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Report and Recommendations from the 3rd GCSS Working Group 3 Workshop

Held in Echuca, Australia from 10 July to 12 July 1997

to the

GCSS Science Panel Sixth Session

Boulder, Colorado, USA
1-7 December 1997

Brian Ryan
CSIRO Division of Atmospheric Research

A MS Word 7 version of this document is also available.

Report and Recommendations

Executive Summary

The third workshop of GCSS Working Group 3 was held in Echuca, Australia from 10-12 July 1997. The objectives of the workshop were as follows:

  1. To analyse a model intercomparison based on a south-eastern Australian extra-tropical layering case, in order to develop a mid-latitude methodology for testing physically based parameterizations of cloud processes in cloud resolving and mesoscale models.
  2. To review progress on other regional case studies that have been identified by Working Group 3 as suitable for investigating the impact of parameterization of cloud processes in climate and numerical weather prediction models.
  3. To develop an outline for GCSS activities in regard to orographic clouds.
  4. To plan for developing a data set archiving procedure.
  5. To summarize our present capabilities of accounting for frontal systems in large-scale climate models and to identify the critical issues that need to be pursued.

The model intercomparison based on the south-eastern Australian extra-tropical layering case was made using a domain at 20 km resolution for limited area models (LAMs), a domain at 5 km resolution using cloud resolving models (CRMs) and single column models (SCMs) over a 300 km domain. The models taking part at 20 km resolution were the CSIRO limited area model DARLAM, the Colorado State University model Regional Atmospheric Modeling System (RAMS) run by CSIRO, a high resolution ECMWF simulation (T213), the Canadian limited area model MC2 and the German REMO/GESIMA limited area model from GKSS. The models taking part at 5 km resolution were the Canadian MC2 model, the German REMO/GESIMA model and RAMS. The SCM models used were the Canadian CCCma, the German ECHAM and the ECMWF SCM.

The model intercomparison showed that:

However the parameterization questions remain unresolved until new CRM analyses are completed.

The workshop considered regional case studies identified by Working Group 3 as suitable for investigating the impact of the parameterization of cloud processes in climate and numerical weather prediction models. In particular preliminary modelling results from BASE, CASP II and PIDCAP, and overviews of FASTEX and SALPEX were discussed at the workshop.

The workshop developed an outline for GCSS activities in regard to orographic clouds and the following strategy to improve the parameterization of orographic clouds for Working Group 3 was recommended:

1. Production of a review paper summarising current knowledge on orographic influences on clouds of relevance to GCSS, and providing details of knowledge gaps.

2. Determination of the global extent, type and frequency of orographically influenced clouds, using data from the International Satellite Cloud Climatology Project (ISCCP).

3. Consideration by each GCSS working group of whether specific case study experiments and/ or analyses are required to address orographically generated clouds that fall within their remit.

4. Studies by individual working groups covering significant gaps identified in the previous task. Such studies should follow the normal GCSS pattern: Analysis to determine and understand underlying processes; high resolution simulations of cloud systems validated by appropriate case studies; development or testing of cloud parameterizations using synthetic data from these high resolution simulations.

5. Working groups should include a section on orographic clouds in their routine reports to the GCSS Science Panel.

The workshop identified and attempted to answer the following questions concerning the parameterization of mid-latitude layer systems.

The deliverables from the workshop were:

1. Introduction

The third workshop of GCSS Working Group 3 was held in Echuca, Australia from 10-12 July 1997. The objectives of the workshop were as follows:

  1. To analyse a model intercomparison based primarily on a south-eastern Australian extra-tropical layering case in order to develop a mid-latitude methodology for testing physically based parameterizations of cloud processes in the cloud resolving and mesoscale models.
  2. To review progress on other regional case studies that have been identified by Working Group 3 as suitable for investigating the impact of parameterization cloud processes in climate and numerical weather prediction models.
  3. To develop an outline for GCSS activities in regard to orographic clouds.
  4. To plan for developing a data set archiving procedure.
  5. To summarize our present capabilities of accounting for frontal systems in large scale climate models and to identify the critical issues that need to be pursued.

Appendix 1 contains a list of conference participants as well as the conference program.

There were 12 presentations made at the workshop. There was a strong emphasis in the workshop on the discussion of papers and the future directions for GCSS Working Group 3. The first two days of the workshop were organized into the following sessions (i) the Australian Cold Fronts Research Program (CFRP) modelling intercomparison, (ii) preliminary modelling results from BASE, CASP II and PIDCAP, (iii) an overview of FASTEX and SALPEX, and (iv) an overview of the workshop achievements and development of a GCSS Orographic Strategy. A brief summary of these sessions is given in section 2.

