Atmospher Sci & Global Chg
Research Highlights

November 2012

Tropical Clouds: From Jekyll to Hyde

Revealing the environmental conditions that push tropical clouds from fair-weather to stormy

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From Jekyll to Hyde, this anvil cloud is an example of tropical clouds that evolve from fair-weather to stormy. Scientists at PNNL used observational data and high-resolution modeling to uncover and rank the key environmental conditions that encourage this transformation.

Results:  It's a suspense story with a world-climate conclusion. Using high-resolution model simulations, two scientists from Pacific Northwest National Laboratory uncovered the relative importance of unique conditions that lead to tropical rainstorm clouds using a novel mathematical approach. Among four key environmental factors, they found that the presence of moisture and vertical wind velocity events, about one hour before the cloud forms, are the prime culprits. The researchers validated the model results with unique data collected by the U.S. Department of Energy's (DOE's) atmospheric instruments.

Why It Matters: Weather is born in the turbulent tropics. A continual cycle of heat and moisture is pulled from the tropical ocean and transported around the globe on belts of atmospheric energy. Tropical clouds are at the leading-edge of these forces. Understanding how they form, and replicating their lifecycle in global climate models, remains an elusive goal for those aiming to project climate changes accurately. Scientists need a better understanding of how different natural forces, like moisture and wind, control the timing of clouds' transition from fair-weather to rainstorm clouds. The unique mathematical technique described in the study integrates field observations and cloud-resolving models to identify environmental variables important for tropical storm-cloud creation. The research will enable better predictions of these climate forces in global circulation models.  

Methods: This research describes a novel mathematical technique to assess the comparative roles of key environmental variables and the distribution of moisture and winds inside of clouds that trigger deep convection in the tropics.

Lead researcher Dr. Samson Hagos and Dr. L. Ruby Leung, atmospheric scientists at PNNL, used a high-resolution model, data from the Atmospheric Radiation Measurement (ARM) Climate Research Facility, NASA-TRMM satellite rainfall measurements, and the NOAA GFS-FNL analysis to develop an analytical method to represent four environmental conditions that trigger deep convection clouds. Four environmental conditions are represented in a vertical profile as vectors:

• moisture
• vertical velocity
• potential temperature
• equivalent potential temperature.

The analysis measures the degree of separation between the clusters of grid points in the vector space corresponding to the "transitioning" and "non-transitioning" convection conditions. The researchers' novel approach identified anomalies in the cloud environmental conditions that are important to trigger the transition to deep convection. The approach also quantified the degree of importance for each environmental variable in comparison to the others.

Identifying these key environmental variables, quantifying their relative importance and adequately representing them in models are among the most important goals of the ARM field campaigns. This research used data from the sites in Darwin, Australia; Manus, Papua-New Guinea; and Niamey, Niger.

While the specific vertical profiles of these variables differ from region to region, the study showed that the moisture and vertical velocity anomalies, occurring about one hour before the triggering of convection, are better predictors of the clouds' transition to deep convection.

What's Next? The researchers plan to incorporate information from the cloud-resolving model simulations that have been evaluated against observations in this study into climate model parameterizations of deep convection using probability distribution functions of environmental factors.

Acknowledgments:  

Sponsors: This work is supported by the U.S. DOE's Office of Biological and Environmental Research Atmospheric System Research program.

Research Team: Drs. Samson Hagos and L. Ruby Leung of PNNL.

Reference: Hagos S and LR Leung. 2012. "Large-Scale Environmental Variables and Transition to Deep Convection in Cloud Resolving Model Simulations: A Vector Representation." Journal of Advances in Modeling Earth Systems 4:M11001. DOI:10.1029/2012MS000155.