Abstract

The Nimbus 6 satellite Earth Radiation Budget (ERB) experiment has continuously monitored the solar radiation input and the reflected shortwave and emitted longwave radiation exitance from the earth-atmosphere system since July 1975. In this paper, the planetary radiation budget parameters observed during the first eighteen months in orbit (July 1975–December 1976) are presented. The results show that the annual mean planetary albedo and longwave radiation flux are 31% and 234 W m^−2> (radiative equilibrium temperature of 254 K), respectively. The earth atmosphere system is observed to be in complete radiation balance over a one-year period to within the experimental error of observation. There is an annual cycle of the mean monthly planetary net radiation which is due predominantly to the annual cycle of incoming solar radiation caused by the time variation of earth-sun distance and the sun's declination. Monthly variations in outgoing longwave radiation due to variation in global cloudiness and snow and ice cover are generally compensated by the simultaneous variations in the planetary albedo so that there is generally little monthly variability of the total radiation to space compared to that of the net radiation.

Abstract

A statistical histogram method is developed to objectively determine sea-surface temperature from satellite high resolution window radiation measurements. The method involves inferring the distribution of surface radiances for the clear atmospheric case from observed histograms of generally cloud-contaminated radiances. The brightness temperature associated with the clear atmosphere modal peak radiance is the statistically most probable surface temperature. The reliability of the inferred surface temperature depends upon the number of cloud-free measurements available to define the clear mode. The method accounts for atmospheric attenuation and instrumental noise and also objectively discriminates cloud-free from cloud-contaminated observations.

The statistical histogram method is applied to 3.8 micrometer window radiation data obtained by the High Resolution Infrared Radiometer flown on the Nimbus 2 and Nimbus 3 satellites. Examples of sea temperatures inferred over both small and large areas are presented. Comparisons with conventional ship observations indicate that both bias and random errors of the inferred sea temperatures are less than 1°C.

Due to the apparent success of this statistical histogram technique, plans have been made to use it to obtain sea-surface temperatures on a global basis daily from operational high resolution infrared radiation measurements.

Abstract

Precipitation particle sizes were measured using a continuous hydrometeor sampler (foil impactor) during penetrations of hailstorms with an armored T-28 aircraft. Data have been analyzed from three penetrations of a storm near Raymer, Colorado, on 9 July 1973 at altitudes between 5.5 and 7.2 km MSL, which correspond to temperatures between about −2°C and −12°C. Other results pertinent to the Raymer storm are discussed in Parts I,II,III and elsewhere in this issue.

Most of the particles were identified as ice particles or ones containing both ice and water; however, significant amounts of liquid particles were found in the updrafts of developing cells at temperatures as cold as −12°C. Particles larger than 5 mm in diameter were typically found along the edges of the updrafts, with the precipitation concentrations being strongly dependent on these larger particles. The downdrafts were composed of ice particles.

Several particle size distributions from one of the penetrations were examined. The distributions are roughly exponential, or bi-exponential when large particles are present.

Abstract

A powerful facility for meteorological analysis called the Man Computer Interactive Data Access System (McIDAS) was designed and implemented in the early 1970's at the Space Science and Engineering Center of the University of Wisconsin-Madison. Hardware and software experience gained via extensive use of that facility and its derivatives have led to a newer implementation of McIDAS on a larger computer with significant enhancements to the supporting McIDAS software. McIDAS allows remote and local access to a wide range of data from satellites and conventional observations, time lapse displays of imagery data, overlaid graphics. and current and past meteorological data. Available software allows one to perform analysis of a wide range of digital images as well as temperature and moisture sounding data obtained from satellites. McIDAS can generate multicolor composites of conventional and satellite weather data, radar and forecast data in a wide variety of two- and three-dimensional displays as well as time lapse movies of these analyses. These and other capabilities are described in this paper.

Abstract

The daily distribution of sulfate concentration over the eastern United States during August 1977 is simulated by a Monte Carlo model using quantized emissions, positioned in accordance with the 1973 EPA SO₂ emission inventory. Horizontal advection within a single well-mixed vertical layer is driven by observed surface winds, speeded by a factor of 2.5 and veered 20°. Direct simulation of regional diffusion is implemented by random perturbation of each quantum's trajectory over each 3 h time step, corresponding to K = 10⁵m² s⁻¹. First order kinetics of SO₂ to SO₄ ⁻ transformation and deposition of SO₂ and SO₄ ⁻ are simulated by probabilistic choice among the chemical and physical pathways over each step. The simulated spatial distributions are compared on a daily basis to measured sulfate concentration from the Sulfate Regional Experiment (SURE) and midday visual range observations, indicating the primary role of airmass residence time over the eastern United States in producing regional pollution. The light extinction coefficient, b _ext and SURE SO₄ ⁻ show remarkable spatial and temporal agreement throughout the month. The correlations of daily average SO₄ ⁻ concentration and b _ext over the SURE region with the Monte Carlo simulation are 0.63 and 0.67, respectively. The correlation between daily average b _ext and measured SO₄ ⁻ is 0.84.

