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HSRL Derived Brightness Temperature

HSRL measured visible optical depth can be utilized to derive infrared radiance, converted to a brightness temperature, and compared to AERI measured values. Simultaneous visible and infrared optical depth measurements using the HSRL and AERI, respectively, yields the relationship between the visible and infrared optical depth at each spectral region,


where tex2html_wrap_inline3031 represents the ratio of visible to infrared optical depths. Here, the `visible' optical depth is assumed to be represented by the HSRL measured 532 nm optical depth. This is a valid assumption because the visible extinction cross-section is due to scattering rather than absorption. The infrared cloud transmissivity, as a function of infrared cloud optical depth, can then be described in terms of the HSRL measured visible cloud optical depth,


Using this information, the HSRL integrated column optical depth can be transformed into an infrared value. This approach uses the cloudy RTE solution, Equation 6, in a forward calculation to derive the IR radiance,



tex2html_wrap_inline3033 = HSRL derived column integrated downwelling
radiance, mW (mtex2html_wrap_inline3035 sr cmtex2html_wrap_inline3037)tex2html_wrap_inline3039;
tex2html_wrap_inline3041 = radiosonde determined Planck radiance,
mW (mtex2html_wrap_inline3043 sr cmtex2html_wrap_inline3045)tex2html_wrap_inline3047; and
tex2html_wrap_inline3049 = HSRL differential visible cloud transmissivity.

The first term in Equation 31 represents clear sky radiance below the cloud, calculated using the FASCOD3P atmospheric transmission model for a given radiosonde profile. The second term is the cloud contribution, attenuated by the clear sky transmittance below the cloud, tex2html_wrap_inline3051, which is also determined by the model. The last term is the cloud reflected upwelling terrestrial and clear sky radiance from below the cloud. The radiance contribution from above the cloud was previously shown to be negligible. Multiple-scattering effects are also neglected for visible scattering.

The previous technique utilizes independent observations of AERI derived infrared and HSRL measured visible optical depth to determine a spectral optical depth ratio and HSRL derived downwelling brightness temperature. Equation 30 suggests an alternative method, where the HSRL measured visible optical depth is used to determine the column radiance. This is accomplished by iterating tex2html_wrap_inline3053 until the value agrees with the AERI measured value. The infrared cloud optical depth follows immediately from Equation 30 after tex2html_wrap_inline3055 is determined. It is expected that the iteration of tex2html_wrap_inline3057 will yield better results because the cloud is effectively weighted by the HSRL measured visible optical depth. The former solution assumes a uniform cloud, governed by HSRL measured cloud boundaries, to determine the infrared optical depth and optical depth ratio.

next up previous
Next: Instrumentation Up: Visible Spectrum Previous: Visible Optical Properties

Daniel DeSlover
Sun Aug 11 10:02:40 CDT 1996