PIRCS

July 22, 1999
IUGG General Assembly, Birmingham, U.K.

Attending: Raymond Arritt, Jens Hesselbjerg Christensen, Michael Fox-Rabinovitz, Filippo Giorgi, Richard Jones, Jack Katzfey, Brent Lofgren, Daniel Luethi, Bennert Machenhauer, John McGregor, John Roads, Markku Rummukainen, Huqiang Zhang.

After introductions and a buffet dinner (courtesy of EPRI) Arritt gave an overview of the agenda including brief plans for analysis and publications from Experiment 1b. It was agreed that while consistency of the regional models with observed large-scale fields is a necessary point, the analysis should focus mainly on other issues.

Some preliminary comments were made on possible followup studies. Machehauer pointed out the need to address questions of predictability, requiring true "climate" runs (as opposed to specific episodes) or ensembles. Jones suggested a second integration with different initial conditions. It was generally agreed that followup studies such as sensitivity experiments should be done as individual efforts and not as a part of the PIRCS intercomparison.

Analysis of Experiment 1b

Discussion then turned to analysis of the 1993 case. Most of the group felt that the analysis should focus on the hydrologic cycle. Katzfey suggested examination of the frequency distribution of precipitation, for which we could get some information from the 3 hourly data. Following from this suggestion, Roads recommended looking at extremes and distributions. Fox-Rabinovitz suggested looking at diagnostics related to the hydrologic cycle such as cloud fraction, precipitable water and vertically integrated fluxes. Katzfey pointed out the utility of satellite data (such as ISCCP) to provide cloud optical depth and cloud water for comparison to the models. Comparison of surface fluxes also was recommended. Jones suggested looking at the question of moisture recycling; i.e., the relative roles of evaporation versus horizontal moisture flux in producing precipitation. Roads suggested looking at the diurnal cycle of precipitation and its association with the low-level jet.

Rummakainen pointed out the importance of critical combinations of variables and variable extremes. It could be instructive to see whether regional models can produce these. Machenhauer noted that surface drying that has often been found in climate scenarios and suggested this could be examined.

Proposed PIRCS Experiment 2

The second major topic of discussion centered around possibilities for a future experiment or experiments. Roads presented a proposal for simulating the El Nino episode of 1997-99. Impacts on Florida and California are of particular interest. The regional models could be embedded within a prediction instead of analysis. Ideally resolution should be 25 km in order for the models to represent terrain effects in California, but this would almost certainly require a domain smaller than the continental U.S. Fox-Rabinovitz asked whether the models should use a free running land surface scheme or prescribed soil moisture. Christensen asked what criteria will be used for validation. The ENSO cycle is driven by the large scale, but are there additional aspects to be considered? How do we quantify how accurate regional models have to be?

Rummukainen asked about the source of driving data. Roads responded that this could be taken from analyses or from forecasts, e.g., short term seasonal predictions. Christensen asked whether an ensemble approach might be useful, while Fox-Rabinovitz asked what is meant by prediction mode. Roads responded that there are many ways of driving the regional models. We can do "pure" prediction as well as pure analysis, or any range between. Examples include analyses with observed SST, or predicted SST, etc. We could use prescribed SST and NCEP analysis, then compare with a prediction having some skill (e.g., IRI). McGregor asked whether an AMIP style approach could be useful.

Giorgi pointed out the need to clarify the objective of the experiment and that Roads' proposal would fit an objective of seasonal prediction. We need to consider what is a natural followon from Experiment 1. The group could do a multiyear run, and look at other mesoscale effects (e.g., topography, western U.S., winter) in more of a natural extension than seasonal prediction. The possibility of an enhanced CO₂ scenario was briefly discussed but it was decided that meaningful runs would have to be too long to be practical.

