Quality assessment of high resolution wind and temperature observation from ModeS

Wind, temperature and humidity observations from radiosonde and aircraft are the main sources of upper air information for meteorology. For meso-scale meteorology, the horizontal coverage of radiosondes is too sparse and aircraft observations through AMDAR (Aircraft Meteorological Data Relay) are insufficient because not all aircraft are equipped with an AMDAR system.

Observations inferred from an enhanced tracking and ranging (TAR) radar of Schiphol airport can fill this gap. This radar follows all aircraft in the airspace visible to the radar with a radius of 270km around the Netherlands for air-traffic management. This ModeS-radar contacts each aircraft every four seconds on which the transponder in the aircraft responds with a message that contains information on flight level, direction and speed. Together with the ground track of an aircraft, meteorological information on temperature and wind can be inferred from this information. Because all aircraft are required to respond to the TAR-radar, the data volume is extremely large, being around 1.5 million observations per day.

The quality of these observations is assessed by comparison to a numerical weather prediction (NWP) model information, AMDAR observations and radiosonde observations. A method to improve the temperature and wind observations is applied to enhance the quality wind and temperature observations, albeit with a reduced time frequency of one observation of horizontal wind vector and temperature per aircraft per minute. Nevertheless, the number of observations per day is still very large.

The temperature observations from ModeS, even after corrections are not very good; an RMS which twice as large as AMDAR is observed when compared to NWP. The quality found for wind after correction and calibration is good; it is comparable to AMDAR, slightly worse than radiosonde but certainly very valuable for NWP