Generation of chilling hours maps using surface observations and satellite data

Abstract

Abstract In complex terrain, the topography influences the air temperature near the surface since cold air pooling and thermally-generated winds might respectively enhance or diminish the nocturnal cooling rate. The characterization of the temperature patterns in agricultural areas is important because many crops need a certain amount of time under cold temperatures. This is usually given by the Chilling Hours (CH, number of hours below a certain temperature threshold). In this work the number of CH has been estimated in the island of Mallorca using in-situ and satellite Meteosat data for the period 2008–2018, taking a threshold value of 7 °C, applying corrections for missing data in both cases, including cloudiness for the satellite data. CH values per year obtained are small for stations near or at the coast (CH below 400) and large at mountain tops (CH above 1500), whereas at the foothills and central plain the values vary between 500 and 1000. Cold events contributing to the number of CH happen mostly between December and March and they may be related to cold synoptic advection (with clear or cloudy skies) or mostly to clear nights with weak local winds that may end in late night fog. Satellite-derived CH maps indicate that the coldest areas are located in the central lowlands between the two major mountain ranges and the elevated area in the center of the island, where cold air pools are usually found. Significant differences between in-situ and satellite CH may exist due to the topographical local features compared to a surface-averaged value from the satellite pixel. Furthermore, existing CH parameterizations making use of daily extreme temperatures are checked and adjusted coefficients are proposed.