Identification of side-door/back-door cold fronts for fire weather forecasting applications

1. Identification of side-door/back-door cold fronts for fire weather forecasting applications Ryan P. Shadbolt Department of Geography Michigan State University, East Lansing, MI Daniel Keyser Department of Earth and Atmospheric Sciences University at Albany, State University of New York, Albany, NY Joseph J. Charney USDA Forest Service Northern Research Station, East Lansing, MI Introduction The objective of this presentation is to demonstrate the ability of a mesoscale numerical weather prediction model (MM5) to resolve side-door/back-door (SDBD) cold fronts in the northeastern United States for the purpose of forecasting their potential impact on wildland fires. We employ the MM5 to simulate two SDBD cold front cases: 23 April 1987 and 14 May 2004. Using the output from these simulations, we assess the ability of the model to resolve the structure of SDBD cold fronts and their pre- and post-frontal environments. Side-Door/Back-Door Cold Fronts We are investigating the potential impacts of SDBD cold fronts on fire weather and fire behavior in the northeastern United States. Fire-weather parameters can vary dramatically in the pre-frontal, frontal-transition, and post-frontal regions of SDBD cold fronts. Our simulations illustrate the ability of the MM5 to diagnose the position of SDBD cold fronts, and to diagnose the potential for variability in the fire-weather ingredients to impact wildfires. Climatologically, SDBD cold fronts occur most often during the spring and fall, which coincides with the seasonal peaks in fire activity in the northeastern United States. Summary This presentation has illustrated diagnostics that assess the structure of mesoscale boundaries that could impact wildland fires in the northeastern United States. A suite of boundary detection techniques is under development that may be used to diagnose these features in real time using operational numerical weather prediction model output. The boundary detection techniques under development can also be used to diagnose and predict the evolution of sea-breeze and coastal fronts, which commonly affect wildland fires in the eastern United States. Future Work We are cataloguing specific cases wherein a mesoscale boundary had an important impact on fire behavior and on fire management decisions in the eastern United States. These cases will be used to assess the performance of the boundary detection techniques under development, and to determine how to incorporate them effectively into operational fire-weather forecasting and fire management activities. Please contact us regarding specific cases or anecdotes of interest: Phone: 517-355-7740 x105 E-mail: jcharney@fs.fed.us Case 1: 23 April 1987 The first SDBD cold front case, documented by Hakim (1992), occurred in Pennsylvania, Maryland, and eastern Virginia. Weather conditions in the mid-Atlantic states changed abruptly from partly cloudy skies, light winds, and 25°–30°C surface temperatures ahead of the front to overcast skies, stronger winds (gusts to 20 m s−1), and surface temperatures 10°–15°C cooler behind the front. An MM5 simulation of this event reveals that the model captured the observed sea level pressure signature. Case 2: 14 May 2004 The second SDBD cold front was located over coastal New England. Surface conditions across the front varied from light southerly winds with warm and dry air to moderate easterly winds with cool and moist air. An objective analysis of the surface potential temperature gradient developed by Dr. Eric Hoffman (Plymouth State University) and a 15-h MM5 simulation highlight the position of the SDBD cold front. Fire-Weather Ingredients This research focuses on the phenomenological aspects of SDBD cold fronts and relates these aspects to operational fire-weather and fire-behavior parameters. Pre- and post-frontal environmental characteristics can impact fire-weather parameters from the perspective of both traditional fire-weather ingredients, and experimental diagnostics and indices that detect mesoscale boundaries relevant to fire-weather interactions. Surface analysis (North America) 1500 UTC 14 May 2004 The primary meteorological ingredients that define environments conducive to rapid fire growth and erratic fire behavior are: temperature relative humidity wind velocity (direction and speed). From a fire management perspective, the ingredients become: warm, dry, and windy. 24-h simulation of sea level pressure valid at 0000 UTC 23 April 1987 Surface analysis at 0000 UTC 23 April 1987 from Hakim (1992) Surface analysis (Zoom) 1500 UTC 14 May 2004 In order to objectively diagnose the location of the front, we examined the surface potential temperature and surface potential temperature gradient. Objective identification of fronts and other boundaries is important for designing appropriate diagnostics and indices to predict the impact of these features on fire weather and fire behavior. Boundary detection diagnostics on model grids reveal finer detail with increasing resolution. Surface potential temperature gradient 1500 UTC 14 May 2004 http://www.atmos.albany.edu/gopher-local/surface/theta/00latest.theta.gif Surface Potential Temperature Surface Potential Temperature The ingredients-based perspective enables consideration of synoptic and mesoscale features associated with rapid changes in surface conditions. The method provides a conceptual basis for the formulation of diagnostics and indices that may be synthesized into experimental products suitable for transition into forecast operations, such as: mixed layer depth ventilation index surface potential temperature gradient surface relative vorticity surface horizontal divergence 36 km grid 12 km grid 4 km grid 36 km grid 12 km grid 4 km grid Hakim, G. J., 1992: The eastern United States side-door cold front of 22 April 1987: A case study of an intense atmospheric density current. Mon. Wea. Rev., 120, 2738–2762. Surface Potential Temperature Gradient Surface Potential Temperature Gradient