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Julius London

Abstract

The average radiation flux and flux divergence in the atmosphere during March is computed as a function of the chief absorbents of terrestrial and solar radiation, and the physical parameters of the atmosphere such as pressure, temperature and cloudiness. The infra-red flux for water vapor is computed graphically from Elsasser radiation charts. The absorption of insolation by water vapor is computed with the aid of an empirical formula. The net radiation-flux divergence is then computed by methods of finite differences.

It is shown that the entire troposphere constitutes an energy source as far as radiative processes are concerned, and that at all latitudes the largest radiational heat loss is in the middle troposphere. It is also shown that the effect of cloudiness is to concentrate this maximum radiational cooling to layers at about the height of middle cloudiness (3–4 km). The net heat loss at this level is found to be about 1.5–2.0 C/day. It is suggested that the major contribution to heating the troposphere, to offset the loss due to radiation, is the release of latent heat by condensation.

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William Krasner and Julius London

A method is devised (based on the theory of Bjerknes and Holmboe on the deepening of cyclones), for the forecasting of the deepening of tropical cyclones. It is shown that the distribution of divergence around an easterlies trough (“easterly wave”), as revealed through consideration of the vorticity equation agrees with this application to the deepening of tropical cyclones. The conclusion is reached that an estward tilt of the pressure, or streamline, axis of a trough or developing tropical disturbance is most favorable for deepening; whereas, in deep easterlies, a westward tilt of the axis would result in filling and dissipation. A technique is also suggested for a quick qualitative evaluation of the direction of slope of the axis of such a pressure or streamline system.

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Takashi Sasamori and Julius London

Abstract

It is assumed that a small temperature perturbation is superimposed on a steady equilibrium temperature distribution in the atmosphere. The time dependent equation for radiative transfer in a plane parallel gray atmosphere is then obtained in a manner following Spiegel (1957) and is extended to the case of line and band absorption. The result shows that the strong absorption regions at the line centers play an important role in radiative damping when the perturbed layer is thin, and that the weak absorption regions between lines becomes important with increasing thickness of the perturbed layer.

It is shown that vertical temperature irregularities generated by adiabatic motion or advection may be smoothed by thermal radiation and the dissipation of these irregularities by radiative processes is comparable to vertical mixing by turbulence, if the scale of the perturbation is large.

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Xiu-De Ling and Julius London

Abstract

A one-dimensional model of the quasi-biennial oscillation (QBO) of ozone in the tropical middle stratosphere is derived based on assumed (observed) zonal wind QBO in a coupled dynamic, radiative/ photochemical system. It is found that the derived vertical variation of the ozone QBO amplitude has two. maxima, one at 32 km and the other at 22 km, and a minimum at 28 km. These are in qualitative agreement with observations. In the height interval 30–35 km, the ozone QBO is closely related to temperature dependent photochemistry and the ozone and temperature variations are out of phase. Below 28 km, where vertical ozone and thermal transports are important, ozone and temperature oscillations are in phase but both am approximately 270° out of phase with the vertical wind variation.

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Jae H. Park and Julius London

Abstract

The distributions of minor atmospheric constituents, principally O3 and O(3P), and their contributions to the heat sources and sinks in the earth’s middle atmosphere (30–100 km) are investigated. The latitudinal and seasonal distributions of radiative heating rates for the region 30–100 km are computed considering the absorption of solar UV by O2 and O3 and the chemical heat release by O(3P) recombination. Absorption of solar radiation by O3 is responsible for most of the radiative heating in the region 30–75 km. Between 75 and 90 km the heating rate is relatively small and is contributed to about equally by absorption by O2 and O3. Above 90 km the heating rate due to absorption by O O2 is of major importance, although non-equilibrium production of O(3P) in the summer results in reduced heating rates at these levels. At 100 km the “effective” heating rate is ∼40K day−1 at high latitudes during the summer. Recombination of O(3P) in the winter polar mesosphere and lower thermosphere results in a significant heat source for that region.

Meridional gradients of computed heating rates in the upper atmosphere are found to be generally larger than previous results. The implications of this distribution with regard to the dynamics of this region is discussed.

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Kenneth L. Pitchford and Julius London

Abstract

The significance of the low-level jet in the development of nocturnal thunderstorms over the Midwest through the production of regions of convergence was investigated for an area centered at Omaha, Nebraska. A composite jet axis representing all non-frontal days during the summer months of 1955, 1956 and 1957 was found to coincide with the line of maximum frequency of thunderstorm occurrence. Correlation between average kinematic vertical velocities and per cent thunderstorm occurrence over the area centered at Omaha supports the view that the occurrence of summer nocturnal thunderstorms is closely related to the production of regions of convergence associated with the low-level jet.

