Brian Ryan
CSIRO Atmospheric Research
The scientific viewpoints in Working Group 3 have matured with the model/model and model/observation intercomparisons leading to the conclusions and recommendations coming from the workshop. However, Working Group 3 is still under represented by the GCM modeling community.
The workshop concluded that the generation of middle-level and high-level cloud is a serious problem for mesoscale models. The mesoscale models underestimate the middle-level cloud generated by frontal systems and the models probably overestimate the amount of high cloud associated with the frontal system.
The model intercomparison identified the importance of the feedback between the diabatic effects from sublimation, melting and evaporation and the prefrontal circulation. In particular, the intercomparison pointed to evidence suggesting that the sublimating cirrus in the model is a trigger for the prefrontal descent that suppresses the middle-level cloud.
The workshop identified the feedback of cirrus on the frontal circulation as a model problem and a process reality.
The workshop addressed the problem of transferring the knowledge gained from case studies of cloud systems to the cloud climatologies generated by NWP climate models. The linking of satellite and large-scale datasets were seen as an integral part of the strategy for the model validation and an essential part of the solution to the problem of parameterizing the climatology of cloud properties and processes in GCMs. The workshop demonstrated the importance of diagnosing model fields that are comparable with satellite products.
The workshop saw SCMs as an important tool for identifying strengths and weaknesses of current GCM packages. However, the limitations on the application of SCMs to middle-level cloud systems are still unclear.
The workshop has made recommendations for future studies (e.g. NEWBALTIC and BRIDGE). Working Group 3 is now better situated to advise on requirements for cloud satellite missions, i.e. WG3 is better able to articulate the arguments for the ground truthing of clouds by using observational (cloud radars) and modeling techniques.
The workshop recommends that a suitable case from FASTEX for Working Groups 2 (cirrus) and 3 (extra-tropical layer cloud) is IOP 16. It was decided that there were no highly recommended cases from FASTEX for Working Groups 1and 4.
The workshop made no firm conclusions regarding a GCSS orographic cloud strategy, although recommendations were made to pursue the development of such a strategy.
The fourth workshop of the GCSS Working Group 3 was hosted by GKSS and held in Geesthacht, Germany from 2-5 June 1998. The objectives and outcomes of the workshop were as follows:
The first day of the workshop (Tuesday 2 June) was spent discussing the draft paper based on the Echuca meeting. At the end of Day 1 a number of writing tasks were assigned so that a final decision on the structure of the paper could be made on Thursday. The second day was devoted to the consideration of the other cases under study by the Working Group, namely CASP II and BASE. This was followed by presentations on NEW BALTEX and FASTEX. The third day of the meeting was structured into three segments, a discussion of the selection of a case study from FASTEX, a presentation on the application of the ISCCP satellite analyses as a methodology for generalizing the case studies undertaken by Working Group 3, and finalizing of the material to be included in the draft Working Group 3 methodology paper. On the final day of the meeting there was a discussion on the development of a GCSS strategy on the treatment of orographic clouds in GCMs. The meeting concluded with the preparation of a draft summary that forms the basis of this report and the identification of a number of action items.
Validation of the LAMs against observations and CRMs
The LAMs for the CFRP case generally gave similar cloud amounts
although there were significant differences in the optical properties.
The ISCCP/model comparisons showed that the correct generation
of cloud fraction is not a sufficient condition to produce radiatively
correct cloud fields in a GCM. Those comparisons and model simulations
showed that the correct prediction of the cloud type is also critical.
When the large-scale forcing was strong the LAM simulations of
both cloud cover and cloud type were very good. However, this
was not the case for the LAM simulations when the forcing was
weak.
