Joint Synoptic and Cloud Variability over the Northeast Atlantic near the Azores

Abstract

Marine boundary layer clouds are modified by processes at different spatial and temporal scales. To isolate the processes governing aerosol–cloud–precipitation interactions, multiday synoptic variability of the environment must be accounted for. Information on the location of low clouds relative to the ridge–trough pattern gives insight into how cloud properties vary as a function of environmental subsidence and stability. The technique of self-organizing maps (SOMs) is employed to objectively classify the 500-hPa geopotential height patterns for 33 years of reanalysis fields (ERA-Interim) into pretrough, trough, posttrough, ridge, and zonal-flow categories. The SOM technique is applied to a region of prevalent marine low cloudiness over the eastern North Atlantic Ocean that is centered on the Azores island chain, the location of a long-term U.S. Department of Energy observation site. The Azores consistently lie in an area of substantial variability in synoptic configuration, thermodynamic environment, and cloud properties. The SOM method was run in two ways to emphasize multiday and seasonal variability separately. Over and near the Azores, there is an east-to-west sloshing back and forth of the western edge of marine low clouds associated with different synoptic states. The different synoptic states also exhibit substantial north–south variability in the position of high clouds. For any given month of the year, there is large year-to-year variability in the occurrence of different synoptic states. Hence, estimating the climatological behavior of clouds from short-term field campaigns has large uncertainties. This SOM approach is a robust method that is broadly applicable to characterizing synoptic regimes for any location.

Midlatitude Oceanic Cloud and Precipitation Properties as Sampled by the ARM Eastern North Atlantic Observatory

“…Southerly and westerly flows demonstrate thicker clouds. Overall, cloud base behaviors are generally consistent with northerly flows favoring shallower boundary layers, thinner clouds, high surface pressure, stronger stability, and large‐scale subsidence (Mechem et al, ). In short, textbook stratocumulus‐topped boundary layers likely occur predominantly during northerly flows.…”

“…The ENA site is located in the midlatitudes and is characterized by substantial multiday variability in synoptic configuration, stability, and cloud properties (e.g., Naud et al, ). Using the technique of self‐organizing maps, Mechem et al () found significant influences of synoptic systems over the ENA, even in June when the lower tropospheric flow is, on average, dominated by the Azores High. Of particular interest is their pretrough state, which occurs 40% of the time and is characterized by southerly or southwesterly flow at the surface.…”

“…In this pretrough state, the Azores lie directly along the transition of the large‐scale vertical velocity from upward and downward motion (their Figure 6). Furthermore, Mechem et al () show that these synoptically forced states (that is, the pretrough and trough states associated with large‐scale ascent nearby or over the Azores) are typically accompanied by winds from the south or southwest and by cloud properties with greater amounts of vertically integrated condensate. Their result strongly suggests that such enhancements in cloud thickness and potential for precipitation are a result of synoptic influences.…”

“…This makes extended observations of cloud or precipitation at the Azores useful for evaluating large‐scale models over a wide variety of cloudiness regimes. Moreover, the Azores is a unique site in the context of previous precipitation studies as it experiences both subtropical and midlatitude synoptic environments (e.g., Mechem et al, ; Rémillard & Tselioudis, ). The generalizability of ENA cloud and precipitation properties also suggests this location as an important anchor for refining remote sensing capabilities, including profiling activities from spaceborne platforms (e.g., Hou et al, ; Stephens et al, ).…”

Spatio‐temporal variations of cloud fraction based on circulation types in the Iberian Peninsula

“…However, there is only one known study relating cloud distribution to synoptic circulation . On an international level, more studies can be found on atmospheric patterns affecting cloud cover (Rmillard and Tselioudis, 2015;de Szoeke et al, 2016;Matuszkoa and Weglarczyk, 2018;Mechem et al, 2018). Therefore, a comprehensive study of cloudiness using data collected from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Terra and Aqua satellites has been carried out relating it to the classification of local-regional synoptic patterns.…”