Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part I: Theory

Abstract

A method is presented for determining the optical thickness and effective particle radius of stratiform cloud layers from reflected solar radiation measurements. A detailed study is presented which shows that the cloud optical thickness (7,) and effective particle radius (r,) of water clouds can be determined solely from reflection function measurements at 0.75 and 2. I6 pm, provided rc B 4 and rc Z 6 pm. For optically thin clouds the retrieval becomes ambiguous, resulting in two possible solutions for the effective radius and optical thickness. Adding a third channel near 1.65 am does not improve the situation noticeably, whereas the addition of a channel near 3.70 pm reduces the ambiguity in deriving the effective radius. The effective radius determined by the above procedure corresponds to the droplet radius at some optical depth within the cloud layer. For clouds having +< L 8, the effective radius determined using the 0.75 and 2.16 pm channels can be regarded as 85%95% of the radius at cloud top, which corresponds in turn to an optical depth 209'~40% of the total optical thickness of the cloud layer.

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

“…Alternatively, the R(2.13 µm) reflectance might simply be less sensitive to the broader DSD shape than the R(3.75 µm) reflectance. Overall, these results demonstrate a feature well known to the cloud remote-sensing community: the bispectral retrieval of r e is not particularly sensitive to v e (Nakajima and King, 1990a). Indeed, comparison of the coupled bispectral retrieval of r e to the polarimetric retrieval of r e confirms that the advantage of retrieving v e changes the bispectral retrieval of r e by less than a micron, so it is appropriate to neglect this level of detail of the DSD for bispectral retrieval purposes.…”

“…These techniques typically infer DSD properties based on an assumed size distribution shape, characterized by an effective radius (r e ) and an effective variance (v e ). One such retrieval method is called the bispectral total reflectance technique, hereafter referred to as the "bispectral technique", which simultaneously retrieves cloud optical thickness (τ ) and r e from a pair of cloud reflectances, typically one in the visible to near infrared (VNIR) and the other in the shortwave infrared (SWIR) or midwave infrared (MWIR) spectral range (Nakajima and King, 1990b). This retrieval technique has been implemented for numerous satellite and airborne instruments, such as the Moderate Resolution Imaging Spectroradiometer (MODIS; King et al, 2003;, the Spinning Enhanced Visible and Infrared Imager (SEVIRI; Roebeling et al, 2006), and the Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (Suomi NPP VIIRS; Rosenfeld et al, 2014).…”

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

“…8a highlights one of the main challenges of ground-based images compared to satellite-based cloud detection. In satellite-based τ c detection, the measured radiance can be used to calculate τ c (Nakajima and King, 1990) as the measured (upwelling) radiance monotonically increases with higher τ c . This same method cannot be used for ground-based imagery as radiance increases for thin clouds peaks and then begins to decrease.…”

Satellite–Based Insights into Precipitation Formation Processes in Continental and Maritime Convective Clouds

“…The effective radius of fully cloudy pixels is calculated using an inversion of a radiative transfer model (Nakajima and King 1990), with the mid-IR (3.7 ^m) cloud reflectance (p3 7) and the viewing geometry as inputs. The quantitative analysis is done only on clouds thick enough to be potential precipitation producers.…”