Warm Rain

Warm Rain Introduction

Warm rain systems (those made up of only liquid water droplets) pose an observational challenge since they are often close to the surface, thin, and not raining very intensely. Yet these systems play an important role in removing moisture from the atmosphere and in cloud feedbacks through their influence on cloud structure. Our group uses observations from NASA’s CloudSat satellite that was designed specifically to detect clouds and light rain to confront this challenge from a near-global perspective. We are also involved with localized observation campaigns designed to provide intensive observations of warm rain so we can better understand the underlying formation mechanisms and interactions with aerosols and the environment.

The Global Distribution of Warm Rain

Warm rain is actually present throughout much of the world. This is very evident in the figure on the left that shows the frequency that warm rain occurs across the world’s ocean as seen from CloudSat. In the midlatitudes, warm rain often occurs in the postfrontal regions of midlatitude cyclones. In the tropics, warm rain occurs in the form of stratocumulus decks and small, sometimes isolated, convection. Some of our work has focused on quantifying the contribution of warm rain to the world’s water cycle and realizing how warm rain formation is affected by the surrounding environment.

Warm Rain During the ORACLES Experiment

The ORACLES field campaign focuses on studying the semi-permanent stratocumulus deck over the southeast Atlantic Ocean. Observations from the 3rd Generation Airborne Precipitation Radar (APR-3), a triple frequency radar, is revealing everything from ultra-thin stratocumulus to moderately raining embedded shallow convection, within these clouds in a wide variety of meteorological regimes and during the biomass burning season. We are using these APR-3 measurements to evaluate long-standing scientific questions involving how, why, and what kind of aerosols suppress (or promote) warm rain.

Aerosol-Cloud-Precipitation Interactions

Albrecht (1989) hypothesized aerosol loading in a warm cloud layer will delay the onset of precipitation increasing the cloud extent and cloud lifetime. Satellite observations of the cloud lifetime effect, or the second indirect effect, have been sparse, but have suggested that models often fail to accurately represent the microphysical changes to the cloud extent and precipitation accurately.

CloudSat and CALIPSO high resolution observations allow us a glimpse into the effects of aerosol on both cloud and precipitation properties. The proposed pathway by Albrecht in 1989 claims precipitation suppression is the main pathway to an increase cloud extent, however it may be that the cloud lifetime effect is the summation of both an increased cloud extent due to delayed precipitation and cloud invigoration due to aerosol-cloud interactions. We are now distinguishing the difference between the two distinct pathways of increased cloud extent and quantifying the total cloud lifetime effect.