Temporal and Spatial Structure of Nocturnal Warming Events in a Midlatitude Coastal City

Abstract

Nocturnal warming events (NWEs) are abrupt interruptions in the typical cooling of surface temperatures at night. Using temperature time series from the high-resolution Vancouver Island School-Based Weather Station Network (VWSN) in British Columbia, Canada, we investigate temporal and spatial characteristics of NWEs. In this coastal region, NWEs are more frequently detected in winter than in summer, with a seasonal shift from slowly warming NWEs dominating the winter months to rapidly warming NWEs dominating the summer months. Slow-warming NWEs are of relatively small amplitude and exhibit slow cooling rates after the temperature peaks. In contrast, fast-warming NWEs have a temperature increase of several kelvins with shorter-duration temperature peaks. The median behavior of these distinct NWE classes at individual stations is similar across the entire set of stations. The spatial synchronicity of NWEs across the VWSN (determined by requiring NWEs at station pairs to occur within given time windows) decreases with distance, including substantial variability at nearby stations that reflects local influences. Fast-warming NWEs are observed to occur either simultaneously across a number of stations or in isolation at one station. Spatial synchronicity values are used to construct undirected networks to investigate spatial connectivity structures of NWEs. We find that, independent of individual seasons or NWE classes, the networks are largely unstructured, with no clear spatial connectivity structures related to local topography or direction.