In the previous blog post, there was a short description of an Agroclimatic factor of interest: Frost-free period, frost dates. In this very post, we focus on the calculation itself, which is run by Jupyter notebook on the EUXDAT e-infrastructure.

The minimum daily temperatures are a necessary variable for determining frost dates. The daily minimum is determined to be the lowest value of the hourly temperatures each day. It is also possible to reduce the calculation to the time around sunrise, as usually the lowest daytime temperatures are just after sunrise [1].

We calculate the days where the minimum is below 0. Subsequently, the last day of each year is determined for the spring period and the first day for the autumn period. The spring period is set as a six-month period from the coldest month, the autumn period as 6 months before the coldest month. For the sake of simplicity, the coldest month is designated as January for the northern hemisphere and July for the southern hemisphere, which is the central month of the meteorologist winter season. [2][3]

The resulting last spring frost date and first autumn frost date are calculated from the annual frost dates with a corresponding probability. The frost-free period is calculated as the period between the last and the first frost date. Similarly, it is possible calculate the dates for a temperature threshold of -2 °C.

There are also occasional frosts in summer in some areas, therefore it is possible to calculate more first/last days of frost in a row as the last / first frost date, or possibly to exclude outliers for more precise determination.

The data, which were used are Copernicus ERA5-Land hourly data, which are available from 1979 to present, representing a wide number of variables. [4]

For our purposes we used following subset of available data:

Variable:                            Temperature (measured 2m above the bare ground)

Horizontal coverage:      Global

Horizontal resolution:     0.1° x 0.1°, Native resolution is 9 km.

Temporal coverage:        January 2001 to present

Temporal resolution:      Hourly

The Figure 1 shows the way, how the frost date agroclimatic factor is calculated.

Figure 1: Diagram of frost dates and frost-free period calculation

References:
[1] C. Donald Ahrens, Meteorology Today, 2006
https://books.google.no/books?id=SpGfKb23Y9QC&lpg=PP1&dq=Meteorology%20Today%20By%20C.%20Donald%20Ahrens&pg=PA117#v=onepage&q=Meteorology%20Today%20By%20C.%20Donald%20Ahrens&f=false

[2] Jeff Haby, theweatherprediction.com
https://www.theweatherprediction.com/habyhints3/980/

[3] Meteorological Versus Astronomical Seasons
https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons

[4] Copernicus. ERA5-Land hourly data from 1979 to present
https://confluence.ecmwf.int/display/CKB/ERA5-Land+data+documentation