Atmospheric Tides

Steve Woolnough (s.j.woolnough@rdg.ac.uk), Julia Slingo, CGAM, University of Reading, Brian Hoskins, Dept. of Meteorology, University of Reading.

The observed surface pressure signal of the semi-diurnal tide in the Tropics is well established; Haurwitz and Cowley (1973) calculated from station data an amplitude of the surface pressure signal associated with the semi-diurnal tide of 1.05mb, Hsu and Hoskins (1989) in an analysis of ECMWF data detected a semi-diurnal tide of similar magnitude. These observations have been supported by more recent studies (e.g. Deser and Smith 1998) and are consistent in both magnitude and structure with the theoretical predictions of e.g. Chapman and Lindzen (1970).

Chapman and Lindzen (1970) estimate a magnitude of the semi-diurnal tide based on specified profiles of ozone and water vapour heating of about 1.15mb of which 0.85mb is forced by heating due to absorption by stratospheric ozone and 0.3mb is forced by absorption by water vapour in the troposphere.


Figure 76. Semi-diurnal harmonic of surface pressure (mb) from integrations of an aquaplanet version of the UM. a) control integration, b) as the control integration expect with the diurnal cycle in solar heating above 100mb replaced by the daily mean heating, c) as the control integration except with no cloud radiation.	Figure 77. Amplitude of the semi-diurnal harmonic of the short-wave heating rates for the integrations described in the figure left.

Figure 76 shows the semi-diurnal harmonic of the surface pressure signal from three integrations of an aquaplanet version of the UM. Figure 76a shows the signal from the control integration, it agrees well with the theoretical predictions and observations of the semi-diurnal tide in both magnitude and structure. Figure 76b shows the same field from and integration in which the diurnal varying solar heating in the stratosphere associated with absorption of ozone has been replaced by a fixed daily mean value, thus removing the forcing of the semi-diurnal tide due to the stratospheric ozone absorption (see Figure 77 for vertical profiles of the semi-diurnal harmonic of heating for each of the three integrations). The horizontal structure and phase of the semi-diurnal tide are relatively unchanged from the control integration. However the amplitude has been reduced from 1.18mb to 0.87mb. This contribution to the semi-diurnal tide from the ozone heating of (0.31mb) is substantially less than that predicted by the theory of Chapman and Lindzen. Figure 76c shows the same field from an integration in which the cloud radiative effects have been removed (compared to the control integration). The semi-diurnal harmonic in the surface pressure is reduced by about 0.1mb compared to the control integration.

These integrations suggest that the theoretical predictions significantly over-estimate the contribution to the semi-diurnal tide of the ozone heating and under-estimate that of the water vapour heating. Some of this discrepancy may be attributable to a better understanding of the atmospheric heating profiles. The vertical distribution of water vapour heating used by Chapman and Lindzen is peaked at the surface with a exponential decay with height, in contrast to the typical tropical profile of heating of Liou (1980), in which the atmospheric heating is peaked at around 3km.

The discrepancy between the theoretical predictions and these model results will be investigated further, by looking at the response of a dynamical core to imposed heatings representing the effects of the ozone, water and clouds.

Deser, C. and C.A. Smith, 1998: Diurnal and semidiurnal variations of the surface wind field over the tropical Pacific Ocean.

Hsu, H.-H. and B.J. Hoskins, 1989: Tidal fluctuations as seen in ECMWF data. Quart. J. Roy. Meteor. Soc., 115, 247-264.

Liou, K.-N., 1980: An introduction to Atmospheric Radiation. Academic Press, 392pp