U.S. Department of Energy Office of Biological and Environmental Research

BER Research Highlights

Origin of Water-Vapor Rings in Tropical Oceanic Cold Pools
Published: September 07, 2015
Posted: September 14, 2015

Both observations and cloud-resolving models have frequently revealed that convective clouds over ocean grow from relatively moist boundary layers near the edges of evaporatively driven cold pools. Despite the relevance of these rings for the initiation and organization of cumulus clouds, there has been considerable debate about their origin. The prevailing hypothesis is that evaporation of rain drops within the sub-cloud layer provides the extra moisture that subsequently gets spread out radially by the gust front of the cold pool. However, the sub-cloud layer could be relatively moist simply because deep convection is favored in such environments in the first place. Or, surface latent-heat fluxes could provide the excess moisture found near cold pool edges. Scientists supported by the Department of Energy’s Atmospheric System Research and Scientific Discovery through Advanced Computing (SciDAC) programs carried out large-eddy simulations of a single cloud and cold pool to determine the individual contributions to the water-vapor field.

Their simulations reveal that the dominating contribution to these water-vapor rings comes from surface latent-heat fluxes. In contrast, the source from evaporated rain drops is rather small inside these rings. During the initial phase of cold pool formation the displaced sub-cloud layer air is relatively moist only because the sub-cloud layer has already been relatively moist before rain started falling into it.

Using a simple vertical velocity equation, the scientists demonstrate that evaporation can only explain roughly one third of the observed perturbation. During the cold pool’s early development, the time a descending air parcel is exposed to the rain shaft below cloud base is set by buoyant acceleration. Using this analytical framework, they show that this exposure time is short compared to the time required to evaporate sufficient moisture into the sub-cloud layer. The reasons for this finding are (a) the small saturation deficit in the sub-cloud layer (thus small evaporation rates) and (b) the sufficiently strong negative buoyancy provided by the weight of rain drops.

Reference: Langhans, W., and D. M. Romps. 2015. “The Origin of Water-Vapor Rings in Tropical Oceanic Cold Pools,” Geophysical Research Letters, DOI: 10.1002/2015GL065623. (Reference link)

Contact: Dorothy Koch, SC-23.1, (301) 903-0105, Randall Laviolette, SC-21, (301) 903-5195, Shaima Nasiri, SC-23.1, 301-903-0207
Topic Areas:

  • Research Area: Earth and Environmental Systems Modeling
  • Research Area: Atmospheric System Research
  • Cross-Cutting: Scientific Computing and SciDAC

Division: SC-23.1 Climate and Environmental Sciences Division, BER


BER supports basic research and scientific user facilities to advance DOE missions in energy and environment. More about BER

Recent Highlights

May 10, 2019
Quantifying Decision Uncertainty in Water Management via a Coupled Agent-Based Model
Considering risk perception can improve the representation of human decision-making processes in age [more...]

May 09, 2019
Projecting Global Urban Area Growth Through 2100 Based on Historical Time Series Data and Future Scenarios
Study provides country-specific urban area growth models and the first dataset on country-level urba [more...]

May 05, 2019
Calibrating Building Energy Demand Models to Refine Long-Term Energy Planning
A new, flexible calibration approach improved model accuracy in capturing year-to-year changes in bu [more...]

May 03, 2019
Calibration and Uncertainty Analysis of Demeter for Better Downscaling of Global Land Use and Land Cover Projections
Researchers improved the Demeter model’s performance by calibrating key parameters and establi [more...]

Apr 22, 2019
Representation of U.S. Warm Temperature Extremes in Global Climate Model Ensembles
Representation of warm temperature events varies considerably among global climate models, which has [more...]

List all highlights (possible long download time)