Atmospheric boundary layer characteristics from Ceilometer measurements part 2: application to London’s urban boundary layer

Long-term measurements of mixed layer height (ZML) are possible with advances in detecting ZML from Automatic Lidars and Ceilometers (ALC) observations. Six years of ALC measurements in central London are analysed using the CABAM (‘Characterising the Atmospheric Boundary layer based on ALC Measurements’) algorithm which provides ZML and an ABL classification by cloud cover and type. The boundary layer dynamics are shown to respond to day-length, cloud cover and cloud type. Median daily maxima range from 707 m (stratiform clouds) to 1704 m (days with convective boundary layer clouds following a clear night). A common approach to ABL classification and clear definition of key ZML-indicators would facilitate inter-city comparison. A simple parameterisation based on empirical coefficients derived from the London measurements is proposed to generalise the description of diurnal and seasonal variations in ZML, including cloud conditions. This has the potential to aid improved understanding of the complex relations between surface air quality and boundary layer dynamics

Detection of sea-breeze events around London using a fuzzy-logic algorithm

We present an algorithm for detecting sea breezes based on fuzzy logic, using changes in variables commonly measured at meteorological stations. The method is applied to one year’s worth of UK Met Office data (2012) measured at several stations around London, UK. Results indicate about a dozen potential events over the year, when matched against corresponding detections at a coastal reference site (Gravesend). In some cases the time lags between corresponding events detected at different stations can be used to characterise the average propagation speed of the sea-breeze front. Advantages and disadvantages of the method are discussed

LUCY: Large scale Urban Consumption of Energy

Model and datasets to document changes in global anthropogenic heat flux (QF) for spatial (30?? × 30?? to 0.5° × 0.5°) resolution and temporal coverage (historical, current and future). See further details in the Lindberg et al. publications linked from this record

Volume for pollution dispersion: London’s atmospheric boundary layer during ClearfLo observed with two ground-based lidar types

In urban areas with high air pollution emissions, the boundary layer volume within which gases and particles are diluted is critical to air quality impacts. With advances in ground-based remote sensing technologies and data processing algorithms, observations of layers forming the atmospheric boundary layer (ABL) are becoming increasingly available at high temporal resolution. Here, mixing height (MH) estimates determined from turbulence measurements of Doppler lidars and aerosol derived mixed layer height (MLH) based on automatic lidar and ceilometer (ALC) observations within the centre of London are assessed. While MH uncertainty increases with shorter duration of vertical stare sampling within the Doppler lidar scan pattern, instrument-related noise of the ALC may result in large MLH errors due to the challenging task of layer attribution. However, when long time series are assessed most of the algorithm- and instrument-related uncertainties average out and therefore become less critical to overall climatological analyses. Systematic differences occur in nocturnal MH from two nearby (3-4 km) sites but MLH estimates at both sites generally agree with MH obtained at the denser urban setting. During daytime, most spatial variations in ABL structure induced by synoptic conditions or land cover heterogeneity at this scale do not exceed measurement uncertainty. Agreement between MH and MLH is clearly affected by ABL aerosol content and cloud 28 conditions. Discrepancies increase with cloud complexity. On average, MH rises ahead of MLH during the morning growth period and peaks earlier in the day. There is a faster afternoon decay of MLH so that MLH and MH converge again around sunset and often have similar nocturnal values. Results demonstrate that turbulence-derived MH and aerosol-derived MLH should not be used inter32 changeably for purposes of model evaluation, interpretation of surface air quality observations or 33 initialisation of chemical transport models

On‐ and off‐line evaluation of the single‐layer urban canopy model in London summertime conditions

Urban canopy models are essential tools in forecasting weather and air quality in cities. However, they require many surface parameters, which are uncertain and can reduce model performance if inappropriately prescribed. Here, we evaluate the model sensitivity of the Single-Layer Urban Canopy Model (SLUCM) in the Weather Research and Forecasting model (WRF) to surface parameters in two different configurations, one coupled to the overlying atmosphere (on-line) in a 1D configuration and one without coupling (off-line). A 2-day summertime period in London is used as a case study, with clear skies and low wind speeds. Our sensitivity tests indicate that SLUCM reacts differently, when coupled to the atmosphere. For certain surface parameters, atmospheric feedback effects can outweigh the variations caused by surface parameter settings. Hence to fully understand model sensitivity atmospheric feedbacks should be considered

