Decoupled stable air above London, UK

There are few long-term research-grade studies of the boundary layer above cities. Canonical urban micro-climatological expectation is that the urban atmosphere, compared with rural surrounds, is unstable both by a greater magnitude and more of the time (due to heat storage, sky-view factor, bowen ratio, and anthropogenic heat). Hence, most often there is an unstable boundary layer coupled to the urban surface below. But how often is there stable flow, and how is it generated?

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).