Abstract
Quantifying the precipitation within clouds is a crucial challenge to improve our current understanding of the Earth's hydrological cycle. We have investigated the relationship between the effective radius of droplets and ice particles (r(e)) and precipitation water content (PWC) measured by cloud probes near the top of growing convective cumuli. The data for this study were collected during the ACRIDICON- CHUVA campaign on the HALO research aircraft in clean and polluted conditions over the Amazon Basin and over the western tropical Atlantic in September 2014. Our results indicate a threshold of r(e) similar to 13 mu m for warm rain initiation in convective clouds, which is in agreement with previous studies. In clouds over the Atlantic Ocean, warm rain starts at smaller r(e) , likely linked to the enhancement of coalescence of drops formed on giant cloud condensation nuclei. In cloud passes where precipitation starts as ice hydrometeors, the threshold of r(e) is also shifted to values smaller than 13 mu m when coalescence processes are suppressed and precipitating particles are formed by accretion. We found a statistically significant linear relationship between PWC and r(e) for measurements at cloud tops, with a correlation coefficient of similar to 0.94. The tight relationship between r e and PWC was established only when particles with sizes large enough to precipitate (drizzle and raindrops) are included in calculating r(e). Our results emphasize for the first time that r(e) is a key parameter to determine both initiation and amount of precipitation at the top of convective clouds.