Pendleton, Oregon Research Region Map Types and Frequencies

Henry FuelbergPete Saunders

Pendleton, Oregon Research Region Map Types and Lightning FrequenciesMap Typing (Pattern Classification)In developing the model for lightning probability forecasts, we created pattern-type lightning frequencies that will be used as candidate predictors. As in Shafer and Fuelberg (2006), we believe these predictors will provide detailed information on local lightning enhancements that are not adequately resolved by the NCEP models. The goal is to capture local interactions between the low-level winds and thermal circulations. A correlation technique described by Lund (1963) and Reap (1994a) was used to develop what we call map-type predictors. The correlation technique was applied to 3-hourly observed 850-hPa geopotential height fields from RUC analyses. The time period is the warm season months (May-September) from 2001 to 2007. We believe that the 850-hPa level provides the best indication of low-level flow in terms of both direction and speed.The RUC-20 and RUC-40 geopotential height values were interpolated onto a coarser 50-km grid in order to smooth the smaller-scale variations and to capture the regional characteristics of the low level flow.Map Typing (Pattern Classification)Only time periods when the geopotential height map could be correlated at 0.8 or greater (Lund 1963; Reap 1994a) were used in creating the map types. However, unclassified maps were assigned to the map type in which they were most correlated for model equation development. This also is represented in the map type means and frequencies.For each map type and 3 hourly period, relative lightning frequencies then were calculated.The unconditional mean number of flashes also was calculated for every 3-h period and map type.Details of these procedures follow.

Map Typing Procedures We used the following procedures to determine pattern type classification:

Gathered RUC-20 and RUC-40 data of geopotential heights at various levels. RUC-40 data were used for the years 2001 through 2004. RUC-20 data were used for the years 2005 through 2007, giving a total of 7 years of height data.

Grid points for the Pendleton, OR research region were subset for the following lat/lon coordinates:

Latitude: 40.0 N through 49.0 N Longitude: -115.0 W through -126.0 W

Map Typing Procedures3-h geopotential height data were extracted from the grib files for each day during the 7 year period to create a total of 8265 maps. Each 3-h period then was interpolated onto a 100-km grid from the initial 20-km and 40-km grids. From here, each 3-h RUC geopotential height map was correlated with every other map in the sample to determine the map types. For example, maps correlated at the R=0.8 threshold were placed into Map Type A. Then, every map that was determined to be of Type A was removed, and a new correlation matrix was created to determine Map Type B at the same threshold. This correlation process then was repeated to produce 5 total map types (A E).

Map Type Frequencies and MeansWe then completed the following steps to relate lightning to the map types that had been created: 1. Read in the list of lat/lon values for each grid point. 2. Read in the time and lat/lon location of each flash in the monthly .lga file. 3. Created a program that determined if the flash lies within the specified radius of each grid point by computing the separation distance (km) between the flash and the grid point. The calculation of separation distance used a formula for the great circle distance on a sphere. 4. If the computed separation distance was