Estimating Rainfall in Arizona - A Brief Overview of the WSR-88D Precipitation Processing Subsystem Jonathan J. Gourley National Severe Storms Laboratory (Western Storms Group) University of Oklahoma School of Meteorology Norman, OK

Outline Description of the Precipitation Detection Function Overview of the Precipitation Processing Subsystem Examples of individual radar performance Suggestions for improvements of radar rainfall estimates Discussion (questions, comments, complaints)

Precipitation Detection Function (PDF) Category 0 - no detectable precipitation - Reflectivity values < 22.0 dBZ, precipitation area < 10 km 2 - Radar stays in Mode B (Clear Air Mode) - No Precipitation Processing Subsystem (PPS) activation Category 1 - significant detectable precipitation - Reflectivity values > 22.0 dBZ, precipitation area > 10 km 2 - Radar switches to Mode A (Precip Mode) - PPS is activated Category 2 - light detectable precipitation - 1 condition for Cat. 0 is not met - Radar stays in Mode B (Clear Air Mode) - PPS is activated

Precipitation Processing Subsystem (PPS) 4 Step Process Preprocessing - 6 functions Rate - 2 (3) functions Accumulation - 3 functions (Adjustment)

PPS: Preprocessing Input: Reflectivity values from 4 lowest elevation angles within a radius of 230 km. Output: Hybrid Scan Partial Occultation Correction Isolated Sample Volume Check Outlier Check Complete Occultation Correction Tilt Test Bi-Scan Maximization

PPS: Preprocessing - Partial Occultation Correction Purpose: To allow elevation angles which are less than 60% blocked to be utilized in precipitation computations. Function: dBZ values are added to range bins beyond the obstruction. The magnitude of the added dBZ value is a function of the percent blockage. 0-10% = +0 dBZ 11-29% = +1 dBZ 30-43% = +2 dBZ 44-55% = +3 dBZ 56-60% = +4 dBZ

PPS: Preprocessing - Isolated Sample Volume Check Purpose: To reduce the amount of spurious noise and low frequency interference. Function: If a reflectivity value exceeds 18 dBZ and less than 2 of the 8 surrounding values (within the same elevation angle) exceed 18 dBZ, then the value is set to 0 dBZ. 0 dBZ

PPS: Preprocessing - Outlier Check Purpose: To reduce the effects of unrealistically high reflectivities such as those caused by hail contamination. Function: If a reflectivity value in a range bin exceeds 65 dBZ, then it is either replaced by an interpolated value from the 8 surrounding values, or it is set to 1 dBZ. 40 dBZ 1 dBZ

PPS: Preprocessing - Complete Occultation Correction Purpose: To attempt to interpolate values beyond the obstruction if the blockage exceeds 60%. Function: If the complete blockage is less than 2˚ in azimuth, then values behind the obstruction are interpolated from bins on both sides at the same range and elevation angle.

PPS: Preprocessing - Tilt Test Purpose: To suppress the effects of ground clutter and anomalous propagation. Function: Between the ranges of 40 and 150 km, the tilt test assesses the validity of the reflectivity obtained from the lowest scan. If ANY of the following conditions are satisfied, then the lower scan is used to build the hybrid scan. Total echo area < 600 km 2 Area averaged reflectivity < 10 dBZ Area (Echoes which are not vertically continuous) > 0.75 Area (Echoes at lowest scan)

PPS: Preprocessing - Bi-Scan Maximization Purpose: To mitigate the effects of beam losses at far ranges. Function: From 10 to 50 km (KIWA!) or 180 to 230 km (KFSX), Bi-Scan Maximization chooses the maximum reflectivity values from the 2 lowest, available scans.

PPS: Preprocessing - Hybrid Scan Purpose: To attempt to represent, as closely as possible, the atmosphere at a constant height of 1000 m above the radar. Function: In constructing the Hybrid Scan, the tilt to be selected for a given range and azimuth shall be the tilt whose beam center is closest to the optimal height (1000 m) above radar level. In addition, the height of the bottom of the range bin must clear any underlying obstruction by 500 ft. Tilts which failed the previous quality control steps are not considered in building the hybrid scan. 82,800 reflectivity values are stored on a 1˚x1 km polar grid.

