Julius London

Global distribution of total cloud cover and cloud type amounts over the ocean

This is the fourth of a series of atlases to result from a study of the global cloud distribution from ground-based observations. The first two atlases (NCAR/TN-201+STR and NCAR/TN-241+STR) described the frequency of occurrence of each cloud type and the co-occurrence of different types, but included no information about cloud amounts. The third atlas (NCAR/TN-273+STR) described, for the land areas of the earth, the average total cloud cover and the amounts of each cloud type, and their geographical, diurnal, seasonal, and interannual variations, as well as the average base heights of the low clouds. The present atlas does the same for the ocean areas of the earth.

Validation of the UARS solar ultraviolet irradiances: Comparison with the ATLAS 1 and 2 measurements

The measurements of the solar ultraviolet spectral irradiance made by the two Upper Atmosphere Research Satellite (UARS) solar instruments, Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and SOLar STellar Irradiance Comparison Experiment (SOLSTICE), are compared with same-day measurements by two solar instruments on the shuttle ATmospheric Laboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM and Shuttle Solar Backscatter UltraViolet (SSBUV) experiment. These measurements from the four instruments agree to within the 2σ uncertainty of any one instrument, which is 5 to 10% for all wavelengths above 160 nm and for strong emission features below 160 nm. Additionally, the long-term relative accuracy of the two UARS data sets is better than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement is credited to accurate preflight calibrations coupled with comprehensive inflight calibrations to track instrument degradation. Two solar irradiance spectra, 119 to 410 nm, are presented; the first combines observations from UARS SUSIM and UARS SOLSTICE taken on March 29, 1992, during the ATLAS 1 mission, and the second combines spectra for April 15, 1993, during the ATLAS 2 mission. The ATLAS 1 mission coincided with the initial decline from the maximum of solar cycle 22 when solar activity was relatively high. The ATLAS 2 mission occurred somewhat later during the declining phase of the solar cycle 22 when solar activity was more moderate.

The Effect of Moonlight on Observation of Cloud Cover at Night, and Application to Cloud Climatology

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 o...

Simultaneous Occurrence of Different Cloud Types

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...

Radiation Budget of the Southern Hemisphere

The earth as a planet receives virtually all of its energy from solar radiation. The incoming solar radiation at the top of the atmosphere is approximately 2.00 calories per minute through a 1-cm2 surface perpendicular to the solar beam when the earth is at mean distance from the sun (Johnson, 1954).4 The heating of the earth, however, is not uniform, primarily due to its spherical shape. Geographic variation of the earth’s albedo and the nonuniform absorptivity of the earth’s surface produce further modifications to the heating distribution. This horizontal differential heating is a potential source of energy to drive the circulation of the atmosphere and the ocean which transports heat from equatorial to polar regions and eventually adjusts the earth-atmosphere system toward an energy balance between the heating by insolation and the cooling by emission of terrestrial radiation. The manner in which radiative processes interact with the general circulation of the atmosphere has been investigated in a number of recent numerical studies (see, for instance, Manabe et al., 1965; Mintz, 1968; Manabe, 1969; Sellers, 1969; Washington and Kasahara, 1970; Kasahara and Washington, 1971) where the computed mean distribution of the radiation fluxes is usually specified. Most of these studies are based on radiation calculations developed from Northern Hemisphere climatology, although some applications to the Southern Hemisphere make use of the results of Gabites (1950).

The Low-Level Jet as Related to Nocturnal Thunderstorms over Midwest United States

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.

On the interpretation of seasonal variations of stratospheric ozone

Insight into the causes of the annual and semi-annual ozone oscillations may be gained from the analysis of photochemical model behavior. In this paper, the monthly variations of the ozone mixing ratio computed by the two-dimensional photochemical model of Garcia and Solomon (1983, J. geophys. Res. 88, 1379) are Fourier-analyzed and compared with SBUV observations of ozone mixing ratio. Remarkably good qualitative agreement between the model calculations and the observations is found. Analysis of computed transport and chemical production and destruction rates reveals the causes of the modelled seasonal ozone variations. It will be shown that at high latitudes and low altitudes, modelled ozone abundances increase in the winter due to transport and decrease in the summer due to chemical destruction. In the middle stratosphere, the calculated annual ozone variation is largely due to the annual variation in the odd-oxygen production rate, and in the upper stratosphere, the computed annual ozone variation is caused by the large calculated annual oscillation in temperature. Comparison between the model and observations suggests that the equatorial semi-annual oscillation above 10 mb is caused mainly by the semi-annual temperature and wind oscillation (SAO). Below 10 mb the computed equatorial ozone variation is caused by the increased rates of odd-oxygen production associated with the semi-annual zenith crossings of the Sun. Finally, the calculated polar semi-annual ozone oscillations are found to be caused by modulation of the radiatively driven middle-stratospheric ozone variation by temperature dependent chemical destruction processes.

Global Trends in Total Atmospheric Ozone

Analyses of the mean monthly global distributions of total ozone for the 13-year period from 1957 through 1970 reveal an upward trend of about 7.5 percent per decade in the Northern Hemisphere and about 2.5 percent per decade in the Southern Hemisphere. The increase seems to have started about March 1961 in the Northern Hemisphere and about September 1961 in the Southern Hemisphere. The cause of these trends is at present unknown.

Infrared Radiative Cooling in the Middle Atmosphere (30–110 km)

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 t...

The Quasi-biennial Oscillation of Ozone in the Tropical Middle Stratosphere: A One-Dimensional Model

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.