All the LAM simulations developed mesoscale structures and captured the basic dynamic and thermodynamic structure of the front. However, all of the LAMs failed to capture some important facets of the cloud structure. In particular, while they all captured the upper level cirrus cloud, none of the LAMs developed any significant middle-level cloud ahead of the front. This feature is evident in both the station points and box average representations for the cloud water. At 20km resolution the mesoscale motions determine the extent of the cloud ice or water mixing ratios. A major deficiency in the LAMs is the excessive development of cirrus cloud and the absence of middle-level cloud. There is strong evidence to suggest that sublimation by the cirrus triggers the mesoscale descent that suppresses the development of middle-level cloud.
The deficiency in the LAMs is supported by both the ISCPP analyses and the surface observations and results in part from the descending air generated by the sublimating cirrus layer. The diabatic effects arising from the sublimation, melting and evaporation are a feature of this case. The 20km LAM simulation demonstrated that these diabatic processes represent both a physical reality and a model problem. The evaporation between 700hPa and the surface is real and confirmed by observation. It is an important thermodynamic feature of the front. However, the sublimation of ice crystals from the cirrus well ahead of the front is too strong and is responsible for suppressing the middle-level cloud. GCMs run in weather prediction mode are approaching this resolution and are likely to experience the same problem.
The CRM simulations show that the increased horizontal resolution and the use of explicit cloud schemes assist in the generation of the middle-level cloud ahead of the front. The CRM simulations show that the evaporative cooling ahead of the front produced circulations not resolved by the LAMS. Furthermore, the MC2 CRM started to develop the observed rainband structure. It is likely that even better simulations would be achieved with increased vertical resolution.
The diagnosis of satellite compatible model outputs produced model fields that could be directly compared to satellite retrievals in the ISCCP data sets. Some of the deficiencies in the model cloud simulations revealed by the model-satellite comparisons were similar to the ones found when the models were compared to the field observations. The satellite retrievals, however, provided the tool to extend the spatial coverage of the comparisons and to validate the model runs over the whole domain covered by the storm system. For example, the low cloud deck upstream of the front in the early stages of its development, and the cirrus cloud deck at the northern tail of the storms during its mature stage, were features that were outside the CFRP observational network. Model-satellite comparisons should be further tuned to produce model fields that are quantitatively equivalent to the satellite retrievals and should be extended to include other satellite retrieved fields such as rain amounts and cloud particle phase and sizes.
Extension of the methodology to improve cloud parameterization in GCMs
The second step in the methodology was to investigate how the
results from the LAM and CRM simulations can be applied to improve
the parameterization of cloud processes in GCMs. Two methodologies
were explored and some preliminary conclusions drawn. The first
technique was to run the GCM at GCM resolution in forecast mode
and then to compare the GCM simulation with the LAM or CRM simulation
average back onto the GCM grid.
The second technique was to use SCMs. The present set of SCM studies shows that great care has to be taken with specifying the forcing given to the SCM. A comparison between the DARLAM simulation and the two SCMs showed that the amount of cloud condensate differs substantially between the three simulations.
The validation of the SCM approach has not been resolved by this paper. However, a strategy is proposed in the paper to resolve the uncertainties.
The synthesizing of the results from all three cases raises the following issues that are being addressed by the Working Group and will be put in the form of a draft paper at the 5th Workshop planned for Reading, UK in 1999. The first issue relates to how we transfer the knowledge from the CRMs to the LAM scales for NWP forecasting and to GCMs for climate forecasting. The second relates to how we can overcome the problems with SCMs identified in the methodology paper.
The synthesis of the three cases showed that these cloud systems can be simulated by state-of-the-art mesoscale and cloud resolving model. There is also evidence to suggest that the CRMs perform better in the three case studies. It is not yet clear how well the climate scale GCMs are able to capture these systems. The cloud survey technique provides the methodology to complete this task. The issue of how SCMs should be forced to parameterize the grid average dynamics and physics in a CRM or LAM is as yet unresolved.
