During California field exercises of the Hydrometeorological Testbed (HMT), a key objective has been to improve longer-range forecasts of so-called “atmospheric rivers” or ARs (narrow streams of mid- to low-level moisture) and other meteorological patterns that produce very heavy rainfall. During efforts to evaluate model forecasts for these exercises the DTC has explored methods that can provide more meaningful verification than standard scores. One such method represents regions of, say, precipitation in model forecasts and observed fields as spatial objects and then quantitatively compares attributes of these objects such as size, location, geographical overlap, etc. Since the landfall of moisture on the Western U.S. coastline is a key factor in AR forecasts, a novel approach for this project has been to define objects within thin domains that follow the coastline (as in the figure), and to choose actual moisture transport as a basis for the fields from which to define objects. The narrow coastline-hugging domain allows the MODE (the Method for Object-Based Diagnostic Evaluation) evaluation to focus on actual landfall of moisture, a key factor in the effort to forecast severe precipitation in California and other regions vulnerable to ARs.
