Vineyard frost protection with upward-blowing wind machines

Upward-blowing wind machines have been commercialized for use in frost protection but little quantitative information exists regarding how their operation alters site temperatures. In particular, their performance relative to conventional wind machines has been debated. To address this need, experiments were conducted on 12 spring frost nights in 2010 and 2011 in a commercial winegrape vineyard where either two upward-blowing wind machines or a single conventional wind machine were operated. Comprehensive measurements of air temperature changes caused by wind machine operation were evaluated on multiple transects at heights of 1.1, 4, 7 and 10 m. All 12 frost nights were characterized by low wind and clear sky conditions, with temperature inversion strengths commonly associated with beneficial wind machine use occurring on 9 of the 12 nights. The operation of the conventional wind machine produced consistently larger and more statistically significant increases in temperature, particularly at the 1.1 m vine level, as compared to the operation of the upward-blowing wind machines which produced very minor increases in temperature at the 1.1 m level under strong inversion conditions and either no change or decreases in temperature under weaker inversion conditions. Based on the summary relationships between temperature changes as a function of inversion strength, under conditions of an inversion gradient of 0.2 degrees C m(-1) the conventional wind machine would be expected to raise target area temperatures by 1.6 degrees C at the vine level, while the upward-blowing wind machines would have no net effect under the same inversion conditions. Smoke tracking of the air flow from the upward-blowing wind machines indicated that the air jet reached 25 m height, and then tended to slowly settle back towards the ground. These results indicate relatively poor performance of this type of low-powered (6.3 kW) upward-blowing wind machine compared to a conventional wind machine under the conditions of this study. (c) 2012 Elsevier B.V. All rights reserved.