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 single-wavelength 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 km2. For comparison, local-scale (0.05–0.5 km2) 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.
Highlights
There is considerable demand for large-area estimates of evapotranspiration, or its energy equivalent, the latent heat flux
During the middle of the day on 11 May 2012 QH_EC is 50–100 W m−2 larger than the scintillometry results; possibly QH_BLS–MWS is limited by saturation here (see Ward et al (2014) for discussion)
The first observations of large-area heat fluxes using the two-wavelength scintillometry technique are presented for the urban environment
Summary
There is considerable demand for large-area estimates of evapotranspiration, or its energy equivalent, the latent heat flux. Longer wavelengths (e.g. microwaves, radiowaves) are more sensitive to humidity fluctuations so that, in combination with infrared, both sensible and latent heat fluxes can be obtained more directly (Hill et al, 1988; Andreas, 1989); with a single infrared instrument estimating the latent heat flux relies on closure of the energy balance. During LITFASS-2003, heat fluxes from aggregated eddy covariance (EC) data compared with two-wavelength scintillometry estimates indicated generally good performance over the 11day measurement period (Meijninger et al, 2006). Other urban sites include London (Gouvea and Grimmond, 2010), Toulouse (Masson et al, 2008), Nantes (Mestayer et al, 2011) and Helsinki (Wood et al, 2013) These studies are concerned with the sensible heat flux. We report on the first large-area sensible and latent heat fluxes obtained from a two-wavelength scintillometer system installed over the urban environment. For each wavelength (or combination of wavelengths) the conversion of the observed refractive index structure parameters to temperature (CT2 ) and humidity (Cq2) structure parameters and the temperature– humidity cross-structure parameter (CT q ) is described in Part 1 (Ward et al, 2015)
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