The morning session on the third day consisted of a discussion of the outline for a paper, future Working Group 3 activities, archiving of special data sets and finally a meeting summary and recommendations. The report summarizes the conclusions reached on the final day by including an overview of the issues arising from the workshop (section 3), a summary of CD data currently archived (section 4), a statement on the deliverables from the workshop (section 5), definition of the goals for a future workshop (section 6) and a time line for future activities for Working Group 3 (section 7).

2. Brief Summary of Presentations

2.1 CFRP Intercomparison

(Ryan/Jakob/Katzfey/Szeto/Rockel/Abbs/Lohmann/Tselioudis/Rotstayn)

One of the objectives of the Workshop was to develop a methodology, applicable to mid-latitude systems, that is able to test physically-based parameterizations of cloud processes in mesoscale and cloud resolving models. The first stage in the development of this methodology was to organize a modelling intercomparison of a cool change that passed over south-eastern Australia over the period 17-18 November 1984. The passage of the cold front took place during the Australian Cold Fronts Research Program (CFRP). The special CFRP observations provided a network of two-hourly rawin sondes from which high resolution time sections and heat and moisture budgets were constructed. Mesoscale observations were obtained from a research aircraft and a network of surface stations.

Two domains were specified for the intercomparison. The first was a limited area domain with a grid resolution of 20 km resolution and the second was a cloud resolving domain with a grid resolution of 5 km. Single column models were intercompared using the CSIRO DARLAM limited area model simulation as boundary conditions.

20 km LAM Domain

Model simulations at 20 km resolution were both intercompared and compared with Research Program (CFRP) observations. The models taking part in the intercomparison were the CSIRO limited area model DARLAM, the Colorado State University model RAMS run by CSIRO, a high resolution ECMWF simulation (T213), the Canadian limited area model MC2 and the German REMO/GESIMA limited area model from GKSS. Two simulations were carried out using DARLAM; the first simulation used the cloud microphysical scheme developed for the mesoscale model and the second simulation used the cloud microphysical scheme developed for the CSIRO GCM.

The initial and boundary conditions for the limited area simulations were taken from the 6-hourly re-analysed ECMWF fields interpolated to a 20 km grid. The location of the south-west corner of the domain was 120E, 50S and the location of the north-east corner domain was 155E, 20S.

Validating fields were analysed for each 6 hours of the 48 hour simulation. The fields for comparison included the mean sea level pressure, precipitation, 850 hPa, 600 hPa and 200 hPa fields of geopotential height, water vapour, cloud and precipitation mixing ratios and the u and v velocity components. East-west sections across the domain were specified to pass through Mt. Gambier (140.783E, 37.733S), time height sections were made at the grid points closest to Mt Gambier (140.783E, 37.733S) and the position of the ship MV Sprightly (139E, 41S) and time-height profiles were averaged over two boxes (300 km by 300 km) centred on Mt Gambier and the ship MV Sprightly. The variables specified for intercomparison in these boxes were the heating and moisture budget terms (Q1 and Q2), mean vertical velocity, mean water vapour mixing ratio, mean condensed water/ice, mean temperature, mean u and mean v. Finally, fields of cloud cover, cloud top pressure (ctp) and optical depth (tau) were specified for intercomparison with ISCCP data.

A comparison of the synoptic fields of mean sea level pressure, moisture and winds from the LAMs showed that all simulations captured this event reasonably accurately. However, some differences were noted. In particular, only the DARLAM model produced the meso-high ahead of the front associated with the evaporation of the precipitation. In addition, most models moved the cold front too rapidly, consistent with the lack of evaporating precipitation. One possible cause for these differences is the relative weakness in the warm air advection from over the Australian landmass in some of the models. The reason for this is not clear, but may be due to an improper specification of either the land characteristics or the initial soil moisture.

The precipitation generated by the various models differed more significantly, with the REMO/GESIMA model producing the most intense rainfall and the RAMS and ECMWF simulations producing the least rainfall. The MC2 simulation was the only model that produced significant post-frontal rainfall.

The cloud water mixing ratios of the various models at 600 hPa were generally similar, but with the REMO/GESIMA simulation producing a very narrow frontal cloud band while the ECMWF run had a weaker and broader frontal cloud band (consistent with its larger grid size). The prefrontal high ice clouds generated by the ECMWF model were generally weaker than in the MC2 and DARLAM runs.

