Regal Riverfont Hotel, St. Louis,
June 10, 1998
Attendees: Chuck Hakkarinen (EPRI); Song-You Hong (NCEP); Bill Lapenta (NASA-Marshall); Glen Liston, Roger Pielke (Colorado State University); John Roads (Scripps); Robert Wilby (NCAR); Ray Arritt, Bill Gutowski, Renato Silva and Gene Takle (Iowa State University)1. Introduction and agenda
1. Introduction and agenda
The meeting was preceded by a buffet dinner sponsored by EPRI. Gutowski introduced the meeting agenda, which focuses primarily on results from PIRCS Experiment 1a and plans for their publication. He also thanked EPRI (through Chuck Hakkarinen) for sponsoring the meeting, and NOAA's Rick Lawford for logistical support.
2. Preliminary results from the models
2.1 Precipitation trends
Gutowski showed preliminary analyses by the Iowa State group that focus on a portion of the Upper Mississippi River basin between 37-47 N latitude and 89-99 W longitude. Considerations in selecting this region include: (1) the region will provide a contrast between 1988 and 1993, since it includes the peak precipitation area of the 1993 flood; (2) the area is relatively large from the standpoint of a regional model (of order 400 grid points) but relatively small in terms of a GCM or the NCEP/NCAR Reanalysis; and (3) the region does not extend into Canada, so that U.S. data sets (such as the gridded hourly precipitation data set of Higgins et al. 1996) can be used for comparison to the models.
Gutowski compared results for total precipitation (convective and stable) from seven of the PIRCS models to observed precipitation from the Higgins data set. The models generally are able to replicate wet and dry spells in the observations, but the amounts may be underpredicted or overpredicted. A simple "consensus" (arithmetic mean of the participating models) tracks the observed precipitation rather well. Liston brought up the point that the Higgins data are on a relatively coarse resolution and that we should generate a higher-resolution analysis for comparison to the models. The Iowa State group noted that they had already begun to develop such analyses. The Higgins data were used because of the short lead time for this meeting (models results having been provided as little as two days before).
Evaluation of model performance for different types of events can give a perspective on how the models handle different types of forcing (e.g., large-scale versus convective or mesoscale). Differences between convective and stratiform precipitation in the models can also give information on how the models respond to different types of forcing. To this end, Takle described the synoptic characteristics of specific precipitation episodes during PIRCS 1a. The early part of the period was dominated by large-scale synoptic events; all models captured the timing of these. A dry period around Julian day 159 also was represented by all models as was a convective event around day 160. Days 165-180 had small convective events popping up across the upper Mississippi Basin region, with all models capturing something, though timing may not always be precise. Pielke noted that the models differ on whether precipitation is represented as convective or stratiform. All models give convective precipitation dominating on days 165-180, though Pielke mentioned that all miss the dry period at day 187. A handout was distributed with some of these results.
The two versions of RSM differ in the relative representation of convective and stratiform precipitation for several episodes. The Scripps version of the RSM includes the physics from the operational, global spectral model, while the NCEP version has modifications to the physics introduced by Hong. Liston commented that all the models apparently capture the lateral boundary forcing as indicated by the timing of specific precipitation episodes. Hakkarinen pointed out that the models perform somewhat more poorly toward the end of the period, and several in the audience mentioned that this may point to the implications of soil moisture feedbacks.
Gutowski showed maps of cumulative precipitation from four models. All have a dry area in the lower Mississippi River basin that does not appear in the observations. The nocturnal fraction in most of the models exhibits strong spatial gradients that may not be physically realistic, and the models underrepresent the nocturnal maximum in the central U.S. Hong pointed out that convection schemes generally tend to initiate precipitation during the daytime. RAMS seems to give the most realistic nocturnal maximum in the central U.S., though its precipitation tends to be too low. Wilby asked whether the spatial patterns should be looked at separately for convective or stratiform episodes. Liston mentioned that to evaluate the daytime versus nocturnal precipitation there are several factors that must be accounted for; e.g., there must a significant amount of precipitation and not just a few isolated events. Hakkarinen mentioned that the RAMS precipitation appears to most strongly aligned with the underlying terrain and coastlines.
2.2 Low-level jets
Arritt presented analyses of the low-level jet in several of the models based on the criteria of Bonner (1968, Mon. Wea. Rev.). In 1988 the profilers had not yet been deployed so there is not a good source of observed data. Therefore the NCEP reanalysis is being used for preliminary evaluations. Comparison with profiler observations shows that the reanalysis itself has biases, including too many LLJs (Bonner's criterion 1) in the southern Great Plains and too few in the northern Great Plains. Incidence of strong LLJs (Bonner's criterion 3) is severely underrepresented in the reanalysis, perhaps because the reanalysis terrain is too coarse. It is noteworthy that mesoscale models tend to produce more LLJs than the reanalysis. For 1988 the region of high LLJ frequency tends to be to north of the climatological distribution, as expected in a dry year.
