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The importance of low-velocity wind turbulence in reducing the rate of frost development

R.J. Cawood

Department of Agriculture, Victorian Institute for Dryland Agriculture, Private Bag 260, Horsham VIC 3401

Identification of frost risk zones in the landscape is of benefit for planning and management of agricultural enterprises. Understanding the factors which hasten, delay or change the severity of frosts is important for assessing the likelihood of damage to crops.

Apart from thermal properties influencing the rate of outward net radiation flux, the stability of the atmosphere in the air layer near the ground is the most important determinant of frost temperature and duration. The objective of this research study was to determine the influence of low-velocity wind turbulence and humidity in the lower boundary layer on the development, severity and duration of frost.

Methods

Field studies of diurnal temperature changes were made at 5 sites in the Kaniva-Nhill region of Victoria using electronic sensors and automatic data loggers. Horizontal wind speed and relative humidity was measured 50 cm above the ground every 6 minutes at the most frost prone site using a sensitive digital anemometer (stall speed 0.25 m/s) and a thermocouple psychrometer (accuracy ± 0.08°C).

Results and discussion

Data from the field sites show differences in the rate of onset, frequency and severity of frosts for different landform units. There were many occasions at all sites during which the rate of temperature decline was either lowered unexpectedly or became negative at the surface and in the adjacent air during the commencement of frost. This change in rate delayed the early onset of frost and reduced the severity of the occurrence.

Further detailed study at the site with the greatest frost frequency showed that low velocity wind turbulence (<3 m/s) caused by boundary layer instability was closely correlated (r > 87%) to positive or zero rates of temperature change.

These wind events occur during stable periods of radiative temperature decline, are more widespread than generally expected and change the frequency and severity of frosts.

Minor disturbances in temperature decline were also associated with latent heat release during condensation and ice formation. Rates of temperature drop were temporarily lowered or zero until latent heat output was complete at the dew point and ice point temperatures.

This research has shown that turbulence influences frost development in broad scale agriculture. In the future, both empirical and mechanical models for temperature decline over land surfaces should take account of this factor to improve their predictive ability.

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