The Division's permanent staff consists of a Division Head, 11 scientists, a similar number of externally funded positions, three technical support staff and a management assistant.
The Division runs the experimental site in Cabauw and participates in experiments at other locations. It operates a system for the observation of clouds and radiation, which includes ground based observations and satellite retrieval systems. The following models are used:
Interactions between land-surfaces and the atmospheric boundary layer
Cabauw has been the major experimental site of the Division for more than 25 years. A review paper of older work was published in 1996. Cabauw data have been used in several international projects. Recent examples are the Project for Intercomparison of Land surface Parameterization Schemes (PILPS) and the ECMWF Re-Analysisproject ERA.
In 1995-1996 a large national experiment was conducted: the Tropospheric Energy Budget Experiment (TEBEX). As part of this experiment, observations were made of fluxes and profiles of momentum, temperature, moisture and CO2 over grassland (Cabauw) and forest (Garderen). Presently, the data base of these experiments has been completed and the emphasis of the ongoing research is on data analysis and on the development of parameterizations.
The observations in and over the forest in Garderen were analysed in detail, with interesting results. There are clear indications for an important role of tree-top cooling during the night in producing convective transport in forests. Also the uptake of CO2 by a forest has been analysed, using several techniques for estimating CO2 fluxes. The eddy-correlation technique appears to be a viable way for estimating these fluxes.
The Division has been active in a European COST-project aiming at the development of a European network of Windprofilers. The Windprofiler-RASS system, which was purchased for TEBEX, is now part of a semi-operational European network.
Research into turbulence and cloud dynamics focuses on the development of parameterization schemes for weather and climate models. The long-term goal is the development of a unified closure-scheme for clouds and turbulence. To this end detailed case studies are made using experimental data and a Large Eddy Simulation Model, which resolves the important scales of clouds and turbulence. Based on these case studies parameterizations are developed, which are tested using a Regional Atmospheric Climate Model and a single column version of this model.
The Division has played an active role in the GEWEX-Cloud System Studies project on boundary layer clouds and shallow convection. This is a very dynamic and fast developing field of research. The Division has a strong position in this field, because it combines experimental and modelling expertise. During the past years several new parameterizations for boundary layer clouds and shallow convection have been developed and tested against experimental data.
The non-local structure of the convective boundary layer has been investigated along several lines. It was demonstrated that the entrainment at the inversion base is dominated by the strongest updrafts, which have very non-local dynamics. A new mechanism has been discovered, which helps to create and maintain meso-scale variability. This mechanism may be called "meso-scale turbulence". It has a strong impact on moisture fields.
In 1997 a colloquium was organised on "Clear and Cloudy Boundary Layers" under the wings of the Royal Netherlands Academy of Sciences. In 1998, well edited proceedings of this colloquium appeared.
Interactions between clouds. aerosols. greenhouse gases and radiation
As part of TEBEX, a Cloud Detection System (CDS) was developed, which consists of a network of commercial Cloud Lidars, Infrared Radiometers and Solarimeters and a Cloud Retrieval System for the analysis of data from Meteosat and NOAA satellites. This CDS has performed very well during TEBEX. Not only total cloud cover, but also information of the vertical profiles of cloudiness and of additional cloud characteristics, can be extracted form this relatively simple observation system. This system seems suited for the incorporation in operational observation systems such as WWW and GCOS.
Data from CDS are being compared with model simulations using RACMO. The model results are promising, but also show significant deficiencies. These intercomparisons are very instructive and emphasise the importance of meso-scale structures in most cloud fields.
In 1996 detailed additional observations were made during three campaigns of the Clouds and Radiation project (CLARA). Data from this project have been used to validate satellite retrieval algorithms. The cloud retrieval system has been coupled to a weather forecast system and is now used in the KNMI's operational weather service.
In a national project a system was developed for the retrieval of water vapour data from GPS data. This system provides frequent high-quality data on the water vapour column. Important progress has been made in the development of both a detailed code for radiative transfer and of a more strongly parameterized code suitable for incorporation in climate models. The latter code has been compared with TEBEX data for clear sky conditions. Model and observations show a remarkable agreement.