Comparisons of the DARLAM and ECMWF simulated clouds with ISCCP observations showed that both models produced approximately correct cloud types in the presence of strong large-scale forcing. In weakly forced situations, however, the DARLAM model had the tendency to lift all cloud tops near the tropopause producing higher and optically thinner clouds than those observed by satellite, while the ECMWF model produced clouds that were also higher and mostly thicker than the observed ones.

5 km CRM Domain

The CRM domain was a 600 km (N-S) by 500 km (E-W) box with the south-west corner 150 km south and 150 km west of the ship MV Sprightly. Models taking part in the CRM simulation were the Canadian MC2 model, the German REMO/GESIMA model and the Colorado State University RAMS run at DAR. The CRM intercomparison was incomplete with only the Canadian MC2 model finalised. The REMO/GESIMA simulation had not been completed and there was a need to re-run the RAMS simulation.

Both the initial and boundary conditions for the 24 h simulation were obtained from the MC2 LAM (20 km horizontal resolution) simulation. It is found that a slightly larger domain than originally specified was needed for the successful simulation of this case (possibly related to the under-prediction of the temperature and strength of the warm-sector airflow in the MC2 LAM simulation). The utilization of higher resolutions and more sophisticated model physics has added some fidelity and structural details to the model cold front when compared to the LAM results. In particular, the evolution and intensity of the frontal structure, the arrival time of the front at the observing stations, the convection and the associated fine-scale pressure jump ahead of the front, the arrival time and intensity of the main precipitation features at the stations and the evaporation of precipitation in the sub-cloud layer compared favourably with observations. The MC2 simulation produced, within its limited domain, cloud types similar to the ones found in the ISCCP observations. Results from this simulation also performed slightly better than the LAM results in the tau-p distribution intercomparison with ISCCP data. On the down side, the detailed structure of the convective bands was not captured in this simulation (4 bands were observed but only 1-2 broader bands were simulated) even with a 5 km horizontal resolution. The boundary-layer clouds behind the cold front were also over-predicted. More complete cloud model results from GESIMA and RAMS will be available later this year and an in-depth intercomparison of results from these models will be performed.

300 km SCM Domain

The SCM domains were set in 300 km square boxes centred on Mt Gambier and the position of the ship MV Sprightly. The Canadian CCCma, the German ECHAM and the ECMWF SCMs were used in the intercomparison. The averaged hourly 2D and vertical advection from the DARLAM 20 km simulation were supplied at each model sigma level to force the SCMs.

Problems arose from the way in which the forcing fields were generated for the SCMs. When mean vertical velocity for the SCM box was near zero, no clouds would form in the ECMWF SCM model because it relies directly on the vertical velocity for cloud formation. For DARLAM, however, clouds would form in those grids where the sub-grid scale updraft velocity is positive giving a net cloud fraction for the SCM box. So the forcing using the box averages was specified ways that were not consistent for all models. The more appropriate methodology would have been to average the horizontal winds over the boxes around the SCM domain and recalculate the vertical velocity for the SCM box. The vertical advection for CCCma and ECHAM would then be calculated from this vertical velocity, so that the ECWMF, CCCma and ECHAM SCM would all be forced with the same advective tendencies.

A second problem identified by the intercomparison was that the ECMWF SCM required horizontal advection from DARLAM and vertical velocity (which it computed from its own vertical advection scheme) to force the SCM, whereas the ECHAM and CCCma SCM needed 3D advection supplied by DARLAM, which is not the same. The forcing of the ECHAM and CCCma SCM with the 3D advection led to a drying of the modelled atmosphere, and the same water vapour mass was only obtained when the ECHAM and CCCma SCM were nudged towards the observed values.

A preliminary conclusion from the SCM intercomparison is that the separation in liquid water path and ice water path is quite different in DARLAM box averages, CCCma, ECMWF and ECHAM SCMs. The optical depth - cloud top height diagrams from CCCma and ECHAM (taken over time as opposed to taken at a single time step for ISCCP) clearly show the dominance of high level clouds, which is not in agreement with ISCCP. It is possible that there are errors in both the model and the ISCCP climatologies.

When wind, temperature and moisture fields are nudged in the CCCma and ECHAM SCMs towards the corresponding fields from the driving model, the SCMs are able to capture the cloud amount and liquid/ice water path during the frontal passage, but the two models gave very different liquid/ice water paths and optical depths in the pre-frontal cirrus.

It was clear from the intercomparison that the prescribing of the forcing needs to be more rigorously specified. A new SCM experiment was designed that used (i) only the ocean as the lower boundary and (ii) was forced by the advection recomputed from "average" 300 km DARLAM values of the wind field to give the box mean vertical motion. Ulrike Lohmann is co-ordinating the re-running of the SCM simulations.