Abstract

A precipitation retrieval algorithm based on the application of a 3D radiative transfer model to a hybrid physical-stochastic 3D cloud model is described. The cloud model uses a statistical rainfall clustering scheme to generate 3D cloud structure while ensuring that the stochastically generated quantities remain physically plausible. The radiative transfer model is applied to the cloud structures to simulate satellite remotely sensed upwelling microwave brightness temperatures T_B 's. Regression-derived relationships between model T_B 's and surface rainfall rates for Special Sensor Microwave/Imager (SSM/I) frequencies are used as the foundation of the retrieval algorithm, which is valid over oceans. A case study calibrates the retrieval algorithm to the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather prediction model and applies the algorithm to SSM/I data obtained during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. Comparisons between the satellite-derived precipitation amounts and radar-derived amounts, at a spatial resolution of approximately 55 km, give correlations of about 0.7 for instantaneous rain rates and 0.634 for monthly accumulations. Although the satellite-derived totals are reasonably well correlated with the radar totals, they also appear to contain a relatively large positive bias, which may in part be due to the ECMWF tuning. However, optical rain gauge measurements are lager than both the satellite- and radar-derived amounts, casting uncertainty into the level of bias of the satellite algorithm. Finally, an important aspect of 3D radiative transfer in precipitating systems is illustrated by demonstrating that satellite viewing angle effects realized in the simulation framework also appear to be present in empirical relations between SSM/I T_B 's and radar-derived surface rainfall rates.

Abstract

In the winter of 1986, two microwave radiometers were operated side by side at a high-altitude weather observation site in the central Sierra Nevada for the purpose of comparing measurements in a variety of ambient weather conditions. The instruments continuously recorded measurements of vertically integrated water vapor and liquid water during storms affecting the area. One radiometer was designed with a spinning reflector to shed precipitation particles while the other radiometer's reflector was fixed. Temporal records of the data show periods of wet weather contamination for the fixed reflector radiometer. The absence (presence) of these contaminated periods is mainly explained by the difference in the design of the radiometers. These contaminated periods led to larger standard deviation in the data from the fixed-reflector radiometer and lower correlation coefficients between the two instruments. Correlation coefficients of 0.83 for the liquid and 0.68 for the vapor values were found for the radiometer-radiometer comparisons. When some of the points suspected of contamination were removed, the correlation coefficients improved to 0.87 and 0.71 for the liquid and vapor channels, respectively. The standard deviations were 0.1 mm and 0.12 cm for the liquid and vapor channels, respectively, of the spinning reflector radiometer. For the fixed-reflector design radiometer, a standard deviation of 0.1 mm for the liquid and 0.26 cm for the vapor was found. Comparison of radiometer vapor and rawinsonde precipitable water resulted in a correlation coefficient of 0.97 for the spinning-reflector radiometer and 0.8 for the fixed-reflector radiometer.

Abstract

Knowledge of cirrus cloud optical depths is necessary to understand the earth’s current climate and to model the cloud radiation impact on future climate. Cirrus clouds, depending on the ratio of their shortwave “visible” to longwave “infrared” optical depth, can act to either cool or warm the planet. In this study, visible-to-infrared cirrus cloud optical depth ratios were measured using ground-based lidar and Fourier transform spectrometry. A radiosonde temperature profile combined with the 0.532-μm-high spectral resolution lidar vertical cloud optical depth profile provided an effective weighting to the cloud radiance measured by the interferometer. This allowed evaluation of cirrus cloud optical depths in 18 infrared microwindows between water vapor absorption lines within the 800–1200-cm⁻¹ infrared atmospheric window. The data analysis was performed near the peak solar and terrestrial emission regions, which represent the effective radiative cloud forcing efficiency of the given cloud sample. Results are also presented that demonstrate the measurement of infrared optical depth using an assumed uniform cloud extinction cross section, which requires generic lidar cloud boundary data. The measured cloud extinction profile provided a more robust solution that would allow analysis of multiple-layer clouds and removed the uniform cloud extinction cross-section assumption. Mie calculations for ice particles were used to generate visible and infrared extinction coefficients; these were compared against the measured visible-to-infrared optical depth ratios. The results demonstrate strong particle size and shape sensitivity across the infrared atmospheric window.

Abstract

Lidar and high spectral resolution infrared radiance observations taken on board the ER-2 on 28 October 1986 are used to study the radiative properties of cirrus cloud in the 8–12 μm window region. Measurements from the High-spectral resolution Interferometer Sounder (HIS) indicate that the spectral variation of the equivalent blackbody temperature across the window can be greater than 5°C for a given cirrus cloud. This difference is attributed to the presence of small particles.

A method for detecting cirrus clouds using 8 μm, 11 μm, and 12 μm bands is presented. The 8 μm band is centered on a weak water-vapor absorption line while the 11 μm and 12 μm bands are between absorption lines. The brightness temperature difference between the 8 and 11 μm bands is negative for clear regions, while for ice clouds it is positive. Differences in the 11 and 12 μm channels are positive, whether viewing a cirrus cloud or a clear region. Inclusion of the 8 μm channel therefore removes the ambiguity associated with the use of 11 and 12 μm channels alone. The method is based on the comparison of brightness temperatures observed in these three channels.

The HIS and lidar observations were combined to derive the spectral effective beam emissivity (ε) of the cirrus clouds. Fifty percent of clouds on this day displayed a spectral variation of ε from 2–10%. These differences, in conjunction with large differences in the HIS observed brightness temperatures, indicate that cirrus clouds cannot be considered gray in the 8–12 μm window region.

The derived spectral transmittance of the cloud is used to infer the effective radii of the particle size distribution, assuming ice spheres. For 28 October 1986 the effective radius of cirrus cloud particle size distribution (r _eff) was generally within the 30–40 μm range with 8% of the cases where 10 < r _eff < 30 μm and 12% of the cases corresponding to r _ref > 40 μm.

Abstract

Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include:

(i) the multiple-line nature of the frontal transition zone (FTZ);

(ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and

(iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere.

The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds.

The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance.

The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.