On the Relationship Between the Marine Cold Air Outbreak M Parameter and Low‐Level Cloud Heights in the Midlatitudes

Abstract

Focusing on conditions of subsidence when low clouds are present, ground‐based observations in both the North Atlantic and the Southern Ocean reveal strong relationships between cloud boundary (base and top heights) and different measures of lower tropospheric instability. The difference in potential temperature between the surface and 800 hPa (a metric called M) provides a stronger relationship than measures of inversion strength such as the lower tropospheric stability and estimated inversion strength. This is because (1) inversion strength itself does not correlate well with cloud boundaries, and (2) M contains information that appears important for cloud boundaries. These include the surface forcing through the use of sea surface rather than near‐surface air temperature and an upper level close to the real cloud top. These results expand upon previous work on the importance of M as a predictor of cloud morphology. However, important differences are found in low‐cloud conditions for the North Atlantic as compared to the Southern Ocean (for a given value of M): stronger inversions, deeper boundary layers, and much larger sea level pressures. Therefore, the relationship between cloud boundaries and M differs between the two regions. A general circulation model provides similar relationships as observed between M and both cloud top height and temperature but tends to place clouds higher and at colder temperatures than observed for a given M. This might cause issues with the representation of precipitation, cloud cover and radiation in the Southern Ocean.

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

“…Meteorological drivers of postfrontal boundary‐layer clouds have been investigated in Naud et al. (2016, 2018, 2020) and Lamer et al. (2020).…”

“…Naud et al (2016) note a positive seasonal‐scale correlation between two metrics of boundary layer stability (which includes LTS) and cloud cover, a finding reminiscent of similar relationships observed in subtropical marine stratocumulus cloud regimes (Klein & Hartmann, 1993). A closer look at daily variations reveals correlations of −0.35 and 0.42 between cloud top height and LTS, and CAO index, respectively, over the Southern Ocean and North Atlantic (Naud et al., 2020). While the slightly stronger correlation for CAO index is partly explained by the choice of pressure level used to compute LTS (700 hPa) and CAO (800 hPa), with the latter being closer to the cloud top, these results suggest that both, atmospheric stability and sea‐air temperature differences contribute to marine low‐cloud variability.…”

“…Interestingly, the correlations slightly exceed those for CAO index (0.3–0.5 for both LCF and H T ). We also calculate correlations for LTS, which is calculated here as the potential temperature difference between 800 and 1,000 hPa, to avoid ambiguities associated with the use of different pressure levels between CAO and LTS (Naud et al., 2020). LTS yields substantially smaller absolute correlations than its CAO and Δ T counterparts in DJF, with maximum negative correlations of −0.3 north of 40°N, and modest values for most of the WNAO domain (−0.2–0.1, Figure 15c).…”

“…Meteorological drivers of postfrontal boundary-layer clouds have been investigated in Naud et al (2016Naud et al ( , 2018Naud et al ( , 2020 and Lamer et al (2020). Naud et al (2016) note a positive seasonal-scale correlation between two metrics of boundary layer stability (which includes LTS) and cloud cover, a finding reminiscent of similar relationships observed in subtropical marine stratocumulus cloud regimes (Klein & Hartmann, 1993).…”

The Impact of Slantwise Descending Dry Intrusions on the Marine Boundary Layer and Air‐Sea Interface Over the ARM Eastern North Atlantic Site

“…Several diagnostic parameters have been proposed that link the coverage of low clouds with the tropospheric stability (Klein & Hartmann, 1993; Naud et al., 2020; Wood & Bretherton, 2006). These indices exploit the fact that a stronger boundary layer inversion is more efficient in capping moisture within the marine boundary layer, permitting greater cloud cover.…”