Questions answered from GMOS session 1

Gridded MOS wind forecast, 9/30/06 at 5:00 a.m. EDT

Hi folks --

Questions that came up yesterday in Gridded MOS session that I wasn't sure of the answer to, and the correct answers, appear below. Thanks to Paul Dallavalle for help on most of these.

1. How big an area does the "lapse rate" calculation have to cover to get to the "60-100 stations"? Isn't this a problem where data is sparse? How does the area covered here related to the OI 30 gridpoint radius of influence?

A radial search is done around each of the 8,500 stations used for the single station part of Gridded MOS until either 100 stations is obtained or the search goes out to the maximum analysis radius (150 km or 30 grid boxes). The "60" figure is not a minimum, but rather a representative average of the number of stations that ultimately get used. In sparsely populated regions, this number will obviously be considerably less.

2. How about situations where synoptic situation might result in different lapse rates over different parts of the 60-100 station area, and averaging will give a bad result (e.g. cold-air damming, warm surge west of mts). Is averaging of marine layer and non-marine layer areas perhaps resulting in the relatively poor results for temperature in the GMOS graphics shown in the presentation from Monterey?

Yes to both. In general, the problem is that we are applying an "isotropic" (symmetric in all directions, in a circle) method to a "anisotropic" (not symmetric in all directions) situation. To deal with this, MDL is working on shrinking the search radius so that the assumption of symmetry/circularity of the region with similar "lapse rates" doesn't do as much damage in these circumstances.

3. In discussing the lapse rate determination process, I said that the data determines the lapse rates. The question arose: what about times when there is a diurnal flip (cooling with height daylight, warming with height during nighttime if no winds allow for radiational cooling)? How well will it do? Isn't it dependent on the single station MOS used for GMOS containing that information?

If the "diurnal flip" in lapse rates exists in the single station data being interpolated to the Gridded MOS 5-km grid, it will exist in the grid as well. This thus depends totally on the single station data accurately reflecting radiational cooling when winds calm down. I did find an example that seems to illustrate that this is working at least to some extent in central NY state, attached (see Elmira NY v. Binghamton NY).

4. How do I explain to my forecasters large differences between values from GMOS and MOS for the same location? (I suggested it was the result of different developmental samples between GMOS and MOS.).

According to Paul, this is not because of differences in times used for developmental samples. The analysis DOES make a difference, in that for any station, the nearest grid box to station is used. Additionally, buried deep inside is an "ensemble forecast", in that the previous cycle's forecast is included in the current cycle (2/3 weight given to current forecast, and 1/3 for prior forecast (12 hrs. old)). This is done for T, Tmax, Tmin, Tdew ONLY.

5. Is Tim Barker's MatchGuidance (here, traditional MOS) smart tool not as good as GMOS?

Paul stated that Gridded MOS is merely another approach we're trying to get gridded forecasts, not a value judgment on MatchGuidance.

6. Some folks wanted to know which specific stations were being used in their CWA for GMOS. Is there a place where that is stored? Can I get the info?

MDL is working on parsing out the list of Gridded MOS sites (the "GMOS dictionary) for each WFO now, as requested at an ISST meeting over the summer. We'll set up a webpage (or MDL will) once that information is available.

Let me know if you need further information or have more question.

Thanks for your interest in Gridded MOS!

Bill