Abstract
Abstract. The North Pacific High (NPH) is a fundamental meteorological feature present during the boreal warm season. Marine boundary layer (MBL) clouds, which are persistent in this oceanic region, are influenced directly by the NPH. In this study, we combine 11 years of reanalysis and an unsupervised machine learning technique to examine the gamut of 850 hPa synoptic-scale circulation patterns. This approach reveals two distinguishable regimes – a dominant NPH setup and a land-falling cyclone – and in between a spectrum of large-scale patterns. We then use satellite retrievals to elucidate for the first time the explicit dependence of MBL cloud properties (namely cloud droplet number concentration, liquid water path, and shortwave cloud radiative effect – CRESW) on 850 hPa circulation patterns over the northeast Pacific Ocean. We find that CRESW spans from −146.8 to −115.5 W m−2, indicating that the range of observed MBL cloud properties must be accounted for in global and regional climate models. Our results demonstrate the value of combining reanalysis and satellite retrievals to help clarify the relationship between synoptic-scale dynamics and cloud physics.
Highlights
Low, stratiform clouds that develop in the marine boundary layer (MBL) are of significant interest to the atmospheric science community because they impact meteorological forecasts and, a host of human activities (e.g., Koracin and Dorman, 2017)
For discussion of the Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals, we focus on N, liquid water path (LWP), and CRESW because these variables most clearly accentuate the connection between large-scale dynamics and MBL cloud properties
Even in the instance where the time difference between a MODIS image and North American Regional Reanalysis (NARR) grid is a maximum, we expect the influence of time mismatch to be minimal because we focus on the synoptic scale over relatively short time periods
Summary
Stratiform clouds that develop in the marine boundary layer (MBL) are of significant interest to the atmospheric science community because they impact meteorological forecasts and, a host of human activities (e.g., Koracin and Dorman, 2017) These cloud types are widespread (coverage on the order of one-third of the globe at any given time; e.g., Hartmann et al, 1992) in the subsiding branch of the Hadley circulation (e.g., Wood, 2012) due to a separation of the cool, moist MBL and the warm, dry free troposphere by a strong (∼ 10 K) and sharp O(100–500 m) thermal inversion (e.g., Parish, 2000). These shifts in the NPH are typically associated with an increase in the offshore component of the wind along the western United States coastline and a clearing of the cloud deck (e.g., Kloesel, 1992; Crosbie et al, 2016), and they may lead to a complete reversal of the alongshore pressure gradient (e.g., Nuss et al, 2000)
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