Summary of Results from the Intercomparison

Parameterization questions remain unresolved until after the new CRM analyses are complete.

2.2 Preliminary results from BASE, CASP II and PIDCAP

(Szeto/Stewart/Rockel)

BASE and CASP II

Two extra-tropical cyclone cases have been identified as excellent candidates for the second and third WG3 case studies. The first case is a rapidly deepening winter cyclone that occurred off the east coast of North America during February 26-27, 1992 and was observed in detail during the CASP II field project. This storm is typical of the systems frequently occurring over the western flank of the North Atlantic storm tracks. Complex interactions of various physical processes (eg., intense frontogenesis and cyclogenesis, convection, conditional symmetric instability (CSI) and mixed-phase cloud and precipitation processes) are common within these storms.

The second case is a weak Arctic oceanic frontal cyclone that occurred over the Canadian Beaufort Sea during late September, 1994 and was observed during the BASE field project. This case is characterized by weak storm dynamics, the dominance of ice-phase cloud processes, layered clouds and light precipitation. Such storm characteristics are believed to be typical of Arctic winter cyclonic systems.

These two cases are therefore representative of layered cloud systems over regions that are of special importance to climate simulations (namely, the mid-latitude storm track regions where eddy exchanges by cyclones play a critical role in the global circulation and the Arctic region where the climatic response to changes in external forcing are amplified). Both standard ground-based, as well as specific aircraft, measurements for these two cases are available for model validation.

Some cloud scale simulations of the CASP II case with the MC2 model had been carried out and the evolution and structure of the warm front (and its associated cloud and precipitation features) of this system were quite successfully replicated in the model. Some 15 km resolution MC2 simulations of the BASE case had also been performed but the results are only of limited success. Simulations of this case with higher model resolution and more sophisticated model physics are currently under way.

PIDCAP

The initial implementation plan for BALTEX foresees intensive observational periods in order to provide basic data sets for the analysis and diagnosis of synoptic-scale systems and extreme events in the BALTEX region. The first intensive observation period was PIDCAP, the BALTEX Pilot Study for Intensive Data Collection and Analysis of Precipitation.

The objectives of PIDCAP include:

The observational period for PIDCAP was August to November 1995. Precipitation data sets being compared include standard data (gauge land stations) and non-standard data (research vessel, specially equipped ships of opportunity, from satellite radiometer SSM/I, radar stations, and GPS stations). Modeling groups (eg. MPIfM, GKSS, DMI, and SMHI) are undertaking runs with different regional models for the same period in the frame of the European Union funded project NEWBALTIC.

During the period of PIDCAP both regional-scale precipitation associated with frontal systems and local convective rainfall connected to showers and thunderstorms occurred frequently. Strong precipitation of several tens of millimeters per day was observed at different stations in all four months. Storm conditions associated with strong rainfall (and also snowfall) were caused by several deep cyclones. The occurrence of two violent storm cyclones in November 1995 suggests the additional inclusion of this particular month into the observational period of PIDCAP.

A detailed description of the weather situations during PIDCAP can be found in the GKSS external report (GKSS 96/E/55) "Weather Patterns and Selected Precipitation Records in the PIDCAP period, August to November 1995" by H.-J. Isemer.

2.3 FASTEX and SALPEX

(Clough/Lean/Wratt)

FASTEX

An overview of the Fronts and Atlantic Storm Track Experiment (FASTEX) and its results was presented. The field experiment took place during January and February 1997 and was very successful. About 10 flights were made to study the mesoscale structure of frontal weather systems, most of them during a particularly active phase of February. These included 50 km resolution dropsonde observations, usually from the UKMO C-130, and simultaneous airborne Doppler radar observations from the NOAA P-3 and NCAR Electra aircraft (which, unfortunately, was unserviceable after IOP 11). The resulting observations show a wealth of mesoscale structure across a set of ocean weather systems more diverse and detailed than any previous observations. A central data repository for the project is based at Meteo France in Toulouse, while participant organisations are building up web pages of detailed aspects.

Comparisons of cross-sections based upon dropsonde observations with results from a 65km resolution version of the UKMO Unified Model for IOP 16 of FASTEX demonstrated the potential of the FASTEX cases for GCSS related studies. The simulations showed good overall agreement on weather system structure, particularly the indications of diabatically forced patterns that sometimes included melting-induced thermodynamic patterns. Further studies will concentrate on higher resolution modelling and diagnosis of the evolution of mesoscale structure and its relation to frontogenetic and microphysical factors.