PPS: Rate Input: Hybrid Scan Output: Rate Scan Average Rainfall Rate Computation Time Continuity Test (Range Corrected Rate Calculation)

PPS: Rate - Average Rainfall Rate Computation Purpose: To exclude unrealistic rainfall rates and provide smooth spatial continuity between adjacent rainfall range bins. Function: Reflectivity values greater than 65 dBZ are not considered to build the rate scan. In addition, rainfall rate pairs along the same radial are successively averaged to create a 1˚ x 2 km polar grid

PPS: Rate - Time Continuity Test Purpose: To check that the total volumetric precipitation rate does not exceed the rates that are expected with precipitation decay or development. Function: Compares total volumetric rate from current scan to previous scan. If the rate and echo area has decreased, then the test computes the maximum decay rate. If the current precipitation rate is less than the expected minimum rate, or if the difference between the two is small, then precipitation decay is assumed to be too large and the scan is rejected.

PPS: Rate - Time Continuity Test (Continued) Function: An analogous procedure is performed for precipitation development. If the rainfall rate and area has increased between scans, then the maximum expected rate is computed based on the previous scan. If the current “inner” volumetric rain rate exceeds the computed maximum rate, or if the difference is small, then the scan is rejected.

Purpose: To correct for underestimation of rainfall rates at long ranges due to signal degradation and partial beam filling Function: Coefficients are set such that no correction is made at the present time. Range Corrected Rate = C1 + C2 x Precip Rate + C3 x Range where C1=0, C2=1, C3=0 Range Corrected Rate = Precip Rate PPS: Rate - (Range Corrected Rate Calculation)

PPS: Accumulation Input: Rate Scan Output: Accumulation Scan Accumulation Computation Threshold Check Product Generation

PPS: Accumulation - Accumulation Computation Purpose: To compute how much rainfall has accumulated between scans based on rainfall rates. Function: For each range bin, rain rates from consecutive scans are averaged and multiplied by the time lapsed between scans to determine the rainfall accumulated. If the time between scans exceeds 30 min, then data are flagged as missing..03”.025” M.05” ∆t = 6 min∆t = 40 min 15 min

PPS: Accumulation - Threshold Check Purpose: To ensure that unrealistic accumulations are excluded in constructing the products. Function: If hourly accumulations exceed 400 mm (15.75”), then they are considered outliers and are interpolated from the 8 surrounding values. No accumulations greater than 800 mm (31.5”) are allowed

PPS: Accumulation - Product Generation Purpose: To generate hydrometeorological output for a variety of uses. Function: 5 products are generated - 1 Hour Precipitation, 3 Hour Precipitation, Storm Total Precipitation, User Selectable Precipitation and One Hour Digital Precipitation Array. 1 Hour Precipitation - moving 1 hour window which updates every scan. Product is not generated if there is missing data. 3 Hour Precipitation - available after each scan, but updated at the top of each hour. Product requires at least 2 “top of the hour” accumulations before it is generated.

PPS: Accumulation - Product Generation (continued) Storm Total Precipitation - available from the first volume scan that produces a category 1 or 2 in the precipitation detection function. It is updated every volume scan until there is at least 1 hour of category 0 precipitation. This product includes periods in which there may have been missing data. User Selectable Precipitation - uses top of the hour accumulations for the previous 24 hours. Product may contain missing data. 1 Hour DPA - accumulations are converted to a 4 x 4 km grid. 256 data levels (.01”-14.38”) are used in the NWSRFS. Grid format allows mosaicking of data from multiple radars.

PPS: (Adjustment) Input: Accumulation Scan Output: Adjusted Accumulation Scan Purpose: To calibrate the Z-R relationship to the current rainfall characteristics in real-time. Function: Each hour, gage reports (and their 8 surrounding values) are compared to radar accumulations in coinciding areas. The algorithm then computes a multiplicative bias based on the difference in the gage-radar sets. This bias is used to adjust the current hourly accumulations. Furthermore, it is forecast to the next hour in the event that insufficient gage data arrive.