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Range Forecasts of the Pre-‐Convective Environment Using SEVIRI Data

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Range Forecasts of the Pre-‐Convective Environment Using SEVIRI Data Enhancing objective short-range forecasts of the pre-convective environment using SEVIRI data Ralph A. Petersen1, Robert Aune2 and Thomas Rink1 1 Cooperative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin – Madison 2 NOAA/NESDIS/STAR, Advanced Satellite Products Team, Madison, Wisconsin ABSTRACT: This paper describes development efforts and evaluation results made with the CIMSS NearCasting system during the last year. Tests of the 1-9 hours forecasts of GOES products were made at US National Weather Servicer (NWS) Forecast Offices, CIMMS, and both the Storm Prediction Center (SPC) and the Aviation Weather Center (AWC) divisions of the National Centers for Environmental Prediction (NCEP). The tests at the two centers focused both on where/when severe convection will occur, as well as all forms of deep convection will and will not occur. The results showed that the prediction period could be successfully increased from 6 to 9 hours, that the analysis improved by increasing the number of observations projected forward from previous, that the NearCast wind fields can provide information about low-level triggering mechanisms and storm severity, that the NearCasts enhanced NWP guidance by isolating which forecasts areas were and were not likely to experience convection and when, and that successful use of the NearCasting tools requires increased forecaster training and education - both about the NearCast system itself and interpreting satellite observations and derived products. INTRODUCTION: This paper provides an update to Petersen et al., 2010. During the past year, primary emphasis of CIMSS NearCasting activities has focused on testing and evaluation of the short-range forecasting system internally at CIMSS, at local US National Weather Servicer (NWS) Forecast Offices, and both the Storm Prediction Center (SPC) and the Aviation Weather Center (AWC) divisions of the National Centers for Environmental Prediction (NCEP). The tests at the two centers had different objectives according to their missions: at SPC, they focused on identifying areas where severe convection would occur, while at AWC, they attempted to identify areas where all forms of deep convection would and would not occur. However, all groups wanted to: a) Increase lead-time, b) Reduce false alarms and increase probability of detection, c) Provide updates / detail to NWP guidance for next several hours, and d) Increase use of satellite products (they currently rely heavily on Numerical Weather Prediction (NWP) models and radar observations). FINDINGS: Summary of Forecaster Evaluations: NearCast products were evaluated from periods ranging from several months to several weeks. Forecaster appraisals of the current and potential value of existing and future products included the following observations/recommendation: 1. Provide information about dynamic triggering, 2. Extend forecast length 3. NearCast fields (especially tendencies) were most useful when used to diagnose initial growth and coverage 4. Clouds limit the usefulness of product at times 5. Nearcasts were most valuable when used in conjunction with observations and other model data (both where convection will and will not occur) - Especially useful in updating / verifying mesoscale NWP and Ensemble guidance, especially during summer when only ~15% of NWP precipitation forecasts are correct 6. More experience was needed using the product to help with interpreting the observed fields and combined parameters. The NearCast model developments and future actions that resulted from these recommendations are described in the following sections Extending NearCast Length and Areal Coverage: A number of research and applications activities have been undertaken to address the issues listed above. The first of these addressed both the issues that forecasters wanted the NearCast guidance extended beyond 6 hours into the future and wanted to have fewer areas blocked by cloud cover. Before evaluation activities were initiated at SPC and AWC, the length of the NearCasts was successfully increased from 6 to 9 hours into the future. Since the NearCast products are generally available within less than one hour after the GOES observing sequence is begun, this effectively expands the maximum useful time of the NearCast products by 60% (from 5 to 8 hours). Extending the length of the NearCast predictions also benefitted the data coverage in both the initial analyses and the NearCast predictions. The NearCasting system is unique in that it uses Lagrangian trajectory techniques to project geostationary satellite observations forward in time to future geographical locations. This is done once every hour as new observations become available, with the information about trajectory location and parameters stored every 15 to 30 minutes. Graphical NearCast products are then produced by ‘binning’ all of the trajectory data from all previous observations valid at the same time onto a regularly spaced grid. As such, data from up to 9 previous observation times can impact the graphical NearCast analyses and predictions. Figure 1: Comparisons of NearCast low-level Equivalent Potential Temperature (θE) analyses for 24 May 2011showing increased analysis coverage when using one hour of on-time observations only (left), on-time observations plus forward projections of data from 4 previous hours (center) and on-time observations plus forward projections of data from 9 previous hours (right). Figure 1 shows the impact of including projections of previous observations in improving the spatial coverage of the NearCast analyses. In this case, the left panel shows an analysis based only on date from 1300 UTC 24 May 2011. The center right panel shows the effect of including both observation at 1700 UTC and projections of data from the previous four observing times (1300, 1400, 1500 and 1600 UTC), effectively tripling the number of ‘data points’ available to the analysis. By including data from nine successive observing times at 2200 UTC, the ‘missing data’ area (black regions over the central and eastern US on the left panel) has been reduced by between 60-80% compared to the analysis made using only on-time data. It should also be noted that forecasters at SPC were especially impressed by the ability of the GOES data and NearCast system to detect and monitor the progression of the ‘dry line’, an important factor in initiating severe convection, eastward from the Texas panhandle into western Oklahoma during the morning and early afternoon. Providing information about dynamic triggering: Although previous examples have shown the ability to identify areas where convective storms are likely to develop, many of the areas characterized as being convectively unstable never experience convection due to either the presence of a strong capping inversion (as described in Petersen et al, 2010) or the lack of a sufficiently strong low-level lifting mechanism. In addition, both thermodynamic and momentum-based parameters are needed to differentiate the probable severity of the ensuing storms. Investigation of the lower- and mid-level wind fields showed that the NearCast models had information and sufficient detail about the low-level convergence and atmospheric shear to isolate areas of substantial lifting and deep-layer shear. The tornado event from 2007 over Poland described in Petersen et al. (2010) was again used for testing. In this case, trajectory forecasts were made using initial winds and geopotential gradients from the NCEP Global Forecast System interpolated to SEVIRI observations locations at 840 and 480 hPa. Figure 2: Example of evolution of NearCast low-level wind (arrows) and divergence/convergence (blue/red) preceding tornadic storm development over Poland on 20 July 2007. Note the formation of a southwesterly low-level jet upstream of the tornado location (white marker) in the 6 hours period to the event, along with the growth and progression of a band of convergence (and corresponding lifting) across the area of the storm immediately prior to the tornado sighting. The results in Figure 2 show the development of a low-level jet during the 6-hour NearCast period with a region of strong convergence (divergence) couplet forming near (upwind) and slightly before the tornado formation. This information about dynamical lifting, combined with the information about convective instability and change in capping inversion presented in the previous report, help to identify where within the areas of convective instability storms are and are not likely to occur. Figure 3: Example of evolution of NearCast low-level moisture flux (white arrows), divergence/convergence (blue/red) and 840- 480 hPa Wind Shear Vectors preceding tornadic severe storm development over Poland on 20 July 2007. Note the continuing low-level moisture supply associated with the south-westerly low-level jet provide energy for sustained convection toward the tornado location (white marker) in the 6 hours period to the event, along with the growth of wind shear immediately prior to the tornado sighting. Figure 3 shows the development of a low-level moisture flux and wind shear during the 6-hour NearCast period before the tornado formation. The development of strong moisture transport and intensifying wind shear in the vicinity (southwest) of the tornadic convection points to the ability of the NearCast trajectories to provide dynamical information about both the thermodynamic and wind fields immediate prior to storm genesis, even when using wind data obtained
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