An Overview of Atmospheric Features Over the Western North Atlantic Ocean and North American East Coast—Part 2: Circulation, Boundary Layer, and Clouds

Abstract

The Western North Atlantic Ocean (WNAO) region represents a complex climate system that comprises a wide range of spatiotemporal scale phenomena: mesoscale continental convection and tropical cyclones, synoptic-scale processes (e.g., frontogenesis), and interannual climate variability (e.g., North Atlantic Oscillation). The region is influenced by the Gulf Stream current system, which gives rise to sharp spatial gradients in sea surface temperature (SST) and is responsible for significant ocean-atmosphere interactions (Small et al., 2008). In this regard, interactions between SST, surface air temperature, and winds yield strong turbulent fluxes that regulate the evolution of the atmospheric boundary layer and the regional atmospheric circulation (Nakamura et al., 2008). The WNAO climate is strongly controlled by the semipermanent North Atlantic Anticyclone, which modulates the air flow patterns and aerosol transport from North America and Africa. The diverse atmospheric and oceanic processes over WNAO are responsible for a variety of cloud morphological types: (i) stratiform boundary-layer clouds preferentially in winter and spring, (ii) shallow cumulus over the ocean during the warm season, and (iii) and deep convective and cirrus clouds associated with fronts, continental convection, and tropical cyclones. Noteworthy is the intense convective

Comment on acp-2021-153

“…As low-level cloud fraction impacted model results of Section 4.2 so substantially, the dynamics of the studied clouds require further characterization. As cloud fraction and CAO index are well related, especially in DJF, aerosol-cloud interactions likely are stronger than other seasons (as implied by Section 3.5) due in part to enhanced surface fluxes and turbulence, and thus more droplet activation with higher cloud supersaturations (Painemal et al, 2021); in contrast, the smaller shallow cumulus clouds in summertime may be less favorable for droplet activation due to factors such as reduced turbulence and more lateral entrainment.…”

“…The following is the list of thermodynamic/dynamic input parameters derived from MERRA-2: vertical pressure velocity at 800 hPa (ω800), planetary boundary layer height (PBLH), cold-air outbreak (CAO) index, relative humidity (RH) in the PBL and free troposphere represented by RH950 and RH800, respectively. CAO index is defined as the difference between skin potential temperature (θskt) and air potential temperature at 850 hPa (θ850) (Papritz et al, 2015). CERES-MODIS cloud parameters include liquid cloud fraction and cloud top height for lowlevel clouds.…”

“…These events are marked by cold air over the warm ocean leading to strong surface heat fluxes, boundary layer deepening, weakened inversion strength, in addition to high and deep clouds (Kolstad et al, 2009;Fletcher et al, 2016;Abel et al, 2017;Brummer, 1996;Naud et al, 2018). Coincident with these features is the Icelandic Low, which is a significant climatological feature of the North Atlantic whereby subpolar low pressure builds in extratropic areas beginning in the fall with westerly winds in the boundary layer that shift more to northerly in the winter (Sorooshian et al, 2020;Painemal et al, 2021). This low-pressure system seems to be stronger on high Nd days resulting in more continental outflow and high number concentrations of CCN; the greater CAO index values near the coast promote high cloud coverage affording more opportunity for cloud processing of particles to ultimately enhance droplet activation.…”

“…Dust in JJA and sea salt in DJF, seasons of which each respective aerosol type is most predominant, exhibited negative relationships with Nd. Such a negative relationship is plausibly related to differences between ACI when calculated using AOD versus AI (Painemal et al, 2021); for instance, coarse sea salt can expedite collision-coalescence and thus reduce Nd, which has the effect of reducing ACI (Eq. 3) and even possibly yielding negative values (Table 4).…”

“…Of special interest in this work is the western North Atlantic Ocean (WNAO) where decades of extensive research have been conducted for topics largely unrelated to aerosol-cloud interactions (Sorooshian et al, 2020), thereby providing opportunity for closing knowledge gaps for this area in a region with a wide range of aerosol and meteorological conditions Painemal et al, 2021). Past work showed different seasonal cycles of AOD and Nd in this region (Sorooshian et al, 2019;Grosvenor et al, 2018), which partly motivates this study to unravel why Nd behaves differently on seasonal time scales.…”