Comparison of the NEXRAD legacy and Lincoln Laboratories echo tops products

NEXRAD echo tops products have proven highly valuable for air traffic controllers in managing the National Airspace System (NAS). The National Weather Service legacy algorithm produces a 4-by-4 km, gridded product in 5-kft height increments. The product is derived from an estimate of the height of 18.5-dBZ reflectivity factor contours. The derivation of echo tops is dependent on NEXRAD radar volume coverage, which is necessarily limited because of time sampling constraints and loss of spatial resolution with range. The legacy algorithm interprets the 18.5-dBZ threshold as the echo top, while the Lincoln Laboratory (LL) algorithm uses a linear interpolation of reflectivity factor between elevation angles to estimate the height of the 18-dBZ reflectivity factor. The LL echo tops product has higher spatial and temporal resolution as well. Results are given in gridded horizontal 1-by-1 km cells with 1-kft height resolution. The legacy update rate for a national composite is 5-min, while the LL is 2½ -min. The comparison involves both qualitative and quantitative assessments. Qualitative comparisons are based primarily on images of the products, while the quantitative comparisons depend mostly on statistical measures. The evaluations are limited to two common regions of spatial extent that offered the potential for comparing different weather regimes. Images of both products are in general quite similar with resolution differences being most noticeable near the edges of storms and as a consequence of the different update rates. Differences in clutter filtering and removal of possible anomalous echoes due to high-flying aircraft are also apparent. The statistical results for the mean echo tops over extended coverage areas are consistent with expectations derived from consideration of the differences in the two algorithms.