Combining ARM and Satellite Data Leads to Insights into Warm Rain Formation

The Science

Warm liquid clouds are of fundamental importance to the global climate since their particle-growth processes (the growth of droplets from cloud particles to drizzle to rain) play a key role in the hydrological cycle of the Earth. Lack of a full understanding of these particle-growth processes and how to best represent them in Earth system models is a source of uncertainty in future climate projections. Previous studies have indicated differences in particle growth processes and in the transition from cloud to drizzle in clouds that form over land and ocean. This study combines satellite observations with detailed ARM ground-based observations to explore the hypothesis that these differences between clouds over land and ocean are caused by different intensities of rising air motion (convective updrafts) in the clouds.

The Impact

This study shows that a stronger updraft increases the height at which significant droplet coalescence begins, and also prolongs the lifetime of falling drops, which promotes larger droplet growth.  These results point to the critical role of the strength of the convective updraft in the warm-rain formation process.   These processes occur at cloud scales smaller than the grid boxes of most weather and Earth system models, so their effects must be represented by numerical formulations known as ‘parameterizations’.  This study indicates important paths to improving these parameterizations, and hence future simulations of global rainfall, in Earth system models.

Summary

This study provides new insight into the role of vertical velocity in the warm-rain formation process using both NASA satellite observations and surface observations obtained from a U.S. Department of Energy (DOE) ARM mobile observing system that operated in the Azores. Results from the satellite data, from the ARM surface dataset, and from a one-dimensional model all produce a consistent result, that the land-ocean differences in the warm-rain formation process can be (at least partly) explained by the land-ocean differences in the intensity of cloud updraft when the aerosol effects are minimized. The present result is important because it is the first clear evidence of cloud motion affecting the cloud-precipitation process in warm clouds. Current global climate models (GCMs) cannot resolve these cloud-scale motions which we show are important in simulating realistic warm-rain clouds.

This article provides the first detailed analysis of the land-ocean difference in warm-rain clouds using the contoured frequency by optical depth (CFOD) method. And for the first time, the role of updraft intensity in the warm-rain formation process has been demonstrated by minimizing aerosol effects; namely, that updraft intensity affects both the height at which significant coalescence begins and the lifetime of falling drops. This study encourages focusing on other important variables such as vertical velocity beside aerosol effects in warm-rain clouds, which represents a new way of understanding the warm-rain formation process that can contribute to climate model improvement in representing warm-rain clouds.

Principal Investigator(s)

Hanii Takahashi
Jet Propulsion Laboratory
[email protected]

Funding

This study was carried out at Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This study was supported by NASA Grants NNN13D455T. K. Suzuki was supported by NOAA’s Climate Program Office’s Modeling, Analysis, Predictions, and Projections Programme with the grant number NA15OAR4310153, JAXA/EarthCARE and JAXA/GCOM-C projects. Data were obtained from the Atmospheric Radiation Measurement (ARM) Program sponsored by the US Department of Energy, Office of Science, Office of Biological and Environmental Research.

References

Takahashi H., K. Suzuki, and G. Stephens. 2017. “Land-Ocean Differences in the Warm-Rain Formation Process in Satellite and Ground-Based Observations and Model Simulations,” Quarterly Journal of the Royal Meteorological Society, 143(705), 1804-1815. DOI: 10.1002/qj.3042