Rains-Showers Paradigm in Organization of Tropical East Pacific Convection Field Project Observations

In his book on monsoon meteorology, Colin Ramage stated that “... most [monsoon] rain falls in one of these two circumstances: either from (1) deep nimbostratus with embedded cumulonimbus when vertical wind shear and lower tropospheric convergence both are large (although rain intensity may fluctuate considerably, skies remain predominantly overcast), or (2) scattered towering cumulus or cumulonimbus, when vertical shear and lower tropospheric convergence both are small. I shall assign the term rains to the former and showers to the latter, realizing of course that sharp demarcation is impossible.” He then notes that rains are associated with cloudy, near-moist-adiabatic soundings while showers occur in more unstable environments with clear air.

We use observations (high-altitude dropsondes deployed from the NSF/NCAR Gulfstream V aircraft and Hiaper Cloud Radar) from OTREC2019 (Organization of Tropical East Pacific Convection) field project to see whether Ramage’s rains-showers paradigm applies in the OTREC region. OTREC performed systematic observations over pre-specified regions (the Eastern Pacific and southwest Caribbean) and randomly selected dates (over two months), at the same time each day, capturing snapshots of all convection.

We find that showers tend to occur for drier initial soundings with greater moist convective instability and high elevations of maximum lifted parcel buoyancy, whereas rains convection is favored by moister, more stable soundings. The saturation fraction, the mean rainfall rate and vertical mass flux were greater in the rains cases than in the showers. The showers cases exhibited more lightning than the rains cases. From HCR radar’s vertical particle velocity, we infer that showers are accompanied with graupel, while rains have mostly aggregates.