Abstract

Frosts are one of the most harmful weather events, causing recurrent and great damage in agriculture. Active control methods, such as wind machines, must be implemented to reduce the impact on crops. A new portable wind machine (PM) has been marketed for use in frost control; however, there are no reports on its effectiveness or its performance relative to a stationary wind machine (SM). Thus, the aim of this work was (i) to evaluate and compare the effectiveness on the temperature modification between the SM and PM in different operating configurations; (ii) to study the spatial and vertical temperature variation produced for both machines in several thermal inversion conditions, and (iii) to characterize the airflow pattern generated by the propellers. The evaluation was done on 12 nights with frost during winter and spring in two seasons in a pear orchard. Maximum airflow speed and spatial and vertical temperature profile measurements around the machines were equally distributed and evaluated at the same time. All frost events were characterized by clear skies, low drift and strong and moderate thermal inversion; therefore, beneficial conditions to operate the machines for optimal frost control performance were expected. The PM is not as effective as SM to remove cold air from the surface, not only showing a lower stability and coverage, spatially and vertically, but also achieving a less temperature increase. In the best operational configuration, the SM achieves temperature increase equivalent to 30% and 50% of the inversion strength (measured between 1.5 and 15 m) over of 5.3 ha and 2.7 ha, respectively, whereas the PM achieves similar temperature increases over 3.0 ha and 0.6 ha, respectively. The narrow angle of operation of PM, about 26, is an important limiting factor in the effectiveness of frost control, so it must be operated with the minimum rotation time. Additionally, the lower height of the PM (8 m) influences the range of the wind gust, especially when facing the rows, and works with colder air of the strata, in comparison with the 13.7 m of the SM. The microclimatic conditions, intra and inter-frost, are highly variable, making it difficult in the case of the PM to implement a correct recommended operating configuration, position and rotation mainly, without a previous site-specific study.