The Cloud-Radiative Forcing of the US landfalling Atmospheric Rivers

Atmospheric Rivers (ARs) are narrow elongated regions with significant 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 coarsely represented in global climate models.

We studied the 3D spatial structures of kinematic and thermodynamic fields from the ERA-interim reanalysis and satellite observations at the time steps before, during, and after the ARs impinged on the southwest and northwest coasts of US in November—March, 2000–2008. There are totally 120 cases of ARs, with 60 cases making landfall on the southwest coast (SW-ARs) and 60 cases on the northwest coast (NW-ARs), as identified by Dettinger et al. (2011).

It was found that the SW-ARs transported less moisture to the US continent than the NW-ARs, yet the former was more likely to produce prolonged extensive ice clouds through indirect effects thus inducing stronger CRF. Specifically, one day before (day-1), and the day when (day+0) the SW-ARs made landfall, a significant increase of ice clouds took place around the landfalling regions with integrated horizontal water vapor fluxes (IVT) approximately 200 kg/m/s. On day+1, IVT propagated eastward and reduced to 153 kg/m/s, collocated with substantial ice cloud coverage. On day+2, the IVT further reduced to 103 kg/m/s, with ice clouds covering extensively over the Central US. In contrast, the IVT associated with the NW-ARs were, e.g., >434 kg/m/s on day-1 and day+0 in the landfalling regions, and were 237 kg/m/s on day+1 in its downstream regions. Less ice-cloud coverage was found along NW-ARs' trajectory in day+2~+3.

A key mechanism for the persistent and extensive ice-cloud coverage in the Central US was attributed to the secondary ARs. These ARs rooted in the Caribbean Sea, drawing moisture towards the Gulf of Mexico and then penetrated deeply into the Central US (hereafter, the Gulf-ARs). Following the AR criteria in Lavers et al. (2013), 51 Gulf-ARs were found day+0~+3 after the SW- and NW-ARs, 32 took place exclusively after the SW-ARs and 15 exclusively after the NW-ARs. Most of the Gulf-ARs were on day+2~+3, and were stronger after the SW-ARs.

Synoptic-scale analyses were conducted to explain why the SW-ARs favored the strong and frequent Gulf-ARs. The Gulf-ARs were developed in connection with a strong pressure gradient between a extratropical cyclone to the east of the Rocky Mountains and a subtropical ridge over the Eastern US. In the SW-AR cases, the strengthening of the extratropical cyclone was supported by a 200-hPa jet streak and thus creating stronger pressure gradient than that in the NW-ARs. In addition, an abnormally strong low-level easterly (CLLJ) was observed in the Caribbean Sea concurrently with the NW-ARs. Presumably, through exporting more moisture from the Caribbean Sea to the tropical Eastern Pacific by the CLLJ, less moisture was likely to be extracted northward to the US by the Gulf-AR.

Details on 1) the interactions between the synoptic-scale processes and the subgrid-scale convection, and 2) the associated CRF impacts will be further discussed at time scales relevant to climate processes in the presentation.

References:

Luo, Q., and W.-w. Tung, 2015: Case study of moisture and heat budgets within atmospheric rivers, Mon. Wea. Rev., doi: 10.1175/MWR-D-15-0006.1

Dettinger, M.D., et al., 2011: Atmospheric rivers, floods, and the water resources of California. Water, 3, doi:10.3390/w3020445.

David A. Lavers and Gabriele Villarini, 2013: Atmospheric Rivers and Flooding over the Central United States. J. Climate, 26, 7829–7836. doi: 10.1175/JCLI-D-13-00212.1