From the discussion:
The UHI is evident from the radiative surface skin temperature field. There is clear diurnal variation in UHIskin, with larger UHI intensity during daytime than during nighttime. There is also a seasonal variation in UHIskin, with a larger signal in summer than in winter. This seasonal variation corresponds to the seasonality of solar radiation, the vegetation growing cycle, and rainfall or soil moisture content.
UHIskin maximizes during the daytime. Therefore, we should not consider UHI exclusively a “nighttime phenomenon” anymore. Jin et al. (2005) suggested the possible physical mechanisms responsible for a stronger daytime UHI. The albedo is reduced in urban regions, partly due to urban canopy effect and partly due to new materials used in urban roads and buildings; consequently, more solar radiation is absorbed at the surface to warm it. In addition, surface emissivity is reduced because of the new materials used in the urban surface, leading to less longwave radiation leaving the surface. As a result, daytime UHI is stronger than that nighttime. Furthermore, urban regions reduce soil moisture and vegetation coverage, which further change the solar radiation redistribution in terms of latent and sensible heat fluxes (e.g, Bowen ratio), and thus increases surface temperature. MODIS provides albedo data (Shaff et al. 2003), emissivity data (Jin and Liang 2006), and vegetation data (Myneni et al. 1995), which can be further used to address these mechanisms.
Journal of Climate 2012 ; e-View
Developing an Index to Measure Urban Heat Island Effect Using Satellite Land Skin Temperature and Land Cover Observations
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A new index of calculating the intensity of urban heat island effects (UHI) for a city using satellite skin temperature and land cover observations is recommended. UHI, the temperature difference between urban and rural regions, is traditionally identified from the 2-m surface air temperatures (i.e., the screen-level temperature, T2m) measured at a pair of weather stations sited in urban and rural locations, respectively. However, such screen-level UHI is affected by location, distance, and geographic conditions of the pair of weather stations. For example, choosing a different pair of rural and city sites leads to a different UHI intensity for the same city, due to the high heterogeneity of the urban surface temperature. To avoid such uncertainty, satellite-observed surface skin temperature measurements (i.e., skin level, Tskin) is recommended to record UHI, known as skin-level UHI or UHIskin. This new index has advantages of high spatial resolution and aerial coverage to better record UHI intensity than T2m. An assessment of skin-level UHI from 10-years of the National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) observations reveals that skin-level UHI has a strong UHI signal during day and night. In addition, there are significant diurnal and seasonal variations in skin-level UHI. Furthermore, the skin-level UHI is stronger during the day and summer than during nighttime and winter, respectively. This new index is important for more uniformly assessing UHIs over cities around the globe. Nevertheless, whether the seasonality and diurnal variations revealed in this work using skin-level UHI index is valid over desert cities, such as Phoenix, needs to be examined.
I would note that the conclusion UHI is stronger during the day is compatible with the finding that water vapor is a negative feedback, as urban areas have lower levels of water vapor. see
ReplyDeletehttp://hockeyschtick.blogspot.com/2012/04/new-analysis-finds-water-vapor-is.html