Spatial and temporal variations of thermal contrast in the planetary boundary layer

Short-lived atmospheric pollutants mainly reside in the planetary boundary layer (PBL). In recent years, the sensitivity of high-resolution infrared sounders to the PBL has been amply demonstrated, most notably through observations of local emission sources of sulphur dioxide (SO₂), carbon monoxide (CO) and ammonia (NH₃). However, sensitivity of infrared sounders to the PBL varies strongly as a function of thermal contrast (TC), the temperature difference between the Earth’s skin temperature and the temperature of the atmosphere. Enhanced contrast, typically seen during daytime when the surface is typically (much) warmer than the air, provides favourable measurement conditions. At night, TC is smaller, and can even become negative, providing again favourable measurement conditions. More generally speaking, TC is highly variable in both time (inter and intraday) and space. Up to now, no study has provided insight in the global statistical behaviour of the TC or answered the question when and where thermal infrared sounders experience optimal measurement conditions.

Here we combine the Copernicus Global Land Services land surface temperature (LST) dataset, derived from geostationary satellite measurements, with air temperatures from the ERA5 reanalysis dataset to obtain a global TC dataset at high temporal (1 hour) and high spatial (31 km) resolution. TC is analysed at two different altitudes, the standard meteorological height of 2 meters, and at half the boundary layer height. In addition to the ERA5-based dataset, we also present an additional TC dataset obtained with data from a global constellation of meteorological stations.

We analyse and present statistics on the dependence of TC as function of time of the day, time of the year and land cover.  These provide constraints on the time windows and boundary conditions (e.g., land cover type) for which the sensitivity of the TIR instruments is best. It also allows optimal planning of overpass times for future infrared satellite sounders or the organization of aerial measurement campaigns for near-surface pollutants. Finally, this unique dataset can be used to statistically assess the measurement sensitivity of infrared sounders.