A new method for measuring the imaginary part of the atmospheric refractive index structure parameter in the urban surface layer

The atmospheric refractive index consists of both real and imaginary parts. The intensity of refractive index fluctuations is generally expressed as the refractive index structure parameter, with the real part reflecting the strength of atmospheric turbulence and the imaginary part reflecting absorption in the light path. A large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of the atmospheric refractive index, from which the sensible and latent heat fluxes can further be obtained, whereas the influence of the imaginary part is ignored or considered noise. In this theoretical analysis study, the relationship between logarithmic light intensity variance and the atmospheric refractive index structure parameter (ARISP), as well as that between the logarithmic light intensity structure function and the ARISP, is derived. Additionally, a simple expression for the imaginary part of the ARISP is obtained which can be conveniently used to determine the imaginary part of the ARISP from LAS measurements. Moreover, these relationships provide a new method for estimating the outer scale of turbulence. Light propagation experiments were performed in the urban surface layer, from which the imaginary part of the ARISP was calculated. The experimental results showed good agreement with the presented theory. The results also suggest that the imaginary part of the ARISP exhibits a different diurnal variation from that of the real part. For the wavelength of light used (0.62 μm), the variation of the imaginary part of the ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols.

A new method for measuring the imaginary part of refractive index structure parameter in the urban surface layer

Atmospheric refractive index consists of both the real and the imaginary parts. The intensity of refractive index fluctuation is usually expressed as the refractive index structure parameter, whose real part reflects the strength of the atmospheric turbulence while the imaginary part reflects the absorption in the light path. The large aperture scintillometer (LAS) is often used to measure the structure parameter of the real part of atmospheric refractive index, and the sensible and latent heat fluxes can further be obtained, while the influence of the imaginary part is ignored, or thought to be a noise. Based on the expression for the spectrum of the logarithmic light intensity fluctuation caused by the imaginary part of refractive index, new expressions for the logarithmic intensity fluctuation variance and the structure function related to the imaginary part of refractive index are derived. Then a simple expression for the imaginary part of the atmospheric refractive index structure parameter (ARISP) is obtained. It can be conveniently used to measure the imaginary part of the ARISP from LAS. Experiments of light propagation were performed in the urban surface layer and the imaginary part of the ARISP was calculated. The experimental results showed a good agreement with the presented theory. The results also suggested that, the imaginary part of ARISP shows a different variation from the real part of ARISP. For the light with the wavelength of 0.62 μm, the variation of the imaginary part of ARISP is related to both the turbulent transport process and the spatial distribution characteristics of aerosols. Based on the theoretical analysis, it can be expected that the method presented in this study can be applied to measuring the imaginary part of the ARISP caused by the trace gas, if the light wavelength is selected within the corresponding gas absorption region.

Influence of Soil Moisture on Urban Microclimate and Surface-Layer Meteorology in Oklahoma City

The influence of soil moisture on the surface-layer atmosphere is examined in this paper by analyzing the outputs of model simulations for different initial soil moisture configurations, with particular emphasis on urban microclimate. In addition to a control case, four different soil moisture distributions within the urban and surrounding rural areas are considered in this study. Outputs from the Global Environmental Multiscale atmospheric model simulations are compared with observations from the Joint Urban 2003 experiment held in Oklahoma City, Oklahoma, and the relevant conclusions drawn in this paper are therefore valid for similar medium-size cities. In general, high soil moisture is found to be associated with colder near-surface temperature and lower near-surface wind speed, whereas drier soil resulted in warmer temperatures and enhanced low-level wind. Relative to urban soil moisture content, rural soil conditions are predicted to have larger impacts on both rural and urban surface-layer meteorological conditions. Dry rural and wet urban soil configurations are shown to have a strong influence on the urban–rural temperature contrast and resulted in city-induced secondary circulations that considerably affect the near-surface wind speed. Dry rural soil in particular is found to intensify the nocturnal low-level jet and significantly affect the thermal stability of nocturnal near-neutral urban surface layer by altering both thermal and mechanical generation of turbulence.