SALPEX

A presentation was given describing the SALPEX'96 field campaign that took place over the period October 11 - November 4, 1996. This addressed orographic cloud generation and precipitation processes over New Zealand's Southern Alps. Data sets for case studies in pre-frontal and frontal conditions were collected from a cloud physics/dropsonde equipped research aircraft, ground-based radars, enhanced atmospheric soundings, raingauges, and river flow and surface observation networks. The intensive observing period included several instances of strong orographic enhancement of precipitation. The initial data processing has been completed, data sets have been distributed to participants, and results discussed at a SALPEX workshop in May 1997. Modellers interested in SALPEX'96 data should contact David Wratt at d.wratt@niwa.cri.nz.

2.4 Orographic Strategy

One of the primary objectives of the workshop was to develop an outline for GCSS activities in regard to orographic clouds. During the workshop the following issues were identified as relevant by the GCSS Working Group 3:

Following the meeting David Wratt prepared a discussion paper based on these issues. This paper is Appendix 2 of this report.

The report recommends that a strategy to improve the parameterization of orographic clouds should include:

1. Production of a review paper summarising current knowledge about orographic influences on clouds of relevance to GCSS, and providing details of knowledge gaps.

2. Determination of the global extent, type and frequency of orographically influenced clouds, using data from the International Satellite Cloud Climatology Project (ISCCP).

3. Consideration by each working group of GCSS of whether specific case study experiments and/ or analyses are required to address orographically generated clouds that fall within their remit.

4. Studies by individual working groups covering significant gaps identified in the previous task. Such studies should follow the normal GCSS pattern: analysis to determine and understand underlying processes; high resolution simulations of cloud systems validated by appropriate case studies; development or testing of cloud parameterizations using synthetic data from these high resolution simulations.

5. Working groups should include a section on orographic clouds in their routine reports to the GCSS Science Panel.

Working Group 3 is already considering cases with significant orographic forcing. These include cases from SALPEX and WISP. In addition, observational initiatives such as the recent work on Arizona orographic clouds by Roger Reinking's group is seen as providing key observations relative to Working Group 3.

The workshop recommended GCSS should encourage researchers undertaking field campaigns on orographic clouds and precipitation to include microphysical and radiometeric measurements for determining cloud radiative properties, as well as measurements for studying the evolution, dynamics and thermodynamics of the air flow generating the clouds.

3. Overview of issues arising from the workshop

The workshop identified and attempted to answer the following questions.

Table 1 shows a break down of the various cases being considered by Working Group 3. They are "typical" cases rather than "extreme" cases.

All the LAM and CRM simulations developed "rich" mesoscale structures. However, the impact of the mesoscale structures on the large scale was unclear at this point in time. The models of 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 life-time and 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.

The CFRP case study has allowed the workshop to develop a methodology for analysing mid-latitude frontal case studies to improve the cloud parameterization in these systems. The difference between the Working Group 3 approach and other GCSS Working Groups is that the LAM is used as an intermediate step between the GCM and the CRM. The LAM is at a resolution that can resolve the frontal structure and is also compatible with the ISCCP analyses. The ISCCP analyses are seen as an important validation tool for the models. The methodology is currently in the form of a draft paper that will be presented to the Science Panel Meeting.

In light of the GCSS objective to improve the representation of clouds in climate models through the use of field data and cloud resolving models, it becomes essential to determine the criteria that will be used to define and judge the improvements in the model cloud fields. In the case of mid-latitude storm clouds it would be ineffective and probably unrealistic to require climate models to reproduce the detailed cloud structures of individual storm systems as they are observed by field experiments around the world. Rather, climate models should be required to capture the mean properties and the time and space variability of the cloud structures produced by mid-latitude storms. This points to the need for a global survey of mid-latitude storm systems that will resolve the mean properties and the variability of the cloud structures associated with those systems.

One approach is to use a combination of satellite observations with conventional meteorological analyses in conjunction with the field experiment results to provide detailed descriptions of the dynamic and microphysical processes in individual storm cases. There is already strong evidence to suggest that the conveyor belt model is a universally applicable conceptual model. The survey strategy would provide a methodology of demonstrating whether the conceptual model can be used to define the mean properties of the frontal systems and to quantify the temporal and spatial variability of the mid-latitude frontal systems. The second component of the strategy is to use the conceptual model as a diagnostic tool to specify the mean properties and variability of the storm cloud structures produced by the present-day climate models. This strategy raises some important questions about conceptual models. For example, how well is the current conceptual model able to represent the mean spatial structure of the system, and is the conceptual model only valid for the mature cloud systems? This modelling diagnostic approach, together with the survey of data sets will define the deficiencies in the conceptual model and point out the improvements needed in the representation of storm clouds in climate models.

