High latitude clouds

ABSTRACT. The current status of models of diffuse interstellar clouds is reviewed. A detmled comparison of recent gas-phase steady-state models shows that both the physical conditions and the molecular abundances in diffuse clouds are still not fully understood. Alternative mechanisms are discussed and observational tests which may discriminate between the various models are suggested. Recent developments regarding the velocity structure of diffuse clouds are mentioned. Similarities and differences between the chemistries in diffuse clouds and those in translucent and high latitude clouds are pointed out. 

TABLE 8. Computed Column Densities for High-Latitude Clouds"... 

Absorption line observations of high latitude clouds are complicated by the fact that almost no bright early -type stars are available as background light sources. However, with the improved sensitivity of the detectors, absorption line observations toward less... 

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

I use the seasonal simulation to explore the sensitivity of this energy balance climate model to such features of the climate system as permanent ice and snow at high latitudes, seasonal ice and snow, cloud cover, carbon dioxide amount, and the distribution of the continents. 

Sloan, L. C and D. Pollard, Polar Stratospheric Clouds A High Latitude Warming Mechanism in an Ancient Greenhouse World, Geophys. Res. Lett., 25, 3517-3520 (1998). 

It was shown by Weaver (2003) that the average annual radiative cooling of clouds in high latitudes has the same order of magnitude as the convergence of vortices-induced meridional heat flux, but of an opposite sign. Since there is a close correlation between CRF and storm track dynamics, we can suppose two ways for the impact of storm tracks dynamics on poleward heat transport ... 

Satellite observations, although lacking the resolution and sensitivity of other methods, give a global view of the phenomenon. Partial conclusions to be drawn from the satellites observations are the high altitude cloud took twenty one days to first circle the Earth moving from East to West its maximum density occurred about eight weeks after the eruption. After the first week in June the aerosol completely encircled the Earth in the latitude band from the Equator to 30 N. 

These modeling results indicate that direct photooxidation combined with photochemically-stimulated microbial degradation can potentially consume about 850 X 10 g C per year in coastal areas. The results indicate that even high-latitude coastal DOC is subject to major UV-induced oxidation in the latitudes of its input to the ocean. These likely are overestimates of the role of photochemistry in oxidizing terrestrially-derived organic matter. As noted earlier, these considerations apply only to the photoreactive component of the DOM (i.e., the CDOM). As noted above, riverine inputs of DOM appear to include a substantial non-reactive component. Estimates for the open ocean are clouded at this point by the previously-discussed findings that bluewater CDOM may be less photoreactive than terrestrially-derived CDOM and much less efficient in the net photoproduction of BLPs. 

The transition between the mesosphere and the thermosphere, called the mesopause, is characterized by a temperature minimum whose location varies significantly with latitude and season (see Figure 3.32). In the tropics during all seasons, and in the winter mid- and high-latitudes, this transition takes place at approximately 100 km altitude. At the winter pole, the mesopause temperature is typically 180-200 K. At mid- and high latitudes during summer, the mesopause is located near 85 km, and the temperature at the pole reaches values as low as 120-140 K (coldest place in the Earth s atmosphere where polar mesospheric clouds are occasionally observed). 

The most abundant ions generally display hydration orders from 2 to 4, but at cold temperatures near the mesopause hydration orders of 8 or 9 are not uncommon. Bjorn and Arnold (1981) observed ions as large as H+(H20)2o near a very cold summer mesopause at high latitude. These authors also suggest that such ions may be important in the formation of noctilucent clouds. 

In the late 1980s, aircraft were flown into the austral and boreal polar stratosphere to investigate the depletion of lower stratospheric ozone at high latitudes. The aircraft measurements revealed that stratospheric air pole-ward of the wintertime zonal-mean jet core is highly isolated from midlatitude air during winter. This isolation, combined with the cold temperatures and subsequent polar stratospheric cloud appearance, allows the chemical environment to become highly perturbed. Stratospheric temperatures below about 195 K at 20 km are required for... 

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