Mid-season climate diagnostics of jet contrail 'outbreaks' and implications for eastern US sky-cover trends

The cirrus-level 'condensation trails' (contrails) produced by jet aircraft often occur as sub-regional-scale 'outbreaks' of multiple contrails, suggested as contributing to post ~1965 climate trends in parts of the US and Europe. Several previously-developed, satellite-image based contrail spatial inventories for the conterminous US (CONUS) revealed regional-scale differences in frequency. However, the use of such geographically-fixed regions was not ideal for climate studies. As a first step towards determining the potential climate impacts of contrail outbreaks for the CONUS, we develop maps of overlapping (in time, space) outbreak occurrences-'overlaps'- by applying GIS to a recent period (2000-2002) satellite-image derived inventory for mid-season months. The higher-frequency outbreak overlap regions undergo substantial between-season variations in magnitude and extent that reflect an association with upper-tropospheric temperature gradients and winds. Overlap maps generated for additional mid-season months in 2008-2009 indicate the inter-annual variability of the outbreak regionalization. To clarify the role of uppertroposphere synoptic meteorological conditions in contrail outbreak occurrence, we form compo - sites-multi-case averages-for the sub-region of maximum overlap frequency in each midseason month. Regional and seasonal variations in the relative roles of 'thermo-dynamic' (here, temperature, humidity) and 'dynamic' (vertical motion of air, horizontal wind) controls in outbreaks are identified. Last, we demonstrate potential utility of the spatial overlap method by deriving fallseason surface station trends (1951-1993) of sky cover variables for contrasting high versus low contrail and overlap frequency grid cells in the eastern CONUS. These suggest a contrail contribution to recent high-cloud increases, notably for the Midwest.