Energy exchange at the micro-scale during the operation of a ventilator for frost protection

A late frost in spring can cause extensive damage and substantial economic losses for agriculture around the world. To mitigate damage, fruit farmers take active measures to raise plant and air temperatures, such as ventilators that mix warm overlying air down to the vegetation. However, up to this point studies on ventilator efficiency have focused on air temperatures. Plant temperatures during ventilator operation remain unknown, while critical for the actual degree of frost damage. With Distributed Temperature Sensing we measured a grid of in-canopy air temperatures in a Dutch pear orchard and thermocouples were installed to determine the temperatures of plant leaves and flower buds. It turns out that before or without ventilator operation, the leaves are cooler than the surrounding air by up to 2 ⁰C. Here we show that over the rotation cycle of a ventilator the temperature difference between plant and air is variable and can be divided into three phases. During the first phase warm air is mixed into the canopy by the ventilator. Air temperatures rise faster than leaf temperatures due to the leaves’ heat capacity and isolating leaf boundary layer. The extent of the temperature rise depends on the distance to the ventilator. Further from the ventilator, the canopy reduces the jet speed and thus vertical mixing. At the peak of the jet, phase II, the high wind speeds break down the leaf boundary layer and enhance convective energy exchange. When the plant temperature approaches air temperature, the convective warming of the leaves stops, and radiative cooling becomes the dominant process. At phase III, after the passage of the jet, the air stabilizes and the leaves cool radiatively until a new equilibrium is reached. Our results demonstrate how leaf-air heat exchange within the canopy differs under varying turbulence conditions. For maximum crop protection and optimal employment of the ventilator both wind speed and air temperature in the canopy should be taken into consideration. Therefore, we expect that optimal settings may vary throughout the growing season as canopy density and the corresponding wind reduction change.