Observation and theory of the diurnal continental thermal tide

Harmonic analysis of summer ASOS data over North America shows sun-following diurnal temperature and pressure oscillations with amplitudes increasing in the western USA, i.e. 5 to 8 degC and 60 to 120 hPa respectively, due to larger sensible heating in the dryer western terrains. The phases of temperature and pressure (i.e. 220deg and 110deg) are constant with longitude after an interfering eastward propagating wave is subtracted. Tidal amplitudes and phases shift significantly with season.

A linear Boussinesq model can capture these observed tidal properties with properly selected parameters. A damping parameter alpha=5~9*10-5 1/s, comparable to the inertia and Coriolis parameters, is needed to explain the temperature phase lag relative to local solar noon (40deg to 50deg). The phase lag between surface pressure minimum and temperature maximum (45deg to 70deg) requires a 3 to 5 hour time delay between surface and elevated heating. The ratio of pressure and temperature amplitude requires a heating depth varying between 550 to 1250 meters; winter to summer. Both the heating delay and depth are consistent with a vertical heat diffusivity of about 10 m2*s-1 in winter, but K-theory give inconsistent summer values. The tide amplitude requires diurnal heating amplitudes in the range of 100 to 200 W/m2.

When the tuned model is applied to an idealized but inhomogeneous continent, the 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 critical latitude and the mean wind speed. Surprisingly, these local circulations do not alter the phases of the temperature and pressure oscillations, in agreement with observations.