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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, Wuhu Feng, Howard K. Roscoe, and Joanna D. Haigh

Abstract

A model simulation forced by prescribed ozone depletion shows strong dynamical modulation of the springtime cooling of the polar stratosphere associated with the Antarctic ozone hole. The authors find that in late spring the anomalous radiative cooling in response to ozone depletion is almost canceled above ~100 hPa by an increase in dynamical heating. Between ~300 and ~100 hPa, however, it is enhanced by a reduction in dynamical heating, resulting in the descent of the cold anomaly down to the tropopause. In early summer increased dynamical heating dominates as the radiative cooling diminishes so that the cold anomaly associated with the delayed breakup of the stratospheric vortex is reduced. The anomalous dynamical heating is driven by changes in the Brewer–Dobson circulation arising primarily from the dissipation of resolved-scale waves. The model changes are broadly consistent with trends from reanalysis and offline diagnoses of heating rates using a radiation scheme. These results help one to understand dynamically induced change in the evolution and timing of the stratospheric vortex in recent decades and will help to enable improved simulation of the Southern Hemisphere climate.

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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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Kelly Helm Smith, Mark E. Burbach, Michael J. Hayes, Patrick E. Guinan, Andrew J. Tyre, Brian Fuchs, Tonya Haigh, and Mark D. Svoboda

Abstract

Drought-related decision-making and policy should go beyond numeric hydrometeorological data to incorporate information on how drought affects people, livelihoods, and ecosystems. The effects of drought are nested within environmental and human systems, and relevant data may not exist in readily accessible form. For example, drought may reduce forage growth, compounded by both late-season freezes and management decisions. An effort to gather crowdsourced drought observations in Missouri in 2018 yielded a much higher number of observations than did previous related efforts. Here we examine 1) the interests, circumstances, history, and recruitment messaging that coincided to produce a high number of reports in a short time; 2) whether and how information from volunteer observers was useful to state decision-makers and to U.S. Drought Monitor (USDM) authors; and 3) potential for complementary use of stakeholder and citizen science reports in assessing trustworthiness of volunteer-provided information. State officials and the Cattlemen’s Association made requests for reports, clearly linked to improving the accuracy of the USDM and the related financial benefit. Well-timed requests provided a focus for people’s energy and a reason to invest their time. State officials made use of the dense spatial coverage that observers provided. USDM authors were very cautious about a surge of reports coinciding closely with financial incentives linked to the Livestock Forage Disaster program. An after-the-fact comparison between stakeholder reports and parallel citizen science reports suggests that the two could be complementary, with potential for developing protocols to facilitate real-time use.

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