Persistent cloud cover over mega-cities linked to surface heat release

Urban areas are a hotspot for the interactions between the built environment, its inhabitants, and weather. Unlike the impact of temperatures through the well-known urban heat island effect, urban effects on cloud formation remain unknown. In this study we show observational evidence of a systematic enhancement of cloud cover in the afternoon and evening over two large metropolitan areas in Europe (Paris and London). Long-term measurements in and around London show that during late-spring and summer, even though less moisture is available at the surface and the atmosphere is drier, low clouds can persist longer over the urban area as vertical mixing of the available moisture is maintained for a longer period of time, into the evening transition. Our findings show that urban impacts on weather extend beyond temperature effects. These prolonged clouds over the city might enhance the urban heat island via night-time radiative forcing

Evaluation of forward-modelled attenuated backscatter using an urban ceilometer network in London under clear-sky conditions

Numerical weather prediction (NWP) of urban aerosols is increasingly sophisticated and accurate. In the absence of large particles (e.g. rain, cloud droplets), information on atmospheric aerosols can be obtained from single wavelength automatic lidars and ceilometers (ALC) that measure profiles of attenuated backscatter (βo). To assess the suitability of ALC profile observations for forecast evaluation and data assimilation, a forward operator is required to convert model variables into the measured quantity. Here, an aerosol forward operator (aerFO) is developed and tested with Met Office NWP data (UKV 1.5 km) to obtain synthetic attenuated backscatter profiles (βm). aerFO requires as input the profiles of bulk aerosol mass mixing ratio and relative humidity to compute βm, plus air temperature and pressure to calculate the effect of water vapour absorption. Bulk aerosol characteristics (e.g. mean radius and number concentration) are used to estimate optical properties. ALC profile observations in London are used to assess βm. A wavelength-dependent extinction enhancement factor accounts for the change in optical properties due to aerosol swelling. Sensitivity studies show the aerFO unattenuated backscatter is very sensitive to the aerosol mass and relative humidity above ~60-80 %. The extinction efficiency is sensitive to the choice of aerosol constituents and to ALC wavelength.Given aerosol is a tracer for boundary layer dynamics, application of the aerFO has proven very useful to evaluate the performance of urban surface parameterisation schemes and their ability to drive growth of the mixing layer. Implications of changing the urban surface scheme within the UKV is explored using two spring cases. For the original scheme, morning βm is too high probably because of delayed vertical mixing. The new scheme reduced this persistence of high morning βm, demonstrating the importance of surface heating processes. Analysis of profiles at five sites on 12 clear-sky days shows a positive, statistically significant relation between the differences of modelled and measured near-surface attenuated backscatter [βm - βo] and near-surface aerosol mass. This suggests errors in near-surface attenuated backscatter can be attributed to errors in the amount of aerosol estimated by the NWP scheme. Correlation increases when cases of high relative humidity in the NWP model are excluded. Given the impact on aerosol optical properties demonstrated, results suggest the use of a fixed, bulk aerosol for urban areas in the UKV should be revisited and the lidar ratio should be constrained. As quality of the observed attenuated backscatter is demonstrated to be critical for performing model evaluation, careful sensor operation and data processing is vital to avoid false conclusions to be drawn about model performance

Attribution and mitigation of heat wave-induced urban heat storage change

When the urban heat island (UHI) effect coincides with a heat wave (HW), thermal comfort conditions in cities are exacerbated. Understanding the surface energy balance (SEB) responses to HWs is critical for improving predictions of the synergies between UHIs and HWs. This study evaluates observed SEB characteristics in four cities (Beijing, Łódź, London and Swindon), along with their ambient meteorological conditions, for both HW and background summer climate (BC) scenarios. Using the Analytical Objective Hysteresis Model (AnOHM), particular emphasis is on the heat storage. The results demonstrate that in London and Swindon not only the amount of daytime heat storage but also its portion relative to the net all-wave radiation increase under HWs. Results further demonstrate that such increases are strongly tied to lower wind speeds. The effects of different UHI mitigation measures on heat storage are assessed using AnOHM. Results reveal that using reflective materials and maintaining higher soil moisture availability may offset the adverse effects of increased heat storage