Analyzing Upstream Orographic Precipitation Enhancement During an OLYMPEX Warm Front Passage Using Observations and High-Resolution WRF Simulations
Frontal precipitation processes in the Pacific Northwest are often modified by interactions with the terrain of the Olympic Mountains, resulting in enhanced precipitation on the windward slopes (west/southwest). While many studies have observed orographic enhancement over the high terrain, understanding of how these processes extend upstream is limited, especially in rapidly-shifting flow regimes. This study analyzed surface precipitation accumulations upstream, near-shore, and adjacent to the Olympics from the 17 December 2015 warm front case during OLYMPEX to examine the role of blocking in upstream orographic enhancement. Using high-resolution Weather Research and Forecasting (WRF) simulations, and observations from the NPOL dual-polarization radar, precipitation accumulations from WRF simulations and OLYMPEX observations are divided into five regions upstream of NPOL and into timeframes relative to landfall to analyze the distribution of precipitation enhancement associated with the warm front. Results indicate that 40 km upstream of the coast saw nearly double 5-min precipitation accumulations compared to regions closer to the coastline, even with radar indicating widespread stratiform precipitation across the region.
High-resolution observed soundings indicate little change in surface thermodynamic characteristics of the low Froude number region ahead of the surface front as it propagated northeastward and eventually stalled, a possible indication of blocking by the terrain. Findings from sounding-derived vertical stability parameters show high levels of low-level stability and significant vertical wind shear which led to the formation of Kelvin-Helmholtz (KH) waves during this case. Future work will thus focus on characterizing the precipitation within these subregions as the frontal flow characteristics evolved.
ENSO-induced Teleconnections on Atmospheric Radiation
The spatial pattern of sea surface temperature changes impacts the net radiative feedback in the Earth’s climate system, and thus the magnitude of global warming - a phenomenon known as the pattern effect. In the observational record, the dominant mode of variability in SST patterns is the El Nino Southern Oscillation (ENSO). ENSO teleconnections and predictability have been a focus of research in climate science, but the relationship between ENSO, clouds, and the Earth's radiation budget has not been fully explored. We use satellite observations of radiation, atmospheric reanalysis, and sea surface temperature reconstructions to examine the impact of the full ENSO cycle on radiation at the top of the atmosphere. Using a recent radiative flux by cloud type NASA product, along with radiative kernel decompositions, we can isolate the role of different processes in mediating the radiative response to ENSO. We find a dominant role for low-cloud radiative effects driven by atmospheric inversion changes, along with long-wave clear-sky water vapor feedbacks.