The five most relevant cases were reviewed in terms of both the Working Group 3 objectives and the possibility of joint study with the other working groups. The most appropriate cases for process studies were expected to result from the systematic survey flight plan using a set of parallel system-relative flight tracks of all three aircraft. In practice the best case is that from Intensive Observing Period (IOP) 16, observations with dropsondes and one Doppler radar aircraft of the early development of a secondary frontal wave. The soundings show substantial mesoscale dynamical structure in mid-troposphere which are thought to be indicative of sublimation effects. Despite a good simulation of the surface pressure evolution the internal precipitation and frontal structure is proving to be a demanding subject for model studies, and so is currently being studied at UKMO as a suitable case for comparisons, given an improved initial analysis.
A second event, IOP 11, was also highlighted. It was a large primary cyclone very thoroughly observed with dropsondes and both Doppler and radar aircraft, and showed a range of interesting internal structures in both warm and cold frontal zones. It will be investigated as a possible example of a mature end of a storm-track system.
Some problems of suspected bias in sonde humidity measurements are currently being investigated, but it is expected that definitive observations will be available within 3-6 months.
The possibility of study by the other GCSS working groups was discussed. Overall it was felt that a FASTEX study for Working Groups 1 and 4 was not practical because study of each type of cloud system requires its own characteristic type of observational configuration and validation information. These were simply not possible in the circumstances of the experiment, which had very different objectives, equipment and expertise. For example, active convection was not a feature of most of the systems observed. It was encountered in rather cold airmasses studied in IOPs 12 and 18, but cirrus tops were normally above the highest aircraft while because of practical difficulties the distribution of observations was unsuitable to obtain the high quality diagnostic information needed for good stratiform cloud system studies.
If any case were to be studied by all groups a reduced set of standards would have to be defined, which would probably render the study much less useful than the individual groups' studies.
The working group therefore recommends that future experimental plans include a strong cloud layering component to their observational strategy. One example of an upcoming experiment is BRIDGE, a major effort within BALTEX, that will measure very well all of the first four fields mentioned above. If feasible, more attention should be paid to the improved measurement of the layering and cloud structure of frontal systems to be observed within this experiment. This might be accomplished, for example, through the use of more cloud radar observations.
For the same reasons, the group is highly supportive of proposed satellite-borne cloud radars. Such instruments will map out the critical cloud layering fields within a range of frontal systems.
One part of the EU funded project NEWBALTIC II is a greater in depth study of LAM simulations for selected cyclones that occurred during the period of August to November 1995 over the Baltic Sea catchment area. The LAMs HIRLAM (from SMHI, Sweden), RACMO (from KNMI, Netherlands), REMO-MPI (Max-Planck-Institute for Meteorology, Germany), and REMO-GKSS (GKSS, Germany) took part in this study. This project can take advantage of the experiences and methods of the Australian CFRP case study.
The workshop concluded that the generation of middle-level and high level cloud is a serious problem for mesoscale models. The mesoscale models underestimate the middle-level cloud generated by frontal systems and the models probably overestimate the amount high cloud associated with the frontal system.
The model intercomparison identified the importance of the feedback between the diabatic effects from sublimation, melting and evaporation and the prefrontal circulation. In particular the intercomparison pointed to evidence suggesting that the sublimating cirrus in the model triggers the prefrontal descent that suppresses the middle-level cloud.
The workshop identified the feedback of cirrus on the frontal circulation as both a model problem and a process reality.
The workshop addressed the problem of transferring the knowledge of cloud systems gained from case studies of cloud systems to the cloud climatologies generated by NWP and climate models. The linking of satellite and large-scale datasets were seen as an integral part of the strategy for the model validation and an essential part of the solution to the problem of parameterizing the climatology of cloud properties and processes in GCMs. The workshop demonstrated the importance of models diagnosing fields that are comparable with satellite products.
The workshop saw SCMs as an important tool for identifying strengths and weaknesses of current GCM cloud packages. However, the limitations on the application of SCMs to extra-tropical layer cloud systems are still unclear.
