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Glenn K. Yue, M. P. McCormick, and W. P. Chu

Abstract

The SAGE II satellite system was launched on 5 October 1984. It has seven radiometric channels and is beginning to provide water vapor, NO2, and O2 concentration profiles and aerosol extinction profiles at a minimum of three wavelengths. In preparation for the use of these data and assuming the aerosol size distribution is lognormal, a parametric study has been conducted of the effect of size distribution parameters and composition of stratospheric aerosols on the expected extinction in the aerosol and water vapor channels. Our calculated results show that aerosol characteristics can be retrieved from the SAGE II data set assuming stratospheric aerosols are distributed lognormally. A simple, fast and operational method of retrieving characteristics of stratospheric aerosols from the water vapor and three-wavelength aerosol extinction profiles is proposed. Some examples are given to show the practicality of the scheme. Possible sources of error for the retrieved values and the limitation of the proposed method are discussed. Our method may also prove applicable to the study of aerosol characteristics in other multispectral extinction measurements.

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Pi-Huan Wang, M. P. McCormick, and W. P. Chu

Abstract

Ozone data from the Stratospheric Aerosol and Gas Experiment (SAGE) have been used in conjunction with meteorological information to study the ozone transport near 55°N due to planetary waves during the late February 1979 stratospheric warming. The results indicate an intense poleward eddy ozone transport in the middle stratosphere between ∼24 and 38 km altitudes and an equatorward transport above an altitude of ∼38 km. It is found that this equatorward eddy ozone transport in the upper stratosphere was accompanied by a poleward eddy heat transport, as expected on the basis of the ozone photochemistry. The results also show an equatorward eddy ozone transport in the lower stratosphere (below ∼25 km), but it is secondary. The transport effect of wavenumber 2 can account for a major portion of the net eddy ozone flux during this late February 1979 warming.

The phase relationship between temperature, meridional velocity and ozone mixing ratio waves has also been examined. Overall, the results agree qualitatively with existing model analyses. In the lower stratosphere, the temperature and ozone waves are found to be nearly in-phase. They are approximately out-of-phase in the upper stratosphere. A transition region is shown in between. However, this transition region is thinner and centered at a slightly lower altitude than that predicted in the model analyses of Hartmann and Garcia, Kawahira and those reported by Gille et al. The reason for this difference is discussed.

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G. S. Kent, L. R. Poole, and M. P. McCormick

Abstract

Airborne lidar measurements of backscattering at 0.6943 μm from polar stratospheric clouds, made in January 1984, are reported. The clouds, whose altitudes and geographical locations coincided with ambient atmospheric temperatures below about 193 K, were observed to cover a greater area of the polar cap than had previously been apparent from satellite measurements. They were seen on three separate flights north of Thule, Greenland (76.5°N, 68.7°W), on one occasion extending continuously from approximately 80°N to the North Pole. Pronounced layering of the clouds was observed and the maximum backscatter enhancement, relative to that from the background aerosol, was between 100 and 200. These values occurred at an altitude of about 20 km, close to the region of minimum stratospheric temperature. Depolarization of the order of 20–50% in the backscattered signal was measured, in support of the hypothesis that the aerosols forming the clouds are frozen. Comparison of the experimentally determined backscattering-temperature relationship with a theoretical model, based on a volcanic aerosol and using best available estimates for water vapor concentration, shows good agreement at the 100- and 70-mb pressure levels. A small systematic error at the 50- and 30-mb levels may be due to inaccurate characterization of the temperature field at these altitudes and locations.

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A. Ghazi, Pi-Huan Wang, and M. P. McCormick

Abstract

Satellite ozone observations made by the Stratospheric Aerossol and Gas Experiment (SAGE) and corresponding meteorological temperature data are used to study the radiative damping processes associated with planetary waves during stratospheric warmings. Ramanathan's model has been adapted fox. the radiative heating and cooling calculations. The derived infrared damping coefficients, based on observed stratospheric ozone and temperature, are compared with the Newtonian cooling coefficients of Dickinson and Fels. It is also shown that the negative correlation between temperature and ozone solar heating in the upper stratosphere accelerates the damping rate due to infrared cooling alone, in agreement with the theoretical analysis and an earlier report based on observations. In addition, it is also found in this analysis that the phase relationship between ozone and solar heating waves is characterized by its behavior in three distinct layers. In the regions above about 2 mb and also below about 10 mb, the waves are closely in-phase. Between approximately 2 and 10 mb, they show a departure from the in-phase relationship which can be attributed to the so-called “opacity effect.” This effect significantly determines the magnitude of radiative damping.

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T. J. Swissler, P. Hamill, M. Osborn, P. B. Russell, and M. P. McCormick

Abstract

We compare a series of 85 dustsonde measurements and 84 lidar measurements made in midlatitude North America during 1974–80. This period includes two major volcanic increases (Fuego in 1974 and St. Helens in 1980), as well as an unusually clean, or background, period in 1978–79. An optical modeling technique is used to relate the dustsonde-number data to the lidar-backscatter data. The model includes a range of refractive indices and of size distribution functional forms, to show its sensitivity to these factors. Moreover, two parameters of each size distribution function are adjustable, so that each distribution can be matched to any two-channel dustsonde measurement.

We show how the mean particle radius for backscatter, rB, changes in response to size distribution changes revealed by the dustsonde channel ratio, N r>0.15/N r>0.25. (N r>x is the number of particles with radius larger than x microns.) In early 1975, just after the Fuego injection, N r>0.15/N r>0.25 was ∼3, and the corresponding rB, was ∼0.5 μm; by early 1980, when N r>0.15/N r>0.25 had increased to eight or larger, rB had correspondingly decreased to ∼0.25 μm. Throughout the 1975–76 Fuego decay, rB always exceeded 0.3 μm; thus, lidar backscatter was influenced primarily by particles larger than those that contribute most to N r>0.15 and N r>0.25. This is in accord with the shorter lidar background-corrected, 1/e decay time: 7.4 months, versus 10.4 and 7.9 months for N r>0.15 and N r>0.25.

The modeling technique is used to derive a time series of dustsonde-inferred peak backscatter mixing ratio, which agrees very well with the lidar-measured series. The best overall agreement for 1974–80 is achieved with a mixture of refractive indices corresponding to aqueous sulfuric acid at about 210 K with an acid-weight fraction between 0.6 and 0.85.

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M. P. McCormick, H. M. Steele, Patrick Hamill, W. P. Chu, and