Proceedings of The National Conference On Undergraduate Research (NCUR) 2006 The University of North Carolina at Asheville Asheville, North Carolina April 6-8, 2006 The Integration of WRF Model Forecasts for Mesoscale Convective Systems Interacting with the Mountains of Western North Carolina Jacob Carley Department of Atmospheric Sciences The University of North Carolina at Asheville One University Heights Asheville, NC 28804 Faculty Advisor: Douglas K. Miller, Ph. D. Abstract This research involves the investigation of mesoscale convective systems (MCS s) and supercell thunderstorms and their interaction with the mountains of Western North Carolina and Eastern Tennessee. The Weather Research and Forecasting (WRF) Model is used to forecast three separate MCS events from 2005. The data from each event is analyzed. This includes, but is not limited to, the investigation of upper air soundings, upper air maps, surface maps, and convective instability. The output from the WRF is then compared to corresponding WSR-88D Doppler radar data from each event in order to evaluate the accuracy of the model forecast. If the WRF is found to have handled the event well and the model forecast is comparable to the radar data then the model is said have had a good forecast. If the model produces an otherwise poor forecast, the model data is analyzed and an analysis is conducted in an attempt to locate potential deficiencies in the model. If deficiencies are found, then an attempt at finding potential viable solutions is made. Keywords: Weather Research Forecasting (WRF), Convective Systems, Western North Carolina 1. Introduction Prior to discussing the details of the research, it is necessary to define what a mesoscale convective system (MCS) is. The American Meteorological Society s Glossary of Meteorology defines a MCS as, A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction. 1 These severe systems of thunderstorms can often cause much damage to communities. They have been known to possess the capability of producing damaging wind, large hail, and even tornadoes. When a MCS system approaches an orographic boundary, such as the Blue Ridge Mountains of Western North Carolina and Eastern Tennessee, it often dissipates but also has the capability to cross over the mountains and cause damage to downwind communities. This is an obvious problem for operational meteorologists attempting to forecast these types of events. The current models may favor the development of a MCS, but the question of whether the severe weather will cross the mountainous terrain still remains a difficult question to answer. This research focuses on the WRF model s ability to forecast the passage of these systems across this complex terrain. The WRF model was developed through the collaborative efforts of many organizations. The organizations that were involved in the creation of this model are as follows: the National Center for Atmospheric Research, the National Oceanic and Atmospheric Administration, the National Centers for Environmental Prediction, the Forecast Systems Laboratory, the Air Force Weather Agency, the Naval Research Laboratory, Oklahoma University, and the Federal Aviation Administration. 2 The WRF model is unique in that it possesses algorithms to place emphasis on terrain and mesoscale events. The model can be run at incredibly high resolutions of up to less than two kilometers between grid points. The WRF also
has a unique customizable ability: it allows the user to choose from a variety of physics packages as well as many other parameters. This customizability, along with its emphasis on terrain and mesoscale features, makes this an optimal choice for research purposes, especially concerning mesoscale convective systems. A set of MCS events will be chosen based upon certain selective criteria: crossing or dissipating. Crossing describes the types of storms that initiate before the Appalachian Mountains and continue across them. The types of storms that fall under the dissipating category initiate before the mountains but diminish upon reaching this orographic boundary. The common factor for each event is that the storms generally originate west of the Appalachian Mountains of Western North Carolina and East Tennessee and attempt to move across these mountains. The main principle is to study how the storms interact with the mountains and evaluate the WRF model forecast for these events. Once the evaluation is complete, an attempt at locating possible deficiencies will take place. If the model is found to have identifiable biases, an attempt will be made to adjust the model in an effort to produce more accurate forecasts. This is a work in progress. 