Spatial and temporal variation in the mixing depth over the northeastern United States during the summer of 1995

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 Ame rican Research Strategy for Tropospheric Ozone-Northeast (NARSTO-NE) 1995 f ield program. Rawinsonde balloon soundings made every 4 h during 13 ozone e pisode days during NARSTO-NE provided the principal source of upper-air dat a, supplemented by virtual temperature profiles from five radio acoustic so under system sites. Forty-four weather stations provided surface data. Dayt ime mixing depths were estimated using a profile-intersection technique. Th e 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 i ncrease occurring within the first 100 km of the coastline. The coefficient of variation of maximum mixing depths averaged over the 13 episode days va ried from 0.65 at coastal stations to 0.19 at inland locations. Greater var iability at the coast may be caused by the interplay of sea-breeze circulat ions with synoptic wind patterns there. The rate of growth of the mixing de pth between 0600 and 1000 EST (UTC - 5 h) averaged 165 m h(-1) for ail stat ions, 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 e stimates from three different methods: rawinsonde virtual potential tempera ture profiles, C-n(2) (the atmosphere's refractive index structure paramete r), and output from running the Fifth-Generation Pennsylvania State Univers ity-National Center for Atmospheric Research Mesoscale Model (MM5) version 1, a widely used nonhydrostatic mesoscale model. Estimates obtained from th e three methods varied by as much as 200 m at night and by up to 500 m duri ng the daytime. Mixing depths obtained from running MM5 were in good agreem ent with estimates from the other methods at Gettysburg, Pennsylvania, an i nland station, but were 10%-20% too low at New Brunswick, New Jersey, a loc ation 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-contin ental airmass boundary with high precision.