The workshop made recommendations for future observational studies (e.g. NEWBALTEX and BRIDGE). Working Group 3 is now better situated to advise on requirements for cloud satellite missions, i.e. Working Group 3 is better able to articulate the arguments for ground truthing clouds by using observational techniques (cloud radars) and modeling techniques.
The workshop recommends that a suitable case from FASTEX for Working Groups 2 (cirrus) and 3 (middle-level cloud) is IOP 16. It was considered that there were no highly recommended cases from FASTEX for Working Groups 1 and 2.
The workshop made no firm conclusions regarding a GCSS orographic cloud strategy, although recommendations were made to pursue the development of such a strategy.
Sid Clough JCMM University of Reading PO Box 243 Earley Gate, Reading, Berkshire RG6 2BB UNITED KINGDOM Yasushi Fujiyoshi Inst. Low Temperature Sciences Hokkaido University N-19, W-8 Sapparo 060 Japan Jack Katzfey CSIRO Atmospheric Research Private Bag 1 Aspendale, Victoria, 3195 Australia Elke Keup-Thiel GKSS Forschungszentrum Max-Plank-Strasse D-21502 Geesthacht Germany Humphrey Lean JCMM University of Reading PO Box 243 Earley Gate,Reading, Berkshire RG6 2BB UNITED KINGDOM Lubomir Levkov GKSS Forschungszentrum Max-Plank-Strasse D-21502 Geesthacht Germany John McBride Bureau of Meteorology 150 Lonsdale Street Melbourne, Victoria 3000 Australia Burkhardt Rockel GKSS Forschungszentrum Max-Plank-Strasse D-21502 Geesthacht Germany Brian Ryan CSIRO Atmospheric Research Private Bag 1 Aspendale, Victoria, 3195 Australia Ron Stewart Climate Processes and Earth Observing Division Atmospheric Environment Service Downsview, Ontario M3H 5T4 CANADA Kit Szeto Climate Processes and Earth Observing Division Atmospheric Environment Service Downsview, Ontario M3H 5T4 CANADA George Tselioudis NASA Goddard Institute for Space Studies 2880 Broadway New York, NY 10025 USA Peter Yau Department of Atmospheric and Ocean Sciences McGill University Montreal, QC Canada H3A 2K6
Ying Zhang GKSS Forschungszentrum Max-Plank-Strasse D-21502 Geesthacht Germany
| BALTEX | Baltic Sea Experiment |
|---|---|
| BASE | Beaufort and Arctic Storms Experiment |
| BRIDGE | Baltic Sea Experiment 1999-2001 |
| CASP II | Canadian Atlantic Storms Program II |
| CCCma | Canadian Centre for Climate for Modeling and Analysis |
| CFRP | Cold Fronts Research Program |
| CRM | Cloud Resolving Model |
| CSIRO | Commonwealth Scientific and Industrial Research Organization |
| CSU | Colorado State University |
| DARLAM | CSIRO Limited Area Model |
| ECMWF | European Centre for Medium Range Weather Forecasting |
| ECHAM | European Centre/Hamburg Model |
| EU | European Union |
| FASTEX | Fronts and Atlantic Storm-Track Experiment |
| GCM | General Circulation Model |
| GCSS | GEWEX Cloud System Study |
| GKSS | Research CentreGeesthacht |
| GISS | Goddard Institute of Space Science |
| GESIMA | Geesthacht Simulation Model of the Atmosphere |
| ISCCP | International Satellite Cloud Climatology Project |
| IOP | Intensive Observing Period |
| JCMM | Joint Centre for Mesoscale Meteorology |
| LAM | Limited Area Model |
| MC2 | Canadian Mesoscale Compressible Community model |
| NEWBALTIC II | Numerical Studies of the Energy and Water Cycle of the Baltic Region |
| NWP | Numerical Weather Prediction |
| RAMS | Regional Atmospheric Modeling System |
| REMO | Regional scale Model used by GKSS |
| SCM | Single Column Model |
| TREX | Torrential Rainfall Experiment |
| UKMO | United Kingdom Meteorological Office |