New possibilities for the remote sensing of clouds, aerosols and gases are opened up by the GOME instrument on board of ERS-2. This instrument not only offers data with a high spectral resolution, but also provides information on the polarisation in three broad spectral bands. A breadboard version of GOME (Gobelin) has been operated on the ground, thus providing equivalent GOME information, with the only difference that the instrument was looking up into the sky. The analysis of this new source of information has just started, but opens a bright future for new research on the role of clouds, aerosols and gases in the radiation budget of the atmosphere.
Regional and global model studies of energy and water budgets
RACMO has been used for the analysis of several flooding episodes in the Netherlands and neighbouring regions. One conclusion is that the model predictions of precipitation amounts are of high quality in winter conditions, when large- scale precipitation dominates over convective precipitation.
RACMO is being used to investigate the role of soil moisture on regional scales. Soil moisture has a strong impact on model simulations of the climate in Europe, as well as on the quality of daily weather forecasts. Small errors in the representation of soil moisture may accumulate and lead to gross errors in surface temperatures and cloudiness, especially during summer. Because of the long memory of soil moisture budgets such errors can persist over whole seasons. In simulations of climate change the correct representation of soil moisture budgets is of crucial importance. This work is in progress.
RACMO is also used to study the mass balance of the ice-cap of Antarctica. The
model uses boundary conditions from ECMWF. The model does a good job in
representing the overall radiation and energy budget. Also the patterns of precipitation
agree well with observations. Model simulations indicate that when warmer boundary
conditions are applied the precipitation increases significantly, which increases the
amount of ice. This indicates that in a warmer climate the amount of ice on
Antarctica increases, which reduces sea level rise.
The observed variability of global mean temperatures has been analysed, using a 1-
dimensional global climate model. Several studies have been performed, including
one or more of the following sources of variability: anthropogenic greenhouse gases
and aerosols, the El Ninõ-Southern Oscillation (ENSO), volcanic eruptions and
variations in solar activity. This work is still in progress, but several conclusions may
be drawn at this state. Natural variability was the primary source of global warming
until about 1950. Since 1950, global warming has been partially masked by a series
of major volcanic eruptions. This suggests that anthropogenic climate change may
have been weaker than previously thought until 1950, but stronger thereafter.
A general outlook for the research of our Division can be found in the conclusions of a recent paper by Gates et al. (An Overview of the Results of the Atmospheric Model Intercomparison Project, 1999, B.A.M.S. 80,1, 29-55). We quote:
" From the analyses presented here and elsewhere, it is clear that much further work is needed to significantly reduce the errors of atmospheric General Circulation Models. Continuing outstanding problems are the parameterization of clouds and their radiative interactions, the parameterization of convection and precipitation, and the portrayal of the interactions between land surface and hydrological processes."
Thus it will be no surprise that all four research themes will be continued, be it in the form of rather different projects. A major shift will be an active participation in the international Baltex-Bridge experiment. Baltex is one of the five GEWEX continental-scale projects. It is designed to explore and model the energy and water budgets of the Baltic Sea drainage basln.
Interactions between land-surfaces and the atmospheric boundary layer
Although much has been achieved in this research field over the past decades, a few important problem areas remain. In the first place stable boundary layers with weak intermittent turbulence are not well understood. The representation of stable boundary layers in weather and climate models still offers great problems, which reduce the reliability of weather and climate predictions of night-time temperatures. In the second place there are uncertainties with respect to available observations of the surface fluxes of heat and moisture. These uncertainties are partially due to the limited accuracy of the observations, but there may be also conceptual problems due to a lack of understanding of the actual processes which are involved. International projects are planned, which may help to shed light on these important problem areas. Our involvement is listed below:
The Cabauw Observatory is planned to become an anchor station in the Baltex- Bridge network. A new experimental research project on the stable boundary layer is being designed, together with the Universities of Wageningen (UW) and Utrecht. International collaboration is considered as well. Together with UW, participation in the international surface energy budget experiment (EBEX) is planned. The implementation of a national windprofiler network is supported.
