Marine Boundary Layer Cloud Observations in the Azores
The recent deployment of the ARM Mobile Facility at the Graciosa Island, Azores, in the context of the Clouds, Aerosol and Precipitation in the Marine Boundary Layer (CAP-MBL) field campaign added the most extensive (19 months) and comprehensive dataset of MBL clouds to date. Cloud occurrence is high (60–80%) with a summertime minimum. Liquid precipitation is frequently present (30–40%), mainly in the form of virga. Boundary layer clouds are the most frequently observed cloud type (40–50%) with a maximum of occurrence during the summer and fall months under the presence of anticyclonic conditions. Cumulus clouds are the most frequently occurring MBL cloud type (20%), with cumulus under stratocumulus layers (10–30%) and single-layer stratocumulus (0–10%) following in frequency of occurrence. A stable transition layer in the subcloud layer is commonly observed (92% of the soundings).. Cumulus cloud bases and stratocumulus cloud tops correlate very well with the top of the transition layer and the inversion base respectively. Drizzling stratocumulus layers are thicker (350–400 m) and have higher liquid water path (75–150 g m-2) than their non-drizzling counterparts (100–250 m and 30–75 g m-2 respectively). The variance of the vertical air motion is maximum near the cloud base and is higher at night. The updraft mass flux is around 0.17kg m-2 s-1, with 40–60% explained by coherent updraft structures. Despite a high frequency of stratocumulus clouds in the Azores, the MBL is almost never well mixed and is often cumulus-coupled.
Marine stratocumulus clouds are ubiquitous over the eastern the subtropical oceans and play a critical role in the boundary layer dynamics and the global climate (e.g., Klein and Hartmann, 1993; Bony and Dufresne, 2005). These prevailing low-level cloud decks are a key component in Earth’s radiation budget (Randall et al., 1984; Ramanathan et al., 1989). The radiative impact of marine boundary layer clouds depends on their macroscopic properties (e.g., horizontal extent, thickness) and microscopic properties (e.g., particle size distribution). Past studies have focused on the cloud macro-structure properties of marine boundary layer clouds and their relationship to large-scale dynamics and thermodynamic state using satellite observations and reanalysis products (e.g., Klein and Hartmann, 1993; de Szoeke and Xie, 2008). Wood and Bretherton (2006), have shown that approximately 80% of the variance in low cloud cover in regions dominated by marine stratocumulus is explained using the estimated inversion strength. However, appreciable complexity and challenges are found on smaller space and time scales, including the cloud micro-scale (spatial scales of tens of meters and temporal scales of a few minutes or less).
Previous field experiments focusing on marine stratocumulus clouds include the Atlantic Stratocumulus Transition Experiment–ASTEX (Albrecht et al., 1995), the East Pacific Investigation of Climate–EPIC (Bretherton et al., 2004), the Dynamics and chemistry of marine stratocumulus–DYCOMS (Stevens et al., 2003), and the VAMOS Ocean-Cloud- Atmosphere-Land Study Regional Experiment–VOCALS-REx (Wood et al., 2011). These field studies advanced our knowledge of marine stratocumulus, providing information on their boundary layer thermodynamic and cloud structure, as well as their diurnal cycle. They have highlighted that stratocumulus clouds can form under a diverse range of conditions, in both deep and shallow marine boundary layers (MBL), and under a wide range of aerosol conditions. Furthermore, the radiative properties and propensity for drizzle from marine stratocumulus clouds depend on several factors including aerosols, liquid water path and dynamics. The aforementioned field campaigns are characterized by intensive observation periods limited in time from a couple of weeks to a month. Thus, previous studies have not been carried out long enough to provide a useful climatology of key MBL and associated cloud properties. The recent Clouds, Aerosol and Precipitation in the Marine Boundary Layer (CAP-MBL) field campaign (www.arm.gov/sites/amf/grw/) that took place in the Azores nicely filled that gap. As part of the campaign, the US Department of Energy Atmospheric Radiation Measurements (ARM) Mobile Facility (AMF) was deployed on Graciosa Island. This AMF deployment is unique compared to previous intensive field campaigns. First, the AMF instrumentation is far more comprehensive and superior to that available in previous ground-based field studies. Second, the campaign is 21 months long and thus provides the opportunity to generate the long data set record required to sample a variety of aerosol, cloud and large-scale environmental conditions. Finally, it is the first marine stratocumulus field campaign with sophisticated cloud radars (profiling and scanning) on a stable (island) platform that enables the use of the Doppler velocity measurements. Thus, the AMF deployment in the Azores produced the most comprehensive data set of MBL clouds to date.
In this study, we select a subset of the deployed AMF instruments to study the observed MBL clouds in more detail. An objective scheme was first developed to identify their occurrence across the entire data set, and to recognize some important subtypes (e.g., cumulus and stratocumulus), with the presence of precipitation also diagnosed (see section 3). The variability and frequency of occurrence of the different cloud and precipitation events is presented with emphasis on the various MBL cloud structures. A further analysis of the MBL emphasizes the differentiation between cumulus and stratocumulus regimes, as well as the presence of decoupling. A statistical analysis of cloud structural and dynamical properties is performed, and related to the thermodynamic profiles
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About the author
Jasmine Rémillard is finishing her Ph.D. under Pavlos Kollias supervision. She is studying stratocumulus clouds, focusing on the detection of drizzle inside the clouds and the retrieval of the cloud droplets sizes. Various ground-based remote sensors (e.g., radars, lidars, and radiometers) helped achieved these goals.
She previously completed a M.Sc. in Atmospheric Sciences (also with Pavlos Kollias) at McGill in 2009, and a B.Sc. in Physics at Université de Montréal in 2006.