|
E. Linacre |
11/'98 |
Origin
Measurements of surface pressure reveal a both a diurnal and a semidiurnal (12h) rhythm underlying any longer-term (synoptic) variation (1). This rhythm is the surface image of a diurnal tide of the entire atmosphere. A wave moves across the upper atmosphere, westward with the speed of the Sun. Bernhard Haurwitz discovered this in 1956 (1) and explained it as due to the warming of the upper atmosphere (mainly the thermosphere) by the Sun. The diurnal sea level pressure variation is entirely hydrostatic, i.e. it is the result of temperature variations aloft. Upper level variations of temperature distort isobaric surfaces. Therefore the upper level wind shows a similar diurnal cycle.
Surface pressure measurements in Taiwan (at 25ºN) are least around 4am and (especially) 4 pm Local Standard Time, and most around (especially) 10am, and 10pm LST; the amplitude of the semidiurnal cycle is about 1.4 hPa. The superimposed diurnal rhythm has about half the amplitude, with a maximum at about 6 am and minimum at 6pm (2).
Variation with latitude, altitude and
season
The amplitudes of both cycles depend on latitude, season and altitude. With regard to latitude, the diurnal cycle has an amplitude around 1.16 hPa at the equator, and elsewhere it is proportional to the cube of the cosine of the latitude angle. So it is only 0.17 hPa at 45°, for instance. At high latitudes only the semidiurnal cycle is noticed. The latitude of highest amplitude is displaced from the equator towards the latitude of the zenithal Sun.
Both cycles are more pronounced at higher altitudes. Automated measurements on the summit of Nevado Sajama at 6542 metres at 18ºS, 69ºW in Bolivia show a daily cycle of 1.7 hPa amplitude, with minima at (especially) 5am and 5pm, and maxima at (especially) 1130am and 1030pm LST (3). The phase of the cycle is 1-2 hours off relative to sea-level measurements.
Tropospheric heating
Diurnal surface pressure variations may also be due to tropospheric heating. Numerical models and surface observations show that deep convection in the Amazon basin acts like a pump (4, 5), lifting the air and heating it (due to latent heat release by condensation) in the afternoon, and sinking and cooling it (by radiative flux divergence mainly in the lower troposphere) around dawn. As a result, the surface pressure over the Amazon basin is lower in the afternoon than at sunrise.
Even PBL heating causes some of the diurnal pressure variation. This is quite obvious along coastlines in warm climates, where the daytime heating of the PBL over land (but not over the ocean) produces a lower sea level pressure inland, and hence a pressure gradient which drives the sea breeze.