2. Data All the MCS events to be studied have been provided courtesy of the National Weather Service in Greenville- Sparatanburg, South Carolina. Of the events provided, three were chosen for this research. The events all took place during 2005. The first event occurred on April 22 nd through the 23 rd, the second event occurred on May 19 th through the 20 th, and the third and final event occurred on July 27 th through the 28 th. 2.1 April 22 nd to the 23 rd of 2005 The April event involves two separate waves of storms, and subsequently, meets both the criteria of crossing and not crossing. The first wave of severe weather crosses the mountains but becomes disorganized rather quickly. The second wave dissipates when it encounters the mountains. According to storm reports from the Storm Prediction Center, emergency managers in Pickens County, South Carolina reported tornado damage associated with the first wave of severe convection. 3 Figure 1. 1200Z Surface analysis of the April 22 nd MCS event by the Hydrological Prediction Center. 4 415
Figure 1 shows the morning synoptic features and frontal positions across the United States. It is worthy to note that this analysis shows an approaching low pressure system and a stationary front draped across Western North Carolina. This stationary front eventually evolves into a warm front and pushes north, leaving Western North Carolina in the warm sector of the frontal system. The quickly approaching cold front that lies to the west of North Carolina is the main feature that brings the severe weather into the area. Figure 2. 1759Z 0.5 Degree reflectivity radar scan of April 22 nd MCS event. Figure 3. 1800Z 5 km WRF run of April 22 nd MCS event showing absolute vorticity at 500 mb. 416
Figure 2 shows the prefrontal MCS crossing the mountains. As indicated by the storm reports after this event, this particular wave of convection was quite severe. This is also made apparent by viewing the strong reflectivity returns of up to 64 dbz. The first wave MCS is the prefrontal precipitation and is associated with a reported tornado in Pickens County. The second wave of convection dissipates upon encountering the mountains. This can possibly be attributed to strong vertical mixing within the boundary layer. This mixing acts to weaken the temperature gradient and thus has a net effect of reducing the frontal intensity. 5 Figure 3 shows the forecasted absolute vorticity at 500 mb by the WRF model at 1800Z. Absolute vorticity is a measure of spin in a parcel of air and is a good indicator of convection. The vorticity in Figure 3 is associated with the first crossing prefrontal wave of convection. As shown above, the model is slow to bring in the convection, generally by about three hours. It also left several residual bull s-eyes of vorticity in the piedmont area of North Carolina. The WRF did a fair job forecasting the prefrontal MCS. While it was about three hours slow to bring it in, it did show it crossing the mountains. The WRF also did a decent job handling the second wave. It was on target with timing, bringing vorticity into the area at around 0100Z, however it did not bring any vorticity across the mountains. When analyzing radar data, it was found that some rain did come into the area, but dissipated very quickly. The WRF had the precipitation stall out in Eastern Tennessee and did not push it far into North Carolina. Overall, the WRF did a reasonable job forecasting this event. While it was slow to bring in convection with the first wave, it did correctly forecast it to cross the mountains. The second wave was the non-crossing wave and the WRF did a great job of forecasting the MCS to not cross the mountains. However, it did miss some convection that did cross on radar. The WRF had no vorticity to represent this convective precipitation. 3. Future Projects 3.1 May 19 th to the 20 th of 2005 Figure 4. 1200Z Surface analysis of the May 19 th MCS event by the Hydrological Prediction Center. 6 417
