Knowledge of the planetary boundary layer (PBL) structure, especially its depth and the mixing layer (ML) height, is important for understanding the various processes in the atmosphere and properly predicting and simulating pollution episodes. Different instruments used for inferring ML height have different strengths and are often appropriate only in certain conditions. Even under optimal conditions, differences among various ML estimates can result from separate instruments using a different variable assumption as to which feature best defines the depth of the mixed layer.

During TexAQS-2000 in Houston, an unusually large number of instruments were deployed with the ability to measure ML height. Available instruments included six boundary layer profilers, three radiosonde sites, an airborne lidar, three other instrumented aircraft, and a microwave temperature profiler. The latter instrument was used for PBL measurements for the first time during TexAQS-2000.

We compare and contrast the various measurements of ML height during selected high ozone days in the Houston area. The various measurements are in general agreement, although the agreement decreases as signatures of PBL structure become ambiguous. No single instrument is adequate by itself, but together the measurements provide a comprehensive depiction of ML height in the Houston area as a function of both space and time.

We find that ML height variations in the Houston area are modulated primarily by the presence of the neighboring Galveston Bay and Gulf of Mexico. During onshore flow conditions, a strong afternoon gradient of ML depth is typically present between Houston and the coast. Under lighter wind conditions, the ML height is subject to larger local-scale variations, and some indication of a deeper ML is found over the center of Houston. Large spatial variations are also present when the environmental stratification is weak. Over Galveston Bay, a residual layer of continental air is typically present over the local marine boundary layer, and this residual layer is depressed downward by the subsidence and low-level divergence induced by the land-sea temperature contrast. Under particular wind conditions, the residual layer has been shown to consist of polluted urban air which undergoes photochemistry over Galveston Bay prior to being advected over land and mixed to the surface.