Objective determination of synoptic states

Science

Studies that explore aerosol–cloud–precipitation interactions must account for the different environments in which these processes occur. This effort demonstrates an objective approach based on self-organizing maps (SOMs) to classify the synoptic state (meteorological environment) that is sufficiently general to be applied to any geographical area.

Impact

The SOM technique is applied to 500-hPa geopotential height fields from ERA–Interim reanalysis to identify dominant weather patterns and associated cloud fields over the northeast Atlantic Ocean near the Azores. SOM analysis based on 500-hPa geopotential heights yields a robust synoptic classification for mid-latitude regions. The Azores lie in an area of substantial variability in thermodynamic environment, vertical motion, stability, and cloud properties, in both winter and summer. For any given month, the distribution of synoptic states can differ greatly from year to year, leading to large uncertainties in estimating the climatological behavior of clouds from short-term field campaigns.

Summary

Marine boundary-layer clouds are modified by processes at different spatial and temporal scales. To isolate the processes governing aerosol–cloud–precipitation interactions, multi-day synoptic variability of the environment must be accounted for. 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 pre-trough, trough, post-trough, ridge, and zonal-flow categories. Because mid-latitude meteorology is largely constrained by quasi-geostrophic dynamics, a robust classification of synoptic state can be obtained from the 500-hPa heights alone.

The SOM technique is applied to a region of prevalent marine low cloudiness over the eastern North Atlantic Ocean 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 stability, vertical motion, and cloud properties. 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 SOM method was run in two ways to emphasize multi-day and seasonal variability separately. Over and near the Azores, the western edge of marine low clouds varies by synoptic state. 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 incurs large uncertainties. This SOM approach is a robust method that is broadly applicable to characterizing synoptic regimes for any location.