Observation and Theory of the Diurnal Continental Thermal Tide

Abstract

Harmonic analysis of summer Automated Surface Observing System (ASOS) data over North America shows sun-following diurnal temperature and pressure oscillations with amplitudes increasing in the western United States (i.e., 5?8 K and 60?120 hPa, respectively) due to larger sensible heating in the dryer western terrains. The phases of temperature and pressure (i.e., 220° and 110°) are constant with longitude after an interfering eastward propagating wave is subtracted. Tidal amplitudes and phases shift significantly with season. A linear Boussinesq model with thermal forcing can reproduce these observed oscillations with properly selected parameters. The model neglects global effects to focus on a single transect across a single ideal continent. A damping parameter α ranging from 5 ? 10?5 to 9 ? 10?5 s?1, comparable to the inertia and Coriolis parameters, is needed to explain the temperature phase lag relative to local solar noon (40°?50°C). The phase lag between surface pressure minimum and temperature maximum (45°?70°C) requires a 3?5-h time delay between surface and elevated heating. The ratio of pressure and temperature amplitude requires a heating depth varying between 550 (winter) and 1250 m (summer). Both the heating delay and depth are consistent with a vertical heat diffusivity of about K = 10 m2 s?1 in winter, but K theory gives inconsistent summer K values. The observed tide amplitude requires diurnal heating amplitudes in the range of 100?250 W m?2. When the model is applied to an inhomogeneous continent, it is possible to obtain a clearer idea of how wide a region must be to approach the tidal (i.e., long-wave) limit. Traveling diurnal heating generates gentle tides over the large uniform interior regions but causes vigorous sea breezes and mountain?plain circulations in regions of heating gradient. These gradient regions have significant vertical motions and are moderately sensitive to the Coriolis force and the mean wind speed. Surprisingly, these local circulations do not alter the phases of the temperature and pressure oscillations, in agreement with observations.