Radar First Echo Detection With Spaceborne Precipitation Radars

The radar “first echo” appears in a fresh growing cloud when hydrometeors grow to large enough sizes to be detected by radar. Using 26‐year observations by the Ku band radars onboard the Global Precipitation Mission and the Tropical Rainfall Measurement Mission satellites, isolated pixels with detectable radar echoes at altitude and without precipitation at the surface are identified as candidate “first echoes.” In general, the appearance of the “first echoes” show three altitude‐related modes in the tropics. The shallow mode at 1.5–2 km is mainly from the growth of warm rain from rapid coalescence. The mid mode between 0°C and −7°C is found mainly over land and coastal regions, that can be explained by the enhanced radar reflectivity by ice particles through heterogenous ice nucleation at relative warmer temperatures, followed by active riming. The deep mode between −12°C and −25°C is frequently found over both land and ocean. Around 12% of isolated echoes, mainly over desert regions, could be the remnant of dissipating precipitation, like virga. Cloud base height and total column water vapor derived from ERA5 reanalysis are found positively related to the “first echo” height. The mid mode first echoes are more likely found in a moist (ice‐supersaturated) environment, a favorable condition for crystal growth following heterogeneous ice nucleation. While the shallow mode over ocean tends to have more sea salt aerosols, large smoke and sulfate aerosol concentrations are often associated with the mid mode, and large dust aerosol concentrations are often involved with the deep mode.