Low-level Jets Enhance Orographic Rainfall in Coastal Mountains

Contact: Marty Ralph

Conceptual representation of orographic rainfall in California's coastal mountains.

The California Land-falling Jets Experiment (CALJET) was aimed at improving quantitative precipitation forecasting in California's coastal mountains through better understanding of physical processes in the low-level jet (LLJ) region and evaluation of the capabilities and limitations of current and potential future operational observations. Although it was known that the LLJ plays a role in producing floods in the region, important scientific questions remained, and gaps in the current observational network made it difficult to monitor the LLJ, even as it hit the coast. Flooding is a particular concern because it causes an annual average of $0.5 to $1 billion dollars in damage in the region, it is a leading emergency management concern, and California is second in the Nation in flood-related fatalities.

Through the use of the NOAA Environmental Technology Laboratory's coastal wind profiler network, we found that hourly rain rates in California's coastal mountains during land-falling Pacific winter storms were most strongly controlled by the upslope wind speed at 1-km altitude (see the conceptual figure on the website given), which corresponds to the altitude of the LLJ located ahead of cold fronts. Also, when a LLJ was present the orographic precipitation process was 50% more efficient than at other times. Overall, roughly half of the hourly variability in coastal mountain rain rates resulted from hourly variations in upslope wind speed at 1 km altitude. Conversely, coastal surface winds were often uncorrelated with rain rates in nearby mountains due to blocking of the low-level airflow, although rainfall on the coast increased when blocking was present. Results were based upon a full season of rain-gauge and 915-MHz wind-profiler observations that greatly increased the number of samples available compared to earlier studies and allowed documentation of the altitude dependency.

These results establish a scientific and practical basis for improving West Coast observations and nowcasting in ways that can aid the issuance of Watches and Warnings by the National Weather Service and help in decision making by forecast users, especially in terms of flooding.

Related Links

• PACJET 2003
• Regional Weather and Climate Applications Division