There are many fundamental aspects of mid-latitude layer cloud systems that are not understood. These include the interaction between frontal development, mesoscale cloud structure and the layer cloud evolution. These research track objectives will be assisted by the development of idealised case studies prepared by WG3.

4 CD Data Archive

A critical aspect of the working group's efforts is the synthesis of information on extra-tropical layer cloud systems in different regions of the world. As a step towards achieving this, CDs have been produced for the CASP II and BASE experiments. These initial working group CDs contain information on the special observations made of these systems, the operational observations, as well as model-derived products. The CDs furthermore have links to the relevant global data sets such as ISCCP that are crucial for the working group. The CDs are being distributed so that others can assess their usefulness, and suggest improvements in later versions.

It is expected that similar products will be produced for all the cases chosen for special investigation by the working group. This will then represent a lasting legacy of our working group efforts.

5. Deliverables from the Workshop

6. Next Workshops

7. Time Line

Nov 1997

Dec 1997

March 1998

Summer/Autumn 1998

ppendix 1 Conference Participants and Program

Conference Participants

Debbie Abbs
CSIRO Division of Atmospheric Research
Aspendale, Victoria, 3195
AUSTRALIA

Sid Clough
JCMM
University of Reading
PO Box 243
Earley Gate, Reading, Berkshire RG6 2BB
UNITED KINGDOM

Humphrey Lean
JCMM
University of Reading
PO Box 243
Earley Gate,Reading, Berkshire RG6 2BB
UNITED KINGDOM

Christian Jakob
ECMWF
Shinfield Park
Reading, Berkshire
RG2 9 AX
UNITED KINGDOM

Jack Katzfey
CSIRO Division of Atmospheric Research
Aspendale, Victoria, 3195
AUSTRALIA

Ulrike Lohmann
Canadian Centre for Climate Modelling and Analysis
University of Victoria
Victoria, BC V8W 2Y2
CANADA

Burkhardt Rockel
GKSS Research Center
Institute for Atmospheric Research
Max-Plank-Strasse
D-21502 Geesthacht
Germany

Leon Rotstayn
CSIRO Division of Atmospheric Research
Aspendale, Victoria, 3195
AUSTRALIA

Brian Ryan
CSIRO Division of Atmospheric Research
Aspendale, Victoria, 3195
AUSTRALIA

Ron Stewart
Climate Processes and Earth Observing Division
Atmospheric Environment Service
Downsview, Onterio M3H 5T4
CANADA

Kit Szeto
Climate Processes and Earth Observing Division
Atmospheric Environment Service
Downsview, Onterio M3H 5T4
CANADA

George Tselioudis
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025
USA

David Wratt
National Institute of Water and Atmospheric Research (NIWA)
Brodie Building
301 Evans Bay Parade
Kilbirnie, Wellington
NEW ZEALAND

GCSS Working Group 3 Workshop Program

Thursday 10th July
Session 1
Welcome/Workshop Objectives				Ryan/Stewart
CFRP Observational Overview				Ryan
CFRP ECMWF re-analysis					Jakob
CFRP 20km intercomparison				Katzfey
(DARLAM/ECMWF/MC2/REMO/RAMS)

CFRP-CRM (MC2)						Szeto
CFRP-CRM (REMO)						Rockel
CFRP-RAMS						Abbs
Discussion CFRP-CRM Intercomparison			Szeto/Rockel

CFRP-SCM (CCCMA/ECHAM/ECMWF)				Lohmann
Discussion of CFRP SCM/CSM Intercomparisons		Lohmann

CFRP simulations and satellite observations		Tselioudis
Discussion of CFRP model results and observations	Tselioudis

Friday 11th July
Session 1
Discussion of CFRP modeling Intercomparison		Ryan

Session 2
BASE and CASP II					Szeto
FASTEX							Clough/Lean
SALPEX							Wratt
PIDCAP							Rockel

Session 3
WG3 Workshop results					Stewart/Teslioudis
GCSS Orographic Strategy				Wratt

Saturday 12th July
Session 4
Discussion of paper outline and tasks			Ryan
WG3 future activities					Stewart
WG3 special data set					Stewart

Session 5
Meeting Summary and Recommendations			Stewart

Appendix 2: Towards a GCSS Strategy for Orographic Clouds

David Wratt (NIWA)

Towards a GCSS Strategy for Orographic Clouds

Introduction

This document answers a request from the GCSS Science Panel, for Working Group 3 to recommend a strategy for addressing orographic influences on clouds. These strategy suggestions grew out of discussions during a July 1997 WG3 workshop at Echuca, Australia.