MM5 gives a pattern of LLJ frequency that is similar to the reanalysis, but with generally higher incidence of LLJs. DARLAM does not extend quite as far north in maximum LLJ frequency and tends to get more strong LLJs. RAMS has a relatively high incidence of LLJs and a tendency for spatial patterns of LLJ occurrence to follow the Bermuda high. Several models show a Y-shaped or "branched" distribution with swaths of high LLJ incidence to the west along the foothills of the Rocky Mountains and to the east along the periphery of the Bermuda high. RAMS output was provided on pressure levels and results may be different if data were used on the native levels of the model. The NCEP version of the RSM gives a somewhat lower frequency of LLJs than the other models, extending rather far north (again, as might be expected in a dry year).
2.3 Additional diagnostics
The Iowa State group has recently begun studying whether and how the regional models generate MCCs. This uses analysis of mesoscale anomalies based on the method of Jones et al. (1995, Quart. J. Roy. Met. Soc.). An example was given of (non-PIRCS) MM5 results from 1993 at 250 mb and 600 mb. There was vertically-aligned cyclonic convergence at 600 mb and anticyclonic divergence at 250 mb that compared well with where the predicted precipitation occurred. Pielke noted that it would be of interest to compare different convective parameterizations in the same model.
Much of the discussion centered around the need to obtain additional data for comparison to the model results. Liston recommended that more work be done on bringing together data sets such as max/min temperature and solar radiation. He asked whether surface radiation could be used from the NCEP reanalysis. Roads mentioned that the reanalysis radiation has errors, but that it still could be useful to compare how the regional models differ from the reanalysis. Liston has processed "Summary of the Day" max/min temperatures in GRADS station format, and he showed maps of the max/min temperatures. Radar observations may be useful at least in a qualitative sense although quantitative rainfall estimates are not available for the PIRCS 1a case. Hakkarinen asked whether there are more details available on the precipitation especially for 1993. Alternative data sources may be available such as Bureau of Reclamation, USGS, Oklahoma Mesonet, and VEMAP. FIFE may have radiation and surface flux data for 1988. Lapenta mentioned that Baldocchi's group at Oak Ridge has long-term radiation data. Global Summary of the Day stations have surface pressure and humidity data.
Pielke suggested calculation of thickness values and comparison with NCEP reanalysis. Roads mentioned that NCEP equitable threat and bias scores for precipitation thresholds could be used to evaluate models. In this vein, Gutowski reported that the Iowa State group is trying to compute histograms of precipitation intensity from cooperative station data. Liston says that surface RH is available on global summary of day; Wilby says these include max/min RH. Liston says first-order Summary of the Day includes additional data (about 300 stations). With regard to the NCEP/NCAR Reanalysis, Hong noted that upper atmosphere variables are fairly accurate but errors in 500 mb geopotential height can be as high as 20 m in southeastern US for 1988. It was pointed out that reanalysis variables in the lower atmosphere are strongly influenced by model physics.
4. Publication of PIRCS results
Gutowski stated that the Iowa State group will do continued analysis and will set up a web page to display the results. After some discussion a consensus was reached that the web page will not be passworded, but will not be publicly announced or linked from the main PIRCS page.
The group as a whole will begin working on papers that report PIRCS. The first will be a general description of the results and procedures, including some examples of analysis products but not in-depth analysis. Shortly following would be a "science paper" with more detailed analysis focusing perhaps on how precipitation is coupled (or not) with mesoscale dynamics. There is a need to show what is the added value of regional models. For example, GCMs can produce low-level jets but still do not properly represent the diurnal cycle of precipitation. Roads mentioned that little attention has previously been given to diurnal variations in the reanalysis.
Liston questioned whether we are ready to release the data sets to others and that we should take time to make sure that the models are at a credible stage. Takle mentioned WGNE and CLIVAR are looking to PIRCS to show added value from regional models. Gutowski noted that PIRCS has ongoing cooperation with MERCURE and that we should proceed in a timely manner in order to share information with them. Hakkarinen mentioned the push for regional impacts for the IPCC TAR and the U.S. national assessment of the state of science of impacts of climate change. New GCM runs also will be done with intent of serving as drivers for regional models. Roads says we need to push for the first paper but for the second we need more of a critical mass of participants, encouraging others to collaborate with us. Roads mentioned Fall AGU as a target date for another meeting.
Pielke suggested the first (overview) paper be submitted for the GCIP special issue of JGR, for which manuscripts are due September 1. Roads mentioned that this would be highly visible. Such a date would be reasonable for the overview paper but the "science" paper would have to be later. Roads suggested a focus on aspects of the hydrological cycle, e.g., evaporation and runoff, and distribution between convective and nonconvective precipitation. Liston recommended that the overview paper include results for upper level heights. Roads says we need to show differences from reanalysis. We could also show differences between 1993 and 1988 in the LLJ, perhaps a sample from one or a few models.
List of acronyms and abbreviations
EPRI - Electric Power Research Institute
GCIP - GEWEX Continental-scale International Project
GEWEX - Global Energy and Water cycle Experiment
LLJ - Low Level Jet
MCC - Mesoscale Convective Complex
MERCURE - Modeling European Regional Climate: Understanding and Reducing Uncertainties (research program sponsored by the European Community)
NCAR - National Center for Atmospheric Research
PIRCS - Project to Intercompare Regional Climate Simulations
TAR - Third Assessment Report (Intergovernmental Panel on Climate Change)
WGNE - Working Group on Numerical Experimentation (World Meteorological Organization)
|Copyright/Trademark Legal Notice|