Post Cold Frontal Clouds at the ARM Eastern North Atlantic Site: An Examination of the Relationship Between Large‐Scale Environment and Low‐Level Cloud Properties

Abstract

Using cloud and environment observations from the Atmospheric Radiation Measurement (ARM) program Eastern North Atlantic site and an automated cold front detection routine, cloud properties in post cold front (PCF) periods are examined and compared to similar conditions of subsidence (non-PCF). PCF periods exhibit stronger subsidence and wind speed than non-PCF periods, with weaker inversions and stronger surface temperature contrasts. Low-level clouds are predominant and are found to have higher cloud base and top heights, colder cloud top temperature, and greater vertical extent and liquid water path during PCF than non-PCF periods. The environmental metric that is best correlated with cloud boundaries for both PCF and non-PCF periods is the difference in potential temperature between the sea surface and 800 hPa, a parameter used to locate cold air outbreak conditions. However, the cloud vertical extent and liquid water path are found to be better correlated with sea-air temperature contrast, a parameter related to turbulent surface fluxes. The strength of the relationships between the cloud characteristics and these metrics does not differ for PCF and non-PCF periods. However, the strength of the metrics differs between PCF and non-PCF periods and can explain cloud property differences. The results suggest both the properties of the boundary layer and the presence of an upper-level cyclone associated with the cold front determine PCF cloud properties.

Relationships Between Precipitation Properties and Large‐Scale Conditions During Subsidence at the Eastern North Atlantic Observatory

“…Cold fronts are identified following the approach described in Naud et al (2018). In a nutshell, the Modeling, Analysis, and Prediction (MAP) Program Climatology of Midlatitude Storminess (MCMS; Bauer & Del Genio, 2006) cyclone location algorithm is used to identify extratropical cyclones that travel in the North Atlantic.…”

“…This idea is further supported by evidence that, among other precipitation properties, rain rate ( RR ) scales with a factor of 3 of cloud thickness ( H ) while only being inversely proportional to droplet number concentration ( N ; i.e., RR ~ H 3 / N ) (VanZanten et al, 2005; Yang et al, 2018). Since a recent study by Naud et al (2018) points out that cloud macrophysical properties, including cloud thickness, are largely influenced by large‐scale conditions, it would not be surprising for rain characteristics to also be influenced by the properties of the large‐scale environment in which they form. That may be especially the case in subsidence regimes where clouds are confined to the shallow boundary layer.…”

The Interaction Between Boundary Layer and Convection Schemes in a WRF Simulation of Post Cold Frontal Clouds Over the ARM East North Atlantic Site

“…We analyze the clouds behind a cold front that passed over the ENA site on 24 December 2015 around 21 UTC. Northerly wind conditions after the passage of the cold front were observed at the site for a total of 48 hr: this timeline is used to define the PCF region (Naud et al, ). The cloud field in the Azores Islands vicinity was captured approximately 18 hr after the passage of the cold front by the National Aeronautics and Space Administration (NASA) Moderate resolution Imaging Spectroradiometer (MODIS; Salomonson et al, ) onboard the Aqua platform (Figure ).…”

“…Since the PCF region is associated with the cold front, a 500‐km‐wide buffer zone was intentionally imposed between the PCF region and the cold front to prevent any potential contamination. Some clouds can reach an altitude above 5 km (e.g., Figure c), but in general these are rare occurrences in PCF regions (Naud et al, ).…”

“…In the midlatitudes, cold air advection over a relatively warm ocean causes a rapid increase in surface fluxes and subsequently the development of convective boundary layer clouds (Abel et al, ; Fletcher et al, ). These conditions typically occur in the wake of an extratropical cyclone's cold front, that is, the post cold frontal (PCF) region (Naud et al, ). Low‐level clouds such as those that occur in PCF regions have a crucial role in regulating the Earth's radiative budget and the moisture budget of the planetary boundary layer (PBL; e.g., Boutle et al, ).…”