Infrared and millimetre-wave scintillometry in the suburban environment – Part 2: Large-area sensible and latent heat fluxes

Abstract

Abstract.A millimetre-wave scintillometer was paired with an infrared scintillometer, enabling estimation of large-area evapotranspiration across northern Swindon, a suburban area in the UK. Both sensible and latent heat fluxes can be obtained using this "two-wavelength" technique, as it is able to provide both temperature and humidity structure parameters, offering a major advantage over conventional singlewavelength scintillometry. The first paper of this two-part series presented the measurement theory and structure parameters. In this second paper, heat fluxes are obtained and analysed. These fluxes, estimated using two-wavelength scintillometry over an urban area, are the first of their kind. Source area modelling suggests the scintillometric fluxes are representative of 5-10 km 2 . For comparison, local-scale (0.05-0.5 km 2 ) fluxes were measured by an eddy covariance station. Similar responses to seasonal changes are evident at the different scales but the energy partitioning varies between source areas. The response to moisture availability is explored using data from 2 consecutive years with contrasting rainfall patterns (2011)(2012). This extensive data set offers insight into urban surface-atmosphere interactions and demonstrates the potential for two-wavelength scintillometry to deliver fluxes over mixed land cover, typically representative of an area 1-2 orders of magnitude greater than for eddy covariance measurements. Fluxes at this scale are extremely valuable for hydro-meteorological model evaluation and assessment of satellite data products.

Characteristics of Evapotranspiration of Urban Lawns in a Sub-Tropical Megacity and Its Measurement by the ‘Three Temperature Model + Infrared Remote Sensing’ Method

“…Compared with the magnitude of ET observed in other landscapes (grasslands, farmlands, and forests), these values were not small (Yan and Qiu, 2016). The daily ET in our study could reach to 6 mm day −1 , much higher than 1-3 mm day −1 observed in other urban areas located in temperate zones ( [4,29,30]). Urban ET is very obvious and cannot be neglected for water and environmental management.…”

Long-term Turbulent Sensible-Heat-Flux Measurements with a Large-Aperture Scintillometer in the Centre of Łódź, Central Poland

“…Basel (Christen and Vogt 2004), Swindon (Ward et al 2014), London (Crawford et al 2017). As highlighted by Ward et al (2015b), there is still a need for long-term scintillometer measurements of sensible and latent heat fluxes in urban areas that capture both the seasonal and inter-annual variability. Therefore, such long-term measurement campaigns could help determine to what extent the observed variability of heat fluxes is caused by different surface characteristics or by weather conditions.…”

Infrared and millimetre-wave scintillometry in the suburban environment – Part 1: Structure parameters

“…One approach for achieving this would be to work with structure parameters directly, rather than fluxes (e.g. Wood et al, 2013).…”

“…Each C 2 n measurement is made up of a combination of the three unknowns: C 2 T , C 2 q and C T q . As C 2 n from large aperture optical or near-infrared scintillometers is almost entirely made up of temperature fluctuations (C 2 T ), the (usually small) contributions from C T q and C 2 q can be approximated using the Bowen ratio, β (Wesely, 1976;Moene, 2003):…”

“…Other small aperture studies include measurements in Basel (Roth et al, 2006) and London (Pauscher, 2010). Large aperture scintillometers are increasingly being used over longer urban paths (Lagouarde et al, 2006;Gouvea and Grimmond, 2010;Mestayer et al, 2011;Wood et al, 2013;Zieliński et al, 2013). These infrared (or optical) scintillometers must rely on the residual of the energy balance if the latent heat flux is to be estimated.…”