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Gridded Model Output Statistics: Improving and Expanding 6A.6 GRIDDED MODEL OUTPUT STATISTICS: IMPROVING AND EXPANDING Kathryn K. Gilbert*, Bob Glahn, Rebecca L. Cosgrove1, Kari L. Sheets, Geoffrey A. Wagner2 Meteorological Development Laboratory National Weather Service, NOAA Silver Spring, MD 1National Center for Environmental Prediction, National Weather Service, Camp Springs, MD 2Wyle Information Systems, McLean, VA 1. INTRODUCTION Since the original implementation, gridded MOS has undergone several adaptations to improve For years, National Weather Service (NWS) consistency as well as regional applicability. MDL forecasters have used Model Output Statistics developers have welcomed assessments of grid- (MOS) guidance (Glahn and Lowry 1972) as an ded MOS from local forecasters in order to make aid in producing text forecast products issued to adjustments to provide guidance which is both the user community. However, the methods fore- representative of the local area, and compatible casters use to generate products have changed with the process for generating forecasts for because of requirements to produce the official NDFD. Some of the improvements resulting from NWS 7-day forecasts on high-resolution grids. forecaster feedback include: removal of unrepre- These gridded, or digital, forecasts are put into a sentative forecasts from the analysis, expansion of National Digital Forecast Database (NDFD; Glahn the grid coverage, and modifications to the and Ruth 2003). NWS forecasters need support- land/water mask. ing guidance available on a grid, at a resolution comparable to the grid resolution used at the local In this paper, we discuss the current state of forecast office. One approach to the problem of the gridded MOS suite, as well as ongoing en- providing guidance on the required grid is to objec- hancements to the operational GFS-based MOS tively analyze forecasts at MOS stations. Initial guidance, and the process used to diagnose po- testing and development resulted in a prototype tential errors in the analysis. Challenges encoun- gridded MOS system over the western CONUS tered in gathering and processing the data neces- (Dallavalle and Glahn 2005). The first set of op- sary to support the development on a high- erational Global Forecast System (GFS)-based resolution grid are described. In addition, current gridded MOS guidance using this method became and future development activities are presented. available in the National Digital Guidance Data- base (NDGD) in August 2006 over the contermi- 2. CURRENT STATE OF GRIDDED MOS nous U.S. (CONUS) on a 5-km grid. Since the initial CONUS implementation in 2006, more 2.1 Development Techniques weather elements have been added and the geo- graphic coverage has expanded to include an There are two methods currently in use to pro- Alaska domain. Gridded MOS has become a use- duce MOS guidance on a grid. The main method ful resource to NWS forecasters for initializing employed to produce the weather fields in the forecasts, particularly in the forecast periods from gridded MOS guidance is based on an objective days 4 through 7. Emphasis is placed in the later technique to analyze the MOS station-based fore- time periods on only making changes to the MOS casts to points. A scheme of successive correc- grids when and where the forecasters see an ob- tions has been extended to analyze site-specific vious opportunity to improve over the guidance. MOS forecasts to the desired grid (Glahn et al. 2009). This scheme is similar to the approach The gridded MOS guidance has demonstrated implemented by Cressman (1959), but it has been many of the positive attributes of its parent station- expanded to account for differences between land based MOS, while including options for tailoring and water forecast points, and it can account for the grid point values to best reflect the terrain. changes in an element’s values due to vertical changes in the elevation. The vertical correction is *Corresponding author address: Kathryn Gilbert, commonly associated with decreasing tempera- 1325 East-West Highway, W/OST22, Sta. 11306, tures as elevation is increasing. However there Silver Spring, MD 20910 e-mail: are cases where the elevation correction can take [email protected] on positive values over limited areas, as with tem- 1 perature inversions. The current gridded MOS vance). Guidance is available in graphical form on software configuration performs some amount of the NDGD web site shown in Figure 1 elevation correction, either positive or negative (http://www.weather.gov/mdl/synop/gridded/sector (snowfall amounts and wind speed may increase s/index.php) and it is also encoded and transmit- with height for example) for every analyzed ted in GRIB2 format. weather element except the probability of precipi- tation and wind direction. For all weather elements except the thunder- storms, MOS guidance is developed for specific The other method uses a generalized operator observing locations. For continuous