The May event involves two items of interest. First, prior to the MCS crossing through Western North Carolina, there is some isolated convection. Second, the MCS crosses the mountains relatively intact, producing one F1 tornado in Smyth County, Virginia. 7 In Figure 4 there is a low pressure system moving southeastward from east central Iowa. As this system approaches North Carolina, it begins to produce deep convection, and as a result, a MCS. As the MCS continues to move toward North Carolina it forms an outflow boundary, allowing it to continue to produce deep convection. 3.2 July 27 th to the 28 th of 2005 Figure 5. 1200Z Surface analysis of the July 27 th MCS event by the Hydrological Prediction Center. 8 The July event involves a very large and linear non-crossing MCS. Figure 5 shows the approaching cold front, the initiator of the MCS. The deep convection forms along the front in a linear fashion. As this MCS advances on the Appalachian Mountains of North Carolina, it quickly dissipates. An interesting item to note is the presence of Tropical Storm Franklin off the coast of North Carolina. This tropical storm may have created subsidence in most of North Carolina, capping off any chance of severe weather and most likely adding to the demise of the MCS as it crossed the mountains. 4. Conclusion The WRF model did a reasonable job forecasting the April MCS event. It will be interesting to see how well the WRF forecasts the May and July events as well. The outflow boundary with the May event poses some interesting questions that need further analysis: does the presence of an outflow boundary increase the likelihood of the successful passage of an MCS and can the WRF correctly detect the presence of such a remote mesoscale feature? The July event produces some topics that require further analysis as well. Does the presence of Tropical Storm Franklin impart a stabilizing effect on the approaching MCS, or is the dissipation due to the more local effects of the orographic boundaries? The WRF model forecast for both the May and July event will prove to be of interest in how it handles these different parameters influencing each event. 418
5. Acknowledgements This research was made possible through the collaboration of several people and organizations. The Science Operations Officer at the Greenville-Spartanburg, South Carolina National Weather Service Office, Mr. Larry Lee, provided the MCS events of interest. The University of North Carolina at Asheville and the National Environmental Modeling and Analysis Center (NEMAC) have provided funding as well as support for the research to take place and continue. The National Weather Service in Blacksburg, Virginia has also provided support for this project. I would like to thank Chad Hutchins for providing help with radar data and Aaron Dicks for assistance with editing. Dr. Doug Miller has been a great faculty advisor and has provided an immeasurable amount of assistance as well. 6. References 1 Glickman, Todd S. (Ed.) (2000). Glossary of meteorology: second edition. American Meteorological Society. Retrieved Feb. 26, 2006, from Allen Press Web site: http://amsglossary.allenpress.com/glossary/search?id=mesoscale-convective-system1. 2 About the Weather Research and Forecasting Model. Retrieved Feb. 26, 2006, from The Weather Research and Forecasting Model Web site: http://www.wrf-model.org. 3 Crisp, Charlie A., Corresponding Author. (2005a). Severe thunderstorm event: April 22, 2005. Index of events. Retrieved Feb. 26, 2006, from Storm Prediction Center Web site: http://www.spc.noaa.gov/exper/archive/events/050422/index.html. 4 Crisp, Charlie A., Corresponding Author. (2005b). HPC synoptic scale analyses loop valid 050422/1200Z 050423/0900Z. Index of events. Retrieved Feb. 26, 2006, from Storm Prediction Center Web site: http://www.spc.noaa.gov/exper/archive/events/050422/hpcsfcloop.shtml. 5 Peng, Linda S., et al. (2001). Boundary layer effects on fronts over topography. Journal of the Atmospheric Sciences, 58, 2222-2239. 6 Crisp, Charlie A., Corresponding Author. (2005c). HPC synoptic scale analyses loop valid 050519/1200Z 050520/0900Z. Index of events. Retrieved Feb. 26, 2006, from Storm Prediction Center Web site: http://www.spc.noaa.gov/exper/archive/events/050519/hpcsfcloop.shtml. 7 Crisp, Charlie A., Corresponding Author. (2005d). Severe thunderstorm event: May 19, 2005. Index of events. Retrieved Feb. 26, 2006, from Storm Prediction Center Web site: http://www.spc.noaa.gov/exper/archive/events/050519/index.html. 8 Crisp, Charlie A., Corresponding Author. (2005e). HPC synoptic scale analyses loop valid 050727/1200Z 050728/0900Z. Index of events. Retrieved Feb. 26, 2006, from Storm Prediction Center Web site: http://www.spc.noaa.gov/exper/archive/events/050727/hpcsfcloop.shtml. 419