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William R. Kuhn and Julius London

Abstract

The infrared contributions to the heat budget by the 15 µ CO2, 9.6 µ O2, and 80 µ H2O bands are evaluated for the upper stratosphere, mesosphere and lower thermosphere as a function of latitude for both summer and winter. Flux divergences are numerically evaluated for a quasi-random band model with the appropriate line-broadening mechanism. A general discussion of the source function applicable to a multi-vibrational level molecule is given, and this formulation is applied to the 15 µ band of carbon dioxide.

The flux divergence of infrared radiation acts to cool the atmosphere in the 30–110 km height region except in the vicinity of the mesopause. Here there is a small, but nevertheless significant heating which increases in value toward the summer pole (∼4K day−1). Centers of cooling appear near the stratopause for low latitudes (∼10K day−1) and in the lower thermosphere over the winter pole. Thermospheric values may vary by a factor of 4 because of uncertainties in the collisional lifetime of the 15 µ transition; the rates of temperature change in this region have been parametrized in terms of the collisional and the radiative rates.

Ozone makes a significant contribution to the cooling in the vicinity of the stratopause (∼3K day−1). The water vapor contribution is approximately 1K day−1for a mixing ratio of 10−6 gm gm−1. Our calculations indicate that both these gases, when compared with carbon dioxide, give a negligible contribution to the flux divergence in the upper mesosphere.

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Capt., USAF Kenneth L. Pitchford and Julius London

Abstract

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Carole J. Hahn, Stephen G. Warren, and Julius London

Abstract

Visual observations of cloud cover are hindered at night due to inadequate illumination of the clouds. This usually leads to an underestimation of the average cloud cover at night, especially for the amounts of middle and high clouds, in climatologies based on surface observations. The diurnal cycles of cloud amounts, if based on all the surface observations, are therefore in error, but they can be obtained more accurately if the nighttime observations are screened to select those made under sufficient moonlight.

Ten years of nighttime weather observations from the Northern Hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6°, light from a partial moon at higher elevation, or twilight from the sun less than 9° below the horizon. It permits the use of about 38% of the observations made with the sun below the horizon.

The computed diurnal cycles of total cloud cover are altered considerably when this moonlight criterion is imposed. Maximum cloud cover over much of the ocean is new found to be at night or in the morning, whereas computations obtained without benefit of the moonlight criterion, as in our published atlases, showed the time of maximum to be noon or early afternoon in many regions. Cloud cover is greater at night than during the day over the open oceans fair from the continents, particularly in summer. However, near-noon maxima are still evident in the coastal regions, so that the global annual average oceanic cloud cover is still slightly greater during the day than at night by 0.3%. Over land, where daytime maxima are still obtained but with reduced amplitude, average cloud cover is 3.3% greater during the daytime. The diurnal cycles of total cloud cover we obtain are compared with those of ISCCP for a few regions; they are generally in better agreement if the moonlight criterion is imposed on the surface observations.

Using the moonlight criterion, we have analyzed 10 years (1982–91 ) of surface weather observations over land and ocean, worldwide, for total cloud cover and for the frequency of occurrence of clear sky, fog and precipitation. The global average cloud cover (average of day and night) is about 2% higher if the moonlight criterion is imposed than if all observations are used. The difference is greater in winter than in summer, because of the fewer hours of darkness in summer. The amplitude of the annual cycle of total cloud cover over the Arctic Ocean and at the South Pole is diminished by a few percent when the moonlight criterion is imposed.

The average cloud cover for 1982–91 is found to be 55% for Northern Hemisphere land, 53% for Southern Hemisphere land, 66% for Northern Hemisphere ocean, and 70% for Southern Hemisphere ocean, giving a global average of 64%. The global average for daytime is 64.6%; for nighttime 63.3%.

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Stephen G. Warren, Carole J. Hahn, and Julius London

Abstract

Cloud observations from land stations and from ships in the ocean are used to investigate the frequency of observation and the co-occurrence of different cloud types, and the geographical and seasonal variations of these co-occurrences. Ground-based observations are used because they provide a more definitive identification of clouds by type than do satellite observations. The clouds are grouped into six types (cirrus + cirrostratus + cirrocumulus, altostratus + altocumulus, stratus + stratocumulus, nimbostratus, cumulus, and cumulonimbus). The results are expressed as frequency of occurrence of different cloud types and as contingency probabilities; that is, given that one cloud type is present, the probability that another particular type is also present is computed. Several sources of bias are identified, and their effects on the results are estimated.

It is found that, on the average at all latitudes and in all seasons, clear skies occur more frequently, by a factor of about 4, over land than over the oceans; cumulus occurs twice as frequently over the oceans than over land but cirrus is reported with a somewhat higher frequency over land.

In general, cirrus and altostratus tend to occur together but altostratus and cumulus do not. The probability of co-occurrence of cirrus and cumulonimbus is much higher in the tropics 30°S–30°N than at mid- to subpolar latitudes. When cirrus or altostratus occurs over land, it is much more likely to be alone than when it occurs over ocean. Some of the reasons for these variations are discussed.

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