UNDER INITIAL CONSTRUCTION. This site will be substantially expanded with graphics beginning in February 2020. Preliminary results from the frost survey of the vineyard field managers are posted at the end. Site visits are delayed until spring 2021.
With global warming, one might expect the potential for frost damage to decrease. However, global warming is leading to increased variability with more temperature extremes. In addition, unseasonably early warm periods can advance the crop phenology such that crops become sensitive to frosts earlier in the season. We proceed using using tall row crops such as vineyards and berry crops.
While some rough rules govern frost potential of a given site, most knowledge of microclimates is valid primarily for sites with very simple topography (uniform slope or flat terrain) and large expanses of uniform surface vegetation, soil type and soil moisture. At more typical sites, the spatial distribution of microclimate becomes more complex and the knowledge of a perceptive field manager becomes useful information. The information below represents general tendencies and exceptions occur.
Uncertainties stem from uncertainties on the exact threshold temperature below which frost damage occurs, which depends on height above ground and the phenology stage. In addition, the minimum duration of temperatures below threshold values is also uncertain.
1. The ground surface cools RADIATIVELY which cools the air adjacent to the surface. Clouds effectively reduce the net radiative loss of energy. Haze and pollution to some degree reduce the net radiative cooling.
2. AIR MOTION causes vertical mixing which brings down warmer air to the surface. The airflow may be too weak for human detection without instrumentation but there is always some airflow and mixing. The weaker the mixing, the greater the cooling at the surface. The cooling is therefore greatest on clear calm nights.
3. Upward heat flux from the SOIL also reduces the surface cooling. This upward heat flux is greater with wet soil and heavier soils such as clay. Organic debris at the surface (leaves, matted grass) significantly reduces the heat flux from the soil causing cooler nocturnal temperatures. However removal of all ground cover may not justify resulting muddy conditions and possible erosion.
Cooling on slopes leads to drainage of cold air down the slope which generates mixing and downward transport of warmer air. Rows oriented down the slope and no ground cover (or only short ground cover) between rows promotes drainage of cold air down the slope and thus reduces frost potential. Brush can slow the downslope motion and thus increase frost potential. Air motion is weak in low lying areas where cold pools form. Cold air drainage may flow over the top of the even colder air at the bottom of the slope.
There are numerous other influences on horizontal variations of nocturnal surface temperatures. Advective frosts with relatively windy conditions where the cold air deep compared to typical slope heights. In these cases, the frost damage may occur part way up the slope where phenology is more advanced compared to the bottom of the slope. The bottom of the slope experiences colder temperatures on most nights.
SHORT TERM FORECASTING
Attempts to to forecast by extrapolating the nocturnal trend omits short term temperature variations (ten minutes to a few hours) that are not part of the diurnal trend but can be several degrees C or greater (more than 5 F). These variations are more or less unpredictable and can be caused by small-scale motions, variations of cloud cover, and other unknown factors. Such variations can be smooth or sudden (microfronts). Errors in short term forecasts consist of errors in the predicted nocturnal trend and unpredicted short-term variations. Ultimately, the user would benefit from a forecast of the probability that the temperature decreases below a threshold value (such as freezing) for a period of time that causes damage.
Dense ground cover can raise the level of coldest temperatures from the ground surface to some level within the ground cover, increasing the probability of frost damage. Organic debris (leaves, dead grass, dry mulch), when dry, can reduce the heat flux from the ground at night and thus increase the possibility of frost conditions.
Onsite temperature measurements that can be read indoors or even used to automatically turn on overhead sprinkling, for example, might benefit vineyards with chronic frost problems. The location of the temperature measurements must recognize rapid variation with temperature with height on clear calm nights and also significant horizontal variation within the vineyard. More later.
SURVEY OF THE VINEYARD FIELD MANAGERS, August 2019
About 25-30% of the vineyards in the Willamette Valley were equally divided between absence of frost damage, minor frost damage and significant frost damage. Only one vineyard reported frost damage as a major factor. These categories are crude and preliminary but attempt to include severity, frequency and spatial coverage. Damaging frost is generally confined to the spring.
We think that the number of vineyards with no damage is larger than that reported because such vineyards are lessly like to respond to the survey.
A wide variety of frost prevention strategies were reported, to be summarized later.