Spatial variability of forward modelled attenuated backscatter in clear‐sky conditions over a megacity: Implications for observation network design

• Published in: Quarterly journal of the Royal Meteorological Society 2022-04, Vol.148 (744), p.1168-1183
• Main Authors: Warren, Elliott, Charlton‐Perez, Cristina, Lean, Humphrey, Kotthaus, Simone, Grimmond, Sue
• Format: Article
• Language: English
• Subjects: Advection; Aerosols; Air quality; Backscatter; Ceilometers; Cluster analysis; Data assimilation; Data collection; Design; Emissions; Empirical orthogonal functions; Gravity; lidar; Megacities; Methods; Modelling; network design; numerical weather prediction; Orography; Principal components analysis; Sensors; Spatial variability; Spatial variations; urban; Urban air quality; Urban areas; Urban climates; Urban weather; Weather forecasting; Wind; Wind roses; Wind speed
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4253

Sensors that measure the attenuated backscatter coefficient (e.g., automatic lidars and ceilometers [ALCs]) provide information on aerosols that can impact urban climate and human health. To design an observational network of ALC sensors for supporting data assimilation and to improve prediction of urban weather and air quality, a methodology is needed. In this study, spatio‐temporal patterns of aerosol‐attenuated backscatter coefficient are modelled using Met Office numerical weather prediction (NWP) models at two resolutions, 1.5 km (UKV) and 300 m (London Model [LM]), for 28 clear‐sky days and nights. Initially, attenuated backscatter coefficient data are analysed using S‐mode principal component analysis (PCA) with varimax rotation. Four to seven empirical orthogonal functions (EOFs) are produced for each model level, with common EOFs found across different heights (day and night) for both NWP models. EOFs relate strongly to orography, wind, and emissions source location, highlighting these as critical controls of attenuated backscatter coefficient spatial variability across the megacity. Urban–rural differences are largest when wind speeds are low and vertical boundary‐layer dynamics can more effectively distribute near‐surface aerosol emissions vertically. In several night‐time EOFs, gravity‐wave features are found for both NWP models. Increasing the horizontal resolution of native ancillaries (model input parameters) and improving the urban surface scheme in the LM may enhance the urban signal in the EOFs. PCA output, with agglomerative Ward cluster analysis (CA), minimises intra‐group variance. The UKV and LM CA shape and size results are similar and strongly related to orography. PCA‐CA is a simple, but adaptable methodology, allowing close alignment with observation network design goals. Here, CA is used with wind roses to suggest the optimised ALC deployment is one in the city to observe the urban plume and others surrounding the city, with priority given to cluster size and frequency of upwind advection. Principal component analysis and cluster analysis are used with forward‐modelled aerosol attenuated backscatter from two numerical weather prediction models over Greater London. This identified regions with common spatial patterns in backscatter variability, which can subsequently help inform lidar deployment in an urban network. Common patterns strongly relate to relative aerosol emission locations, winds, and orography, factors that should be