An Improved BRDF Hotspot Model and its Use in VLIDORT to Study the Impact of Atmospheric Scattering on Hotspot Directional Signatures in the Atmosphere

Abstract. The term “hotspot” refers to the sharp increase of reflectance occurring when incident (solar) and reflected (viewing) directions coincide in the backscatter direction. The accurate simulation of hotspot directional signatures is important for many remote sensing applications. The RossThick-LiSparse-Reciprocal (RTLSR) Bidirectional Reflectance Distribution Function (BRDF) model is widely used in radiative transfer simulations, but it typically requires large values of numerical quadrature and Fourier expansion terms in order to represent the hotspot accurately. In this paper, we have developed an improved hotspot BRDF model that converges much faster, making it more practical for use in atmospheric radiative transfer simulations of top-of-atmosphere (TOA) hotspot signatures. Using the VLIDORT RT model, we found that reasonable TOA hotspot accuracy can be obtained with just 23 Fourier terms for clear atmospheres, and 63 Fourier terms for atmospheres with aerosol scattering. We carried out a number of hotspot signature simulations with VLIDORT to study to the impact of molecular and aerosol scattering on hotspot signatures. We confirmed that (1) atmospheric scattering tends to smooth out the hotspot signature at the TOA, but has no impact on hotspot width; and (2) the hotspot signature at the TOA in the near-infrared is larger than in the visible, and has an obvious increase with the solar zenith angle. As the hotspot amplitude at the TOA with aerosol scattering included is smaller than that with molecular scattering only, the amplitude of hotspot signature at the surface is likely underestimated in the previous analysis based on the POLDER measurements, where the atmospheric correction was based on a single-scatter Rayleigh-only calculation. We also draw attenuation to a scaling factor of 3π/4 which has been applied to the Ross-Thick kernel with hotspot correction.