The Cloud-Radiative Forcing of North American landfalling Atmospheric Rivers

Abstract:

Atmospheric Rivers (ARs) are narrow elongated regions with strong horizontal water vapor flux associated with extratropical cyclones. Upon making landfall, conspicuous mid-to-high-latitude stratiform cloud decks with high reflectivity are observed along with the ARs in satellite imagery. The cloud-radiative forcing (CRF) associated with these clouds has only been preliminarily established (e.g., Luo and Tung 2015). Their climatological impacts are not understood, yet the related cloud microphysics and radiation processes are poorly represented in global climate models.

We studied the correlations between observed variables including the ECMWF-Interim horizontal water vapor fluxes (IVT) integrated from 1000—300 hPa, CERES-derived cloud water path and CRF, and MODIS cirrus reflectance before, during, and after the ARs impinged on the southwest and northwest coasts of North America (NA) in Nov—March, 2000–2008, with 60 ARs affecting the southwest coast (southwest ARs), and 60 ARs affecting the northwest coast (northwest ARs, Dettinger et al., 2011). Anomalies were calculated by subtracting the average over all time steps.

For the southwest ARs, a significant increase of ice clouds took place around the landfalling regions with IVT anomalies >130 kg/m/s on landfalling day-1 and day+0. On day+1, a substantial increase of ice clouds with 50% reduction of IVT anomalies was found along with the ARs. On day+2 to day+3, positive IVT anomalies existed over the central and eastern US. These anomalies could be attributed to the southwest ARs and the secondary ARs that rooted in the Gulf of Mexico and made landfall over central and eastern US. Many parts of the NA continent were covered under ice cloud decks.

The IVT anomalies for the northwest ARs were >250 kg/m/s on day-1 to day+0, and approximately 120 kg/m/s on day+1. Nevertheless, the northwest ARs were not observed to make landfall concurrently with the secondary landfalling ARs from the Gulf of Mexico. Only a small part of the NA continent was covered by a slight increase of ice clouds along with the ARs on day+1 to day+3.

Differences in the amount and spatial distribution of continental ice clouds in different AR scenarios thereby imply different regional CRF; such implications will be further discussed at time scales relevant to climate processes in the presentation.