Thermal risk assessment for viticulture using monthly temperature data

Temperature extremes affect grapevine physiology, as well as grape quality and production. In most grape growing regions, frost or heat wave events are rare and as such conducting a risk analysis using robust statistics makes the use of long term daily data necessary. However, daily climate data suffers many constraints such as typically having a short-term history, with uneven spatial coverage worldwide, and their homogenization to account for changes in climate sensors or changes in site location are challenging. In contrast, monthly data sets offer a much more robust spatiotemporal coverage. Furthermore, data at monthly time steps is relevant for climate projection analyses over the 21st century.

Therefore, the current study evaluates the relevance of estimating thermal risks for viticulture using monthly data. Daily minimum (Tmin) and maximum (Tmax) temperature data were collected from 369 weather stations in Europe (European Climate Assessment & Dataset) and 1218 weather stations in the USA (United States Historical Climatology Network) for the period from 1972 to 2008. For the whole period and for each station, the average yearly number of winter freeze days (Tmin < -17°C), spring frost days (Tmin < -1°C), and heat stress days (Tmax > 35°C) were calculated. In addition, frequencies of years with at least one spring frost event, the date of the last spring frost event at 90% probability (i.e. the quantile 0.9) and frequencies of years with at least one winter freeze event were calculated.

These thermal risk indicators, analyzed on a daily time step, exhibited strong relationships with maximum and minimum monthly average temperatures during the 1972-2008 period. Winter freeze risk is strongly linked to January average monthly minimum temperature, while spring frost risk is related to April minimum monthly temperature. The average number of heat stress days is strongly correlated to July maximum temperature. Using WorldClim 5 arc-minute resolution climate grids, a winter frost risk map for the 1950-2000 period is proposed. The results suggest that grape growing region limits are strongly restrained by winter freeze risk hazards.