Where Does It Start to Precipitate? - “First Echoes” Seeing from the Space

The “first echo” appears in a fresh convective cloud when hydrometeors grow to large enough sizes and number concentrations that can be detected by radar. Depending on the sensitivity of radar and the properties of convective cloud, it may be used to imply the growth of hydrometeors in various strengths of the updrafts. Using 10-year observations by the Ku band radar onboard the Global Precipitation Mission core satellite, isolated pixels with detectable radar echoes at altitudes without reaching at surface are identified as candidates of the “first echo”. In general, the heights of the “first echoes” show three modes in tropics. The shallow mode at 1.5-2 km is mainly from the growth of warm rain cells over ocean and the Amazon. The deep mode at 6-9 km is as frequent over both tropical land and ocean. The mid mode at 4.5-6 km is found mainly over land and ocean surrounding maritime continents. Though it is known that convection over Africa is more intense, no significant difference is noticed in the properties of the deep mode “first echo” from Africa to other regions. Examining the thermodynamic environments of these “first echoes” from ERA5 reanalysis, cloud base height and total column water vapor are found positively related to the “first echo” height. Deep mode is more likely with an easterly near surface wind over tropical ocean. While shallow mode over ocean tends to have more sea salt aerosols, high smoke and sulfate aerosols are more likely associated with mid mode, and high dust aerosols are found with the deep mode. These results help us to understand the growth of the hydrometers at the early stage of convection.