Diurnal Cycles in HIRS Brightness Temperatures

The High-resolution Infrared Radiometer Sounder (HIRS) instruments onboard National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites have sampled the Earth's atmosphere and surface since late 1978. HIRS is a cross-track scanning radiometer that measures infrared and visible radiances in 20 channels. It was originally developed with weather forecasting applications in mind, providing information on atmospheric temperature and humidity, but has in more recent years also been utilized in climate studies. The HIRS data record features characteristics that influence its homogeneity. One such characteristic is the drift in the local equator crossing time causing diurnal sampling biases in the data record. This study aims to investigate the diurnal cycles in HIRS brightness temperatures by looking at the period 2002--2007, which is particularly well-covered with up to five satellites (NOAA14--MetOp) simultaneously flying HIRS instruments. In order to describe the diurnal cycle in brightness temperatures, we fit a second order Fourier harmonic to monthly gridded HIRS data. These fits can serve as a basis for correcting the whole HIRS record for diurnal sampling biases, and on the other hand, also for testing climate model performance against HIRS measurements.