Discussion

The goals of GCSS include improved understanding of the role of clouds in the climate system, and improved representation of clouds in climate models. Orographic cloud research tasks relevant to these goals include:

1. Identifying whether orographically generated or modified clouds significantly affect global or regional radiational forcing of the climate system.

2. Ensuring parameterizations of orographic clouds in climate models are adequate for quantifying such influences

3. Ensuring orographic cloud parameterizations in climate models provide adequate input for simulating the precipitation component of global and regional water cycles.

Research on the influences of mountains on weather and climate has a long history (Barry, 1992; Smith, 1979; Blumen, 1990; Kuettner, 1982; Bougeault et al, 1997). Process studies have addressed airflow deviation and gravity wave generation, mesoscale modifications to weather systems, cloud formation, and precipitation mechanisms. However, such studies have often been oriented towards weather forecasting rather than to improvements in global climate models. Recent orographic related work relevant to climate models has focussed particularly on momentum fluxes (through studies and parameterization of mountain drag, e.g. Lott and Miller, 1997), although some work is also underway on subgrid parameterization of orographic clouds and precipitation (e.g. Leung and Ghan, 1995).

Orographic mechanisms that generate stratiform and convective clouds are discussed by Banta (1990). Stratiform clouds form in areas of upward motion forced by flow of moist air over hills and mountains. Convective clouds may form if the air in the cross-mountain flow is potentially unstable (e.g. Cotton and Anthes, 1989), and can also be triggered by convergence and updrafts in thermally forced daytime upslope circulations. High cloud sheared off the top of orographically triggered cumulus may extend tens or even hundreds of kilometres downwind of mountains, and altocumulus clouds often occur in the rising portions of mountain waves many kilometres downwind of the mountains. Precipitation over mountains, and downward motion and evaporation can also lead to cloud-free and sometimes arid conditions downwind. Thus orographic clouds and the processes that generate them occur at both sub-grid and near-grid scales for climate and mesoscale models, and fall within the ambit of all of the GCSS Working Groups.

To the best of our knowledge there has been no systematic survey of the global extent and frequency of the various types of orographic clouds, and their significance to global or regional energy balances. In addition, any successful simulation at either global or regional scales of the various components of the hydrological cycle will be dependent on appropriate parameterization of orographically modified clouds and the resulting precipitation. This is because the influence of hills and mountains on precipitation (through both enhancement of precipitation and formation of rain shadows), and the contribution of mountain induced precipitation to river flows, are out of proportion to the extent of the globe covered by mountains.

Strategy Suggestions

To address the issues outlined above, we suggest the following strategy:

1. Production of a review paper summarising current knowledge about orographic influences on clouds of relevance to GCSS, and providing details of knowledge gaps.

2. Determination of the global extent, type and frequency of orographically influenced clouds, using data from the International Satellite Cloud Climatology Project1 (ISCCP).

3. Examination of the effects of orography on clouds in global models including comparisons with the ISCCP analyses recommended above. This work could include sensitivity tests of the effects on modelled cloud fields of varying the orography in global models.

4. Consideration by each working group of GCSS of whether specific case study experiments2 and / or analyses are required to address orographically generated clouds which fall within their remit.

5. Studies by individual working groups covering significant gaps identifed in the previous task. Such studies should follow the normal GCSS pattern: Analysis to determine and understand underlying processes; high resolution simulations of cloud systems validated by appropriate case studies; development or testing of cloud parameterizations using synthetic data from these high resolution simulations.

6. Working groups should include a section on orographic clouds in their routine reports to the GCSS Science Panel.

7. GCSS should encourage researchers undertaking field campaigns on orographic clouds and precipitation to include microphysics and radiometer measurements for determining cloud radiative properties, as well as measurements for studying the meteorological processes generating the clouds.

1 GCSS WG3 members are already discussing actions to address this suggestion.

2 Past or planned measurement campaigns such as ALPEX (Kuettner, 1982) , PYREX (Bougeault et al, 1997), MAP (Kuettner, 1995), SALPEX (Wratt et al., 1996), WISP (Rasmussen et al., 1992), Arizona Winter Storms (Klimowski et al., 1997) may provide appropriate data for case studies. GCSS WG3 is documenting and examining cases from some of these studies.

