Comparison of Freezing-Level Altitudes from the NCEP Reanalysis with TRMM Precipitation Radar Brightband Data

Abstract

A global climatology of the altitude of the freezing level (0°C isotherm) is computed using 20 yr of 6-hourly output from the National Centers for Environmental Prediction (NCEP) reanalysis system. Mean statistics discussed include monthly means and climatological monthly means. Variance statistics include the standard deviation of the 6-hourly values with the month and the standard deviation of the monthly means. In the Tropics, freezing levels are highest (?5000 m) and both intramonth and interannual variability is lowest. Freezing levels are lower and variability is higher in the subtropics and midlatitudes. In 1998 there are unusually high freezing levels in the eastern Pacific Ocean relative to the 20-yr climatology, consistent with elevated sea surface temperatures associated with the 1997?98 El Niño. Freezing levels return to near-climatological values during the last half of 1998. The individual monthly means for 1998 and the 20-yr climatology are compared with monthly means of the altitude of the bright band (melting layer) retrieved from Tropical Rainfall Measuring Mission (TRMM) precipitation radar data. Differences between TRMM and NCEP typically range from about ?300 to ?900 m. Differences are somewhat larger over landmasses and in zonal bands centered on ±20° latitude.