The topic of verification data for the 1993 experiment was briefly revisited. Fox-Rabinovitz mentioned that his group has run the 1993 experiment using their stretched grid model in data assimilation mode with 60 km resolution over the U.S. These results include comprehensive diagnostics and are available for use as verification. Roads agreed that this could be useful for the second paper and asked whether it is it better than the NCEP reanalysis. Fox-Rabinovitz replied that the stretched grid analyses are a bit better than the standard (coarser resolution) GEOS (not NCEP) analyses, according to his evaluation for summers of 1988 and 1993. Giorgi suggested comparing the 1988 and 1993 model-predicted precipitation to reanalysis precipitation. There was brief discussion of the ways in which reanalysis precipitation is affected by the "spinup" problem.

Roads said we need to ask what are people willing to do. Roads suggested reconsidering what data should be archived as the data volume can be a burden on some participants; for example, whether it might be sufficient to save monthly averages. Fox-Rabinovitz felt there is a clear need to save 6 hourly values in order to resolve the diurnal cycle and mesoscale features. Arritt pointed out that data volume could be reduced by saving only certain levels and pre-computing some of the required quantities such as heights of standard pressure surfaces. Katzfey recommended we save and examine cloud fields (fraction, optical depth).

Stretched grid modeling

Fox-Rabinovitz presented an update on stretched grid modeling. His group is working on developing the stretched grid version of the new NASA/NCAR model. This version will be used for regional climate studies. Several groups are doing stretched grid modeling with about 3-4 groups aggressively involved. This could be a new component for PIRCS. Fox-Rabinovitz is running the model both in "simulation mode" and "assimilation mode." For the PIRCS experiment, they are running in assimilation mode with data withheld over the U.S. He will submit the 1993 results for the PIRCS comparison.

NCAR Community Regional Climate Model

In the absence of Ruby Leung, Arritt briefly discussed plans for a possible regional modeling option within the NCAR MM5, presented at a meeting on 24 June 1999 at NCAR. The modified OSU land-surface scheme (similar to the Eta model) is being added to make MM5 more suitable for extended runs. Giorgi mentioned that the relatively simple standard MM5 radiation scheme could be a problem as well as the land surface.

Geographical regions for Experiment 2

The group returned to recommendations for PIRCS Experiment 2, focusing on the region or regions to be simulated. This was motivated by comments by Christensen that while the original PIRCS goals addressed aspects of interest to all, not just U.S. participants, the project seems to have progressed to a U.S. focus. This could discourage participation from groups outside of the U.S. Fox-Rabinovitz pointed out that modelers once were preoccupied with Indian monsoon and that now ENSO over the U.S. is considered an interesting event.

Christensen pointed out that the program needs overall scientific objectives and must also be of benefit to individual groups; do we learn anything new? We need to revisit what is the fundamental reason for doing intercomparisons and stress the new science. Giorgi felt that a natural next step is to do multi-year perfect boundary condition experiments and for this reason likes the 1997-99 ENSO idea.

Arritt brought up the possibility of doing the ENSO for two regions. North America could be run as an extension of Experiment 1 with a second region being simulated for contrast. Giorgi asked what this region might be, and the possibility of Brazil was mentioned. Other regions were also mentioned. It was pointed out that there already lots of people doing regional climate simulations for East Asia. Africa was brought up as a possibility. After discussion, a consensus emerged that South America would probably be the most appropriate region to simulate in addition to (or instead of) North America. This is partly because there are some similarities to North America (e.g., the low-level jet as a frequent mesoscale feature) and data from other studies could be used for validation. Christensen reiterated the need to keep in mind the ultimate goal of the project and that it is hard to use separate simulations as an ensemble. We could approach the two regions as 2a and 2b. Giorgi pointed out the need to make contacts with other investigators and institutions; e.g, Carlos Nobre, PACS, IAI. Roads mentioned funding and suggested we collaborate more closely with AMIP.

Arritt volunteered to summarize the discussions into a plan for PIRCS Experiment 2, with feedback from the rest of the Iowa State group and from the PIRCS community at large.

Next PIRCS meeting

The topic of the next PIRCS meeting was brought up. The most likely venues are the AGU Fall Meeting in San Francisco (December 1999) or the AMS annual meeting in Long Beach (January 2000). Some felt we should meet at both places.