Calculating daily mean air temperatures by different methods: implications from a non-linear algorithm

Abstract

Daily mean air temperature is used as an independent variable in algorithms describing many biological applications. These algorithms are usually of a non-linear nature. The questions addressed in this study were: is there a difference in the daily mean air temperature calculated by different methods and what is the impact of the various calculation methods on a non-linear algorithm? The empirical coefficient in the non-linear algorithm used in this study was determined from a daily mean air temperature based on the mean of 24 hourly mean temperature values. Daily mean air temperature was calculated by five methods: mean hourly (Hourly); three equally spaced hourly mean observations weighted with the last observation (Weighted); 3 h mean temperatures (Mean 3 hour); the algorithm used in the CERES family of crop simulation models (CERES), and the mean of the maximum and minimum daily temperatures (Max/Min). It was assumed that the Hourly method best represented the daily mean air temperature and the other methods were compared to it. Two forms of air temperature were used in a non-linear algorithm; a sequential approach where the algorithm was run as many times as the number of individual temperature values used in each method, the results then averaged; and a single daily mean air temperature value. This non-linear algorithm was evaluated over a wide range of locations, ranging in elevation from 2.4 to 1252 m and annual precipitation from 108 to 1820 mm. There was little difference in daily mean air temperatures between the different methods. However, there were large differences in responses from the non-linear algorithm when using any sequential approach when compared to the single daily mean temperature values. The Mean 3 hour method worked well in all locations. The CERES method worked well except for two locations characterized by high mean annual maximum and minimum temperatures. These results do not mean that the sequential approaches are inappropriate, just that the temperature method used to determine empirical coefficients in the non-linear algorithm must be consistently used in all applications. These results are a guide to different methods used to calculate daily mean air temperature and the range of possible results when used in a non-linear algorithm. Although a specific example was used in this study, the results are relevant to any non-linear algorithm containing empirically determined coefficients.