A Harmonic Approach for Calculating Daily Temperature Normals Constrained by Homogenized Monthly Temperature Normals

NOAA released the new 1981-2010 climate normals in July 2011. These included monthly and daily normals of minimum and maximum temperature. Monthly normals were computed from monthly temperature values that were corrected for biases (i.e., homogenized) due to changes in observing practices over the course of the normals period (station moves, changes in observation time, and changes in instrumentation). Daily temperature observations, however, are not homogenized, leading to inconsistencies between the daily and monthly normals. Here, we offer a constrained harmonic technique that forces the daily temperature normals to be consistent with the monthly temperature normals. This approach replaces the cubic spline interpolation of monthly temperature normals that was used to compute NOAA's 1971-2000 daily temperature normals. It effectively passes the homogenization applied at the monthly scale down to the daily scale, resulting in a smooth annual cycle devoid of day-to-day sampling variability and inter-month discontinuities.