Turbulence and cloud dynamics
The availability of fast computers and cloud resolving models and the development of new observation systems like cloud radars, cloud lidars, micro-wave sounders and new satellites, have accelerated the progress of research into cloud dynamics in recent years. The recent discovery of meso-scale turbulence is an example. This process may help to understand the interactions between small-scale turbulence, convective clouds and large-scale dynamics. These interactions play a crucial role in weather and climate systems. This is a booming research field, which is internationally co- ordinated in GEWEX Cloud System Studies. We have a great research potential in this field, because we combine experimental and model activities. Specific plans are listed below: (N.B. a significant fraction of these plans can only be realized, if the present vacancy is filled)
Active participation in GEWEX Cloud System Studies is continued. Research into meso-scale processes will be intensified. Research with the Large Eddy Simulation Model will be continued. The implementation of a non-hydrostatic meso-scale model is also considered. An existing model will be adopted or the LES will be modified. Contributions to the development of a unified closure scheme for clouds and turbulence is continued in active co-operation with ECMWF and possibly the HIRLAM community. Participation in modelling activities within Baltex-Bridge is planned.
Interactions between clouds, aerosols, greenhouse gases and radiation
Radiative processes drive the climate system. Clouds and aerosols heavily modulate radiative transfer and have to be observed and modelled accurately, in order to achieve credible descriptions of the climate system. Because clouds and aerosols are highly variable in time and space, rather detailed observations are needed. This requires the development of better global observing systems and consequently better model descriptions of the influence of clouds and aerosols on radiative transfer. In recent years we have built advanced systems for observation and analysis and we have gained significant experience in this research field. In the coming years we will further develop and exploit these capacities in close international collaboration. Specific projects are:
The analysis of TEBEX data will be continued. Observations with the Cloud Detection System will be embedded in the Baltex- Bridge project. A Cloud Radar will be purchased and installed at Cabauw. This will greatly enhance our capacity to observe the structure of clouds. Further development of cloud retrieval methods for existing and new satellites. This work is embedded in the international Climate SAF project. The implementation of an operational nation-wide CDS is supported. Participation in research with GOME and SCIAMACHY is continued as well as contributions to the Earth Radiation Mission
Regional and global studies of energy and water budgets
The hydrological cycle has links with almost all compartments of the climate system. Rainfall feeds rivers and the biosphere over land. Ice-caps and glaciers are created by snowfall, which impact on sea levels. Evaporation may create deserts. Evaporation cools oceans and increases their salt concentration. Water vapour condenses into clouds, which affect radiative transfer. Latent heat release influences pressure distributions and atmospheric circulations etc. It is clear that the hydrological cycle plays a key-role in the climate system. The hydrological cycle has different characteristics over different regions and in different seasons. Therefore, regional climate studies are an appropriate way to investigate the role of the hydrological cycle in the global climate system.
Therefore we will continue our research into soil-moisture budgets in Europe (to be
embedded in Baltex and in co-operation with ECMWF). Moreover we will continue
to support research into the mass balance of ice-caps. We will also continue the
analysis of global and regional radiation budgets. In co-operation with others we will
also investigate possible interactions between regional radiative forcings and the large-
scale dynamics of the atmosphere.
Close interactions exist with the GEWEX research community. With respect to climate modelling and radiative forcings there are links to CLIVAR as well. Participation in Baltex will be a major task in the coming years. Co-operation exists further with ECMWF, ESA MPIM, Weather Services and several Universities etc.
Nationally we collaborate closely with the Universities of Utrecht, Delft and Wageningen. We also collaborate with other universities and with many institutes like RIVM, ECN and TNO
Within KNMI we want to establish a more effective collaboration with the Observations and
Modelling Department both with respect to regional modelling and with respect to
observations.
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