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  • Author or Editor: J. D. Haigh x
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Wenyi Zhong and J. D. Haigh

Abstract

Reference transmissivities based on line-by-line calculations have been computed for a wide range of homogeneous paths of water vapor. A new approach is employed in which wideband emissivities are directly fitted to the line-by-line reference calculations without using the intermediate step of narrowband models. A significant improvement in accuracy is obtained over previous schemes. Compared with line-by-line computed fluxes and cooling rates (without continuum absorption) for the standard middle-latitude summer (MLS) profile, the maximum error in fluxes is 1.5 W m−2 agreement is within 1% in fluxes and within 0.11 K/day, or 5%. in cooling rate. Unlike most published water vapor continuum schemes, which use the Roberts et al. model, the authors have reformulated the treatment of the water vapor continuum by producing a new parameterization based on the semiempirical model of Cough et al. This results in ∼7.5 W m−2 difference in calculated radiative fluxes at the tropopause, and maximum difference in fluxes can approach 15 W m−2 in the troposphere for the MLS atmosphere.

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J. E. Russell and J. D. Haigh

Abstract

A method is described for the retrieval of cirrus cloud temperature and optical depth using thermal infrared data from the Along-Track Scanning Radiometer. The method utilizes above-cloud and nearby clear-sky thermal infrared data at a single wavelength and two different viewing angles and assumes that the cirrus cloud is nonscattering, isothermal, and semitransparent. The sensitivity of the method to small uncertainties in the input parameters is calculated. The effect on the retrieval of vertical inhomogeneity is investigated using idealized models of cirrus cloud vertical structure. It is shown that a vertical temperature structure within the cloud, alone and in conjunction with vertical inhomogeneity in absorption coefficient, can cause large errors in the retrieved quantities for a wide range of cloud types. However, these investigations show that retrieved quantities remain within usable limits for the majority of expected cirrus clouds. For example, for clouds with a lapse rate of 9 K km−1 and a linear absorption coefficient profile with gradient ranging from −2 to +2, optical depth can be retrieved to an accuracy of better than 20% and temperature to within 10 K of the midcloud temperature, for clouds of thickness 2 km or less and optical depths between 0.8 and 4.

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S. Pawson, R. S. Hardwood, and J. D. Haigh

Abstract

Radiative dissipation coefficients for the observed, large-scale temperature waves in the middle atmosphere are presented and discussed. These have been calculated using LIMS measurements of the temperature, ozone, and water vapor distributions in a broadband radiative heating model. The total dissipation rate is determined by contributions due to thermal emission by the three gases considered and the absorption of solar radiation by ozone, which is important in the high stratosphere. The relative contribution of each gas to the total dissipation coefficient is discussed; this scales approximately with the contribution to the radiative balance of the atmosphere. In the winter hemisphere, the results are comparable with linearized estimates of the radiative dissipation coefficient, consistent with the deep vertical structure of the waves. Tropical dissipation rates are markedly different; in the middle and high stratosphere the analytical results of Fels are confirmed for the observed waves. Some evidence is tentatively presented for wave amplification by radiative processes in the low stratosphere, arising from absorption by the 9.6-μm bands of ozone.

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Andrew Orr, Thomas J. Bracegirdle, J. Scott Hosking, Thomas Jung, Joanna D. Haigh, Tony Phillips, and Wuhu Feng

Abstract

The authors report a hypothesis for the dynamical mechanisms responsible for the strengthening of the Southern Hemisphere circumpolar winds from the lower stratosphere to the surface due to the ozone hole. A general circulation model forced by stratospheric ozone depletion representative of the ozone hole period successfully reproduced these observed changes. Investigation of the dynamical characteristics of the model therefore provides some insight into the actual mechanisms. From this the authors suggest the following: 1) An initial (radiative) strengthening of the lower-stratospheric winds as a result of ozone depletion conditions the polar vortex so that fewer planetary waves propagate up from the troposphere, resulting in weaker planetary wave driving. 2) This causes further strengthening of the vortex, which results in an additional reduction in upward-propagating planetary waves and initiates a positive feedback mechanism in which the weaker wave driving and the associated strengthened winds are drawn downward to the tropopause. 3) In the troposphere the midlatitude jet shifts poleward in association with increases in the synoptic wave fluxes of heat and momentum, which are the result of a positive feedback mechanism consisting of two components: 4) increases in low-level baroclinicity, and the subsequent generation of baroclinic activity (associated with a poleward heat flux), are collocated with the jet latitudinal position, and 5) strengthening anticyclonic shear increases the refraction of wave activity equatorward (associated with a poleward momentum flux). Finally, 6) confinement of planetary waves in the high-latitude troposphere is an important step to couple the stratospheric changes to the tropospheric response.

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