Towards operational use of aircraft‐derived observations: a case study at London Heathrow airport

Mode‐Selective Enhanced Surveillance (Mode‐S EHS) aircraft reports can be collected at a low cost and are readily available around busy airports. The new work presented here demonstrates that observations derived from Mode‐S EHS reports can be used to study the evolution of temperature inversions since the data have a high spatial and temporal frequency. This is illustrated by a case study centred around London Heathrow airport for the period January 4–5, 2015. Using Mode‐S EHS reports from multiple aircraft and after applying quality control criteria, vertical temperature profiles are constructed by aggregating these reports at discrete intervals between the surface and 3,000 m. To improve these derived temperatures, four smoothing methods using low‐pass filters are evaluated. The effect of smoothing reduces the variance in the aircraft derived temperature by approximately half. After smoothing, the temperature variance between the altitudes 3,000 and 1,000 m is 1–2 K; below 1,000 m, it is 2–4 K. Although the differences between the four smoothing methods are small, exponential smoothing is favoured because it uses all available Mode‐S EHS reports. The resulting vertical profiles may be useful in operational meteorology for identifying elevated temperature inversions above 1,000 m. However, below 1,000 m they are less useful because of the reduced precision of the reported Mach number. A better source of in situ temperature observations would be for aircraft to use the meteorological reporting function of their automatic dependent surveillance system.