variables equation applied over the entire domain. This such as temperature and wind speed, the histori- technique combines all of the stations into a single cal sample of observations and model data is region to increase the sample size and stability of adequate to derive single-station MOS equations the equations, which is particularly important for for each projection, cycle, and season. For many some relatively rare and non-continuous weather discontinuous elements, a larger sample com- elements. A single MOS equation is developed for posed of several stations combined outweighs the a given cycle and projection, which may then be slight loss of specific characteristics at an observ- applied to any station or grid point. The general- ing site. Variables such as the probability of pre- ized operator method is used to generate the cipitation and snowfall amounts are developed for probability of a thunderstorm equations currently groups of stations with similar characteristics available in NDGD (Shafer and Gilbert 2008; within a geographic area. These groups of sta- Hughes and Trimarco 2004). Thunderstorm fore- tions are referred to as regions. Using relative cast equations are developed on a 40-km grid and frequencies or other geoclimatic variables specific the resulting forecasts are then interpolated to the to each observing site provides additional site- 5-km NDFD grid. These gridded MOS thunder- specific information. storm forecasts represent the probability of one or more cloud-to-ground lightning strikes within a 2.3 Cycle Averaging 40-km area centered around each 5-km grid point during the given time period. Cycle averaging was implemented to reduce the impacts of model flip-flops and pulsing in small 2.2 Available Weather Elements and Projec- areas between cycles. These oscillations between tions cycles may be attributed to several factors; the numerical weather prediction models themselves The goal of gridded MOS is to provide guid- have some temporal inconsistencies between ance on high-resolution grids, which corresponds model runs, samples available for the 0000 and to all of the elements in NDFD, and provide this 1200 UTC MOS developments vary for some ele- guidance at the same time and space resolution ments, not every station has a forecast for a par- needed by the NWS forecasters, while maintaining ticular element at all projections, and finally even a level of accuracy and skill comparable to the when stations and sample sizes are matched be- station-based MOS guidance. GFS-based MOS tween cycles, there is no guarantee the unique grids are currently produced on a 5-km grid over regression equations will yield consistent fore- the CONUS and a 3-km grid over Alaska from the casts. 0000 and 1200 UTC GFS model runs. Unless otherwise noted, the following gridded elements An "ensemble of two" approach minimizes the are available in NDGD on the hour every 3 hours oscillation from run to run for an analyses made from 6 to 192 hours in advance: daytime maxi- from only one run. It is a simple 50%/50% weight- mum (max) and nighttime minimum (min) tem- ing of the current cycle and the previous cycle perature, 2-m temperature, 2-m dewpoint, relative 12 hours earlier. So, for instance, the 36-h fore- humidity calculated from the temperature and cast from one cycle is blended together with the dewpoint, sky cover, wind direction, wind speed, 48-h forecast from the previous cycle. Tests have wind gusts, probability of precipitation over 6- and shown that cycle averaging makes little difference 12-h time periods, total expected precipitation in the accuracy or bias of the gridded MOS fore- amount over 6- and 12-h time periods (CONUS cast (Ruth et al. 2009). only, valid out to 156 hours in advance), probabil- ity of thunderstorms over 3-, 6-, and 12-h time pe- Cycle averaging is used for all of the station- riods, and total snowfall amount over a 24-h time based gridded MOS elements except the wind period (CONUS only, valid out to 132 hours in ad- speeds, gusts and direction. The gridded wind 2 averaging was turned off after forecasters noticed 4. CHARACTERISTICS cases of unrealistic wind patterns, described in more detail in Section 5. Gridded MOS has several strengths, one being a lack of discontinuities between WFO boundaries, 3. POST-PROCESSING evident in some forecast fields in NDFD. The use of vertical elevation differences to adjust some A limited amount of internal grid consistency gridded MOS fields results in realistic features checking (i.e. quality control) has been imple- which correspond well to the influencing terrain. mented. Before the station values are offered to The lapse rate used in the gridded MOS analysis the analysis software, the station forecasts are is not limited to a standard lapse rate, but rather checked for consistency. Some inconsistency is the data themselves determine the amount of ver- introduced to the grids due to the analysis proce- tical correction to apply with changing elevation.
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