Modeling zenith-angle dependence of outgoing longwave radiation: Implication for flux measurements

As shown in previous studies, outgoing-longwave radiation (OLR) can be in error by similar to 7% when evaluated from directional measurements by applying time-averaged angular distribution models (ADMs) to account for the emission anisotropy. In order to develop an insight into the problem of assessing hemispheric emission from directional measurements, we formulate the directional (monochromatic) greenhouse factor g(d), specified as the ratio of a longwave radiance measured above the atmosphere to that emitted from the surface at the same zenith angle theta(r). The explicit expressions for g(d) involve two atmospheric parameters, the optical thickness and the temperature-profile parameter. Oar analysis indicates that under clear conditions a narrow-band radiance, if measured at theta(v) approximate to 57 degrees in atmospheric windows (low values of optical thickness) and at theta(v) approximate to 47 degrees in absorption bands, determines the hemispheric-average radiance to within about 1%. For broad spectral bands, whether under clear or cloudy (solid cover, or ''randomly scattered'' clouds) conditions, the same finding applies at theta(v) approximate to 50 degrees. Thus, the zenith angle of equivalence theta(eq) varies by about +/-5 degrees for different values of the optical thickness; it varies only slightly for different temperature profiles (even though different temperature profiles produce quite different patterns of radiance us. theta(v), that is, different ADMs apply). Measurements at or near theta(eq) therefore constitute direct assessment of OLR, without resorting to ADMs to adjust for the variations of emission with view angle (anisotropy). The existing OLR data should be reexamined accepting measurements only within the range 45-60 degrees of view zenith angles. (C) Elsevier Science Inc., 1997.