References

Banta, R.M., 1990: The role of mountain flows in making clouds. Atmospheric Processes in Complex Terrain, W. Blumen, Ed., Meteorological Monographs Vol 23, No 45, Amer. Meteor. Soc., 229 - 283.

Barry, R.G., 1992: Mountain Weather and Climate. (Second Edition). Routledge. 402 pp.

Blumen, W. (Editor), 1990: Atmospheric Processes over Complex Terrain. Meteorological Monographs Vol 23, No 45, Amer. Meteor. Soc., 323 pp.

Bougeault, P., Benech, B., Bessemoulin, P., Carissimo, B., Jansa Clar, A., Pelon, J., Petitdidier, M. and Richard, E., 1997: PYREX: A summary of findings. Bull. Amer. Meteor. Soc. 78, 637 - 650.

Cotton, W.R. and Anthes, R.A., 1989. Storm and Cloud Dynamics. Academic Press, 882 pp.

Klimowski, B.A., Becker, R., Betterton, E.A., Bruintjes, R.T., Clark, T.L., Hall, W.D., Kropfli, R.A., Orr, B.W.,Reinking, R.F., Sundie, D. and Uttal, T., 1998: The 1995 Arizona Program: Toward a better understanding of winter storm precipitation development in mountainous terrain. Bull. Amer. Meteor. Soc. (accepted).

Kuettner, J.P., 1982: An overview of ALPEX. Ann. Meteor., 19, 3 - 12.

Kuettner, J.P., 1995: Plans and experiment design for the coming international Mesoscale Alpine Programme (MAP). Preprints, Seventh Conf. on Mountain Meteorology, Breckenridge, CO, Amer. Meteor. Soc., 102 - 107.

Leung, L.R. and Ghan, S.J., 1995: A subgrid parameterization of orographic precipitation. Theor. Appl. Climatol. 52, 95 - 118.

Lott, F and Miller, M.J., 1997: A new subgrid-scale orographic drag parameterization: Its formulation and testing. Q.J.R. Meteor. Soc. 123, 101 - 127.

Rasmussen, R.M., Politovich, M.K., Marwitz, J.D., Sand, W., McGinley, J.A., Smart, J.R., Pielke, R.A., Rutledge, S. , Wesley, D., Stossmeister, G., Bernstein, B.C., Elmore, K., Powell, N., Westwater, E.R., Stankov, B.B. and Burrows, D. 1992: Winter Icing and Storms Project (WISP). Bull. Amer. Meteor. Soc. 73, 951-974.

Smith, R.B., 1979: The influence of mountains on the atmosphere. Advances in Geophysics, B. Saltzmann, Ed., Vol 21, Academic Press, 87 - 230.

Wratt, D.S., Ridley, R.N., Sinclair, M.R., Larsen, H., Thompson, S.M., Henderson, R., Austin, G.L., Bradley, S.G., Auer, A., Sturman, A.P., Owens, I., Fitzharris, B., Ryan, B.F.and Gayet, J.-F., 1996: The New Zealand Southern Alps Experiment. Bull. Amer. Meteor. Soc. 77, 683 - 692.

Case Breakdown3

Case Study	Field Obs.	Diagnostic Study	ISCCP	  Re-analysis
CFRP		Y			Y		Y		Y
BASE		Y			Y		Y		N
CASP II		Y			Y		P		N
FASTEX		Y			N		N		N
SALPEX		Y			N		N		N
WISP		Y			?		?		?
PIXAP		Y			Y		N		Y
TRE GCI		N			N		N		N

Case Studies	Climatology	Forcing(W/S)	Precip(L/H)	Sfc	Mesoscale
CFRP			Y	W->S		L		L/O		Y
BASE			N	W		L		L/O/I		N
CASP II			N	S		H		L/O/I		Y
FASTEX			N	?		?		O
SALPEX			N	?		H		L/O/M
WISP			N	S		H		L/M		Y
PIXAP			N					L/O/M
TRE GCI			N					L/O


Case Study	LAM		CRM		SCM
CFRP		Y		Y		Y
BASE		Y		Y		N
CASP II		Y		Y		N
FASTEX		Y		N		N
SALPEX		Y		N		N
WISP		?		?		?
PIXAP		Y		N		N
TRE GCI		N		N		N

3 In the tables the following notation Y(yes), N(no) and P(part information available) is used in classifying the case studies. Forcing is classified as W(weak) and S(strong) and Precipitation as H(heavy) or L(light). Sfc(surface conditions) are classified as L(land ), O(ocean), I(sea ice) and M(mountain). The observed presence of mesoscale stucture in the case studies ( Mesoscale) is indicated by Y(yes) and N(no).