Adriana Bernardi

An observational study of heat fluxes and their relationships with net radiation

Experimental evidence indicates that the diurnal behaviour of the fluxes of heat into the ground and into the atmosphere versus the net flux of radiation can be modelled by closed curves, the hourly values folowing one another in either a clockwise or counter clockfashion. A general formulation to express the different heat fluxes as a function of net radiation Φ is proposed. This relationship between the different heat fluxes and Φ can be expressed as a sum of three terms: the first indicates a direct proportionality to Φ, the second gives the deviation from linearity and depends on ∂Φ/∂t, and the third gives the value of the flux when Φ = 0. The formulae are then expressed as a function of time and the ratios between the heat fluxes and Φ are evaluated. A comparison with the approximations generally used shows that the latter may be considered as particular cases of the more general equations proposed here.

The diurnal trend in surface mixing ratio at Padova, Italy

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.

Computing the flux of moisture from net radiation and soil wetness

A study has been made of the time evolution of the flux of moisture E over grassland. The parameterization of E has been examined in order to formulate an equation depending on the net radiation flux and on bulk parameters which depend on the daily average meteorological situation and soil conditions. In particular the gradient of soil moisture has been recognized as playing a fundamental role in the time evolution of E(t). A time-dependent equation is proposed to compute in a statistical sense the irrigation needs or to forecast the hourly values of E(t), the maximum value E0 and the time at which this maximum happens.