The diurnal trend in surface mixing ratio at Padova, Italy

Abstract

The diurnal cycle of the mixing ratio MR is discussed for Padova, sited about 30 km from the sea. In the morning a noticeable increase of the MR can be noted until the nocturnal inversion is completely eroded; afterwards convective activity mixes the humid sublayer with dry air aloft, so that an abrupt drop in MR follows the break-up of the inversion; the minimum is generally reached after noon. The minimum is primarily due to the enhanced activity of the thermals at mid day. During the day, increases in wind speed are generally accompanied by the arrival of new air masses, so that no general statements about the MR can be made without knowledge of the local dynamic climatology. Clouds cause an increase in MR, due to the out-of-phase relation between the dampings of the turbulent diffusion coefficient for water vapour and of the evaporation rate of the soil. In the afternoon the MR increases again until after sunset, when the sea breeze (final phase) transports humid marine air inland practically unaltered.