A 2-year dataset of data from two ultrasonic anemometers was analyzed. The area of study in west-central London (UK) has a mean building height of H = 21.6 m. A rooftop site (18 m) was at 0.85H and the tower-top level (190 m) was at 8.8H. A double-rotated streamline co-ordinate rotation was conducted on 20 Hz ultrasonic anemometer data to give 30-min averages of many variables: including locally-derived Monin-Obukhov length, L (as used to assess the stability). These observations are complemented by profiles of potential temperature taken from NWP (numerical weather prediction) simulations that include an intricate urban characterization: the MetOffice-Reading Urban Surface Energy Scheme (MORUSES).
Expectation was confirmed for the majority of occasions: the two levels had unstable or neutral flow (neutral defined as |z/L|<0.1). On extremely rare occasions (< 3%), flow at both levels was stable. However, in about a quarter of the dataset, the flow at 8.8H was stable whilst at 0.85H was not stable. We suggest that the higher-level flow became decoupled from the flow below during those occasions: the upper-level stable air was probably advected from the upstream rural stable boundary layer.
The use of a high reference site in London should thus be used with care: particularly with application to emergency response and air quality; e.g. in London-based research programmes such as DAPPLE (Dispersion of Air Pollution and its Penetration into the Local Environment) and REPARTEE (the REgent's PARk and Tower ExpEriment).