40) and the new SCM. In the second part I will discuss the question whether the SCM compared to an offline SVAT model might be the superior tool to simulate near surface processes.

Joel Noilhan (CNRM)
Impact of soil moisture initialisation in NWP and hydrological models at mesoscale

Root zone and near surface soil moisture determine to a large extent the simulation of the Bowen ratio and runoff, which impact the boundary layer and streamflow simulations in atmospheric and hydrological short term forecasts. Therefore, there is a strong need for accurate initialisation of soil moisture in NWP and hydrological models at the regional scale. The paper will discuss numerical experiments carried out with research models at Météo-France on various methods of assimilation: (i) estimation of soil moisture from optimum interpolation based on screen level atmospheric parameters (ii) estimation of soil moisture from off - line simulations using the ISBA surface scheme forced by observed atmospheric quantities on the long term. After a short presentation of the off - line land data assimilation SAFRAN - ISBA - MODCOU (SIM) model, a numerical experiment with the MésoNH atmospheric mesoscale model will be discussed where the impact of both types of initialisation on the short range forecast is analysed.

The second part of the talk will be devoted to the ability of the quantitative forecast precipitation (ARPEGE and ALADIN NWP models) to be used to predict riverflows with the ISBA-MODCOU hydrological model. Using forecasted precipitations at 1, 2 and 3 days, two sets of streamflow forecasts were made: a control forecast for which the initial condition of soil moisture is taken from the previous 1 day forecast simulation, and an initialised forecast, for which the initial soil conditions are set according to the offline SIM run which uses analysed precipitation. The result clearly show that the errors on the discharge can reach 20% in the control forecast, but that this error is reduced significantly when the error from the previous forecasted rainfall is removed from a re-initialisation of the soil moisture and the snowpack according to the off - line run with observed precipitation.

Bart van den Hurk (KNMI)
Impact of aerodynamic land-atmosphere coupling on assimilation of surface heating rates