Convective Systems in the Atmospheric Rivers: Case studies

This work performs case studies on the convective systems embedded in two “Atmospheric Rivers” (ARs) that impacted the North America in January 2009. The ECMWF Year of Tropical Convection data, Climate Prediction Center Morphing Technique precipitation, and Global Precipitation Climatology Project One-Degree Daily precipitation data set were used to construct the three-dimensional spatial structures of kinematic and thermodynamic fields. Our case 1 was west-east oriented, and case 2 was north-south oriented. In both cases, an AR was formed by merging a primary integrated water vapor (IWV) band and a secondary IWV band. While this process serves as a moisture source of the AR, it initiates precipitating convective systems that act as moisture sinks. By computing the apparent heat source (Q1) and apparent moisture sink (Q2), we were able to show the locations of these precipitating convective systems as they propagated towards the upstream AR. By comparing the Q1 and Q2 profiles, we found that in the early stages after an AR was initiated, cumulus type of heating was the dominated heating mechanism. In the late stages when an AR weakened, stratiform heating dominated. The evolution of these convective systems determined the magnitudes and locations of heating/cooling and drying/moistening, thus impacting the ambient thermodynamic fields and the moisture transport process of ARs. Our two cases had different fractional precipitating convective systems coverage, with precipitation in case 1 covered ~48% of the AR, and precipitation in case 2 covered ~81% of the AR, we thus compared the integrated thermodynamic impacts of these two cases. Our results showed that case 1 had less vertically integrated heating and drying than case 2. Northeastern Pacific was a heat sink and moisture sink in case 1, but it was a heat source and moisture sink in case 2.