Spatial and Temporal Variation in the Mixing Depth over the Northeastern United States during the Summer of 1995

Abstract

A study of the temporal and spatial variations of mixing layer height over the Ozone Transport Region of the northeastern United States for the summer of 1995 is presented using meteorological data obtained from the North American Research Strategy for Tropospheric Ozone-Northeast (NARSTO-NE) 1995 field program. Rawinsonde balloon soundings made every 4 h during 13 ozone episode days during NARSTO-NE provided the principal source of upper-air data, supplemented by virtual temperature profiles from five radio acoustic sounder system sites. Forty-four weather stations provided surface data. Daytime mixing depths were estimated using a profile-intersection technique. The height of the surface inversion was used as a measure of the depth of the turbulent boundary layer at night. For the 13 ozone episode days, the average maximum mixing depth ranged from less than 500 m offshore to greater than 2000 m inland, with most of the increase occurring within the first 100 km of the coastline. The coefficient of variation of maximum mixing depths averaged over the 13 episode days varied from 0.65 at coastal stations to 0.19 at inland locations. Greater variability at the coast may be caused by the interplay of sea-breeze circulations with synoptic wind patterns there. The rate of growth of the mixing depth between 0600 and 1000 EST (UTC ? 5 h) averaged 165 m h?1 for all stations, ranging from 20?60 m h?1 at coastal sites to more than 350 m h?1 at inland stations. Ventilation coefficients were about 50% lower on ozone episode days than on nonepisode days from 0700?0900 EST. For the ozone episode of 13?15 July a comparison was made of mixing depth estimates from three different methods: rawinsonde virtual potential temperature profiles, C2n (the atmosphere?s refractive index structure parameter), and output from running the Fifth-Generation Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model (MM5) version 1, a widely used nonhydrostatic mesoscale model. Estimates obtained from the three methods varied by as much as 200 m at night and by up to 500 m during the daytime. Mixing depths obtained from running MM5 were in good agreement with estimates from the other methods at Gettysburg, Pennsylvania, an inland station, but were 10%?20% too low at New Brunswick, New Jersey, a location within 30 km of the Atlantic coast. The discrepancy may be caused by the model?s 12-km grid spacing being too coarse to locate the marine?continental airmass boundary with high precision.