Ed Szoke, 325 Broadway, R/E/FS1, Boulder, CO 80303.
About a year ago one version of the RAMS model, a parallelized version known locally as the Scalable Forecast Model (SFM, Hart et al., 1996), was made available to the local (Boulder, BOU) WFO via the Advanced Weather Interactive Processing System (AWIPS, Wakefield, 1998) workstation, enhancing the usability of the model to the forecasters. The SFM is run at 10-km grid spacing, initialized with LAPS, with boundary conditions provided by the Eta model, and run out to 18 h four times per day on a computer at the BOU WFO. The model uses the Schultz (1995) explicit microphysics scheme.
An explicit simulation is clearly an issue of some concern for convective simulations at a grid spacing as large as 10 km, and with this in mind, we believe that the best potential for adding value to a forecast in convective scenarios would include the early stages of events forced by topography (diurnal and/or organized upslope) or some other organized feature. Thus, the forecaster might be able to successfully use the SFM model to predict when and where thunderstorm development might occur, for example, on many days where terrain heating might drive a diurnal upslope flow. As expected, on many days convection in the SFM tended to organize more than would occur in actuality given the available vertical wind shear, with larger scale, more organized and persistent outflow boundaries produced by the model. There were, however, a few remarkably accurate forecasts on a limited set of days when there was sufficient vertical wind shear and instability to support supercell storms. The four days of successful simulations, out to 10 h and beyond in some cases, occurred during the 1999 convective season in the BOU WFO forecast area. Here, we mainly show one of these cases, and try to discern whether such successful simulations may represent some real forecast skill for the model in a type of event where the actual scale of convection (a persistent, relatively large updraft that occurs with a supercell storm) most closely matches the model's natural scaling when using 10-km grid spacing, or whether these solutions were just a matter of "shear" luck. We recognize the limitations present with our rather small sample, and indeed had hoped to collect more supercell cases before the deadline for this paper, but thus far it has been a rather pathetic severe weather season in the year 2000 in northeastern Colorado.
It is certainly not surprising to have fewer model predicted storms than observed, since the actual scale of updrafts with individual cells on lower shear days will be far less than what could be captured with a model employing a 10-km grid spacing. The amount of organization in the model appears to increase some with vertical wind shear, as would occur in the real world, and as shown in this case results in a rather poor solution after a few hours. When this model was introduced to forecasters at the BOU WFO, this type of behavior was explained, and it was emphasized that the best manner to use the model for forecasting summer convection might be: a) in the general location and timing of initial convection, b) where the model ends up producing and not producing storms (generally as occurred for this case), and c) realize that once storms form processes such as gust front interactions, etc., will be on scales that the model cannot forecast in its current grid spacing.
Output from the 1500 UTC 10 June 99 run of the SFM will be examined for this case. The model did a good job of forecasting the location and approximate timing of the initial cells that developed along the Front Range around noon local time (1800 UTC), as seen in the 4 h forecast in Fig. 4. The main, long-lived storm on this day evolved from the cell west-southwest of Denver (DEN) in Fig. 4, and over the next several hours moved east, first right across Denver International Airport (DIA), producing several cm of hailfall, and more severe hail reports as well as a brief tornado on its way to near the Kansas border by 0300 UTC 11 June.
A sequence of SFM prediction beginning at hour 6 and extending through hour 12 are presented in Fig. 5. Although the details of the observed reflectivity are impossible to discern from Fig. 5, it is apparent that the SFM did a remarkably excellent job with the location and timing of the main storm as it trekked across eastern Colorado. At 6 h (Fig. 5a) and 9 h (Fig. 5b) the model forecast is almost exactly on top of the observed storm, and by 12 h is only slightly behind the storm position. In terms of reflectivity, the SFM consistently produced a core of greater than 50 dBZ echo, while the observed core was stronger, generally exceeding 60 to 65 dBZ, but this is not necessarily unexpected with the 10-km SFM grid spacing. It is interesting that at 12 h the SFM forecast storm has not accelerated past the observed storm (as in the example in Fig. 2), but is actually slightly behind the actual storm. Given that the storm motion is also predicted correctly, which is off the hodograph in Fig. 3, the implication may be that the SFM is indeed predicting a supercell-type storm in this case, so this excellent prediction may in fact be more than just "shear luck."
Figure 5. As in Fig. 4, but for sequence of SFM predictions from the 1500 UTC 10 June 99 run, for 6 h (a), 9 h (b), and 12 h (c). Arrow points to the observed reflectivity core in each image. The 10 dBZ SFM reflectivity contour is highlighted with a thin line, and the 50 dBZ contour with a thick line.
Two other consecutive supercell days (25 and 26 June 99) were somewhat different, with storms forming at the northern edge of the domain in southeastern Wyoming and moving east-southeast into extreme northeastern Colorado. For these two days the model forecasts were not as accurate as on 10 June, but still had the general idea of area of storm formation and movement. On these two days the rather small domain of the model likely influenced the accuracy of the forecast, since the echoes developed near the northern edge of the SFM domain. For such a case the prediction will be less influenced by the LAPS analyses and more by the boundary conditions provided by the eta model. These cases from 25-26 June 99, and hopefully some from the 2000 storm season, will be shown at the conference.
While we have shown a remarkably accurate prediction with the 10 June 99 case, the SFM had rather mixed reviews by the operational forecasters at the BOU WFO. As noted by Szoke et al. (1998), a significant reason for somewhat limited use of the model is the wide variation in the verification of the SFM forecasts, both in the winter but especially in the summer convective season. This is undoubtably in significant part a result of the fairly gross 10-km grid spacing, which results in the problems noted in our first example in Fig. 2. Although limitations of the model and discussion of when it might be most useful have been presented at forecaster workshops, it takes time for forecasters to discern when a prediction can be believed (as in the 10 June case) and when it is likely to go awry rather quickly. Indeed, one of the authors worked a shift on 11 June 99 and was only tempted to pay close attention to the SFM forecast after reviewing the success on the previous day. We hope to decrease the grid spacing in the future, either with the SFM or whatever local model is run, in concert with a finer resolution LAPS initialization, which should improve the overall utility of the local model.
Hart, L., C. Baillie, M. Govett, T. Henderson, and B. Rodriguez, 1996: FSL's Scaleable Modeling System: A tool for the parallelization of NWP models. Preprints, 11th Conf. on Numerical Wea. Prediction. Norfolk, VA, Amer. Meteor. Soc., 443-446.
Szoke, E.J., J.A. McGinley, P. Schultz, and J.S. Snook, 1998: Near operational short-term forecasts from two mesoscale models. Preprints, 12th Conf. on Numerical Weather Prediction. Phoenix, Arizona, American Meteor. Society, 320-323.