Calculation of longwave radiation fluxes in atmospheres

Abstract

A technique for the computation of longwave radiative quantities using the line-by-line approach has been developed in the Soviet Union. The method has been applied to obtain fluxes and cooling rates for standard atmospheric profiles used in the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM) sponsored by the World Meteorological Organization. The sensitivity of the result to changes in the vertical quadrature scheme, the angular integration, and the spectral line shape is evaluated. Fluxes and cooling rates in the troposphere are in general agreement with those obtained with different line-by-line models. Results from parameterized models, including a wideband statistical model and one employing the integral transmission function, have been compared to the line-by-line results. Flux errors in the simplified schemes are of the order of 10 W/m 2. The sensitivity of these models to changes in atmospheric profiles, or to an increase in CO2 amount, is similar to that of the line-by-line calculations. INTRODUCTIONRadiative heat exchange is a basic component of atmospheric energy transfer. The numerical models for weather forecasting, climate theory, cloud generation, and a number of mesoscale dynamic processes all invoke radiation codes to compute this quantity.The radiative flux calculation algorithms in these models differ appreciably from each other and are always an approximation to the exact solutions of the radiative transfer equations. These approximations are needed because (1) the radiative calculations have to be made quickly compared to the calculation time of the model as a whole; (2) there is insufficient information in dynamic models to calculate the fluxes, and (3) the spatial, particularly the vertical, resolutions of the models are not sufficiently detailed.The calculated fluxes in each model differ as a consequence of the distinctiveness of the specific algorithms, the initial data applied, the spectral resolutions, etc., of the model. We may thus ask how the results of the simulation of the dynamic processes in the atmosphere are affected by the differences in radiative flux calculation techniques in models. The second principal feature of this technique is the shift in frequency of the line centers to the nearest frequency points (nodal points) at which the absorption coefficients are evaluated. This approach permits the use of a uniformly spaced frequency grid for the calculation, and permits substantial improvement in both the accuracy and the computational efficiency of the numerical frequency integration. A deficiency in the direct application of (1)-(7) to flux computations is that, according to (2), a systematic underestimation of absorption occurs for "weak" lines, i.e., for the lines whose contributions to e(v) are less than e. In certain spectral regions, particularly the 8-to 13-/am "window" region, this underestimation can markedly affect the accuracy of the calculation. In order to compensate for this effect, the simple technique given below is used.If the ith line ...

Radiation and temperature effects of the intensive injection of dust aerosol into the atmosphere

“…The results of calculations by the approximate method [13,14] have shown that the values of ascending fluxes up to 6 km high are calculates within an accuracy of 1-3%. The maximal deviation of ascending flux values in different climatic zones are 8-14 W/m 2 .…”

“…For a cloudless atmosphere, we calculated ITF by atmospheric gases (Н 2 О, СО 2 , and О 3 ) and aerosol. The accuracy of the method for calculating long infrared radiation fluxes was esti mated by comparison with line by line calculations [13,14]. The respective results of a comparison between solar and thermal radiation fluxes for cloud less and clouded atmosphere are presented in [13].…”

“…Approximated methods for calculating inte grated fluxes of solar [12,13] and thermal [13,14] radiation were used with the integrated transmission function (ITF) of the atmosphere involved. The method of calculating integrated solar radia tion fluxes in the short infrared region of 0.2-4.0 μm is based on the solution of radiation transfer equation in the δ Eddington approximation.…”

A k‐distribution technique for radiative transfer simulation in inhomogeneous atmosphere: 1. FKDM, fast k‐distribution model for the longwave

“…The first algorithm [ Fomin , 1994] was independently tested [ Kuntz and Hofner , 1999] and the properties of the second one based on the “linear in τ” approximation [ Wiscombe , 1976] for vertically inhomogeneous atmosphere are well known. It is also worthy of note that a large family of calculations obtained with earlier versions of FLBLM was published long ago as benchmark calculations for the validation of radiation codes being used in climate models [ Feigelson et al , 1991; Fomin and Gershanov , 1997]. Of course these calculations have been used for FKDM validation.…”

<title>Line-by-line radiative parameters databases in atmosphere sensing and modeling applications</title>

“…In doing so to allow for an actual atmosphere heterogeneity the modified scaled content approximation is considered that exploits a set of effective parameters based upon the CurtisGodson approximation. Thus in order to simulate the JR photon optical path-length from the path coordinate 1=1' towards the atmosphere top (1=0) within each Av the scaled content for each i is calculated using u(O,l')= Jp(l)dl (11) where p ( I is the scaled density (g/rn3) defined as Al (1) = I I ii g (9(l)) i (12) re P,! ) t\ e g'(O) ) and ] is the reference pressure ofthe path evaluated separately for each I by the formula like P = P(l)w'(l)dl I w'(l)dl (13) The following notation are used in expressions (1 1)-(13):L is the path length ( in km); e(l), P(l) are the temperature and pressure at current path coordinate 1, respectively; p' (1) is the actual density, and n,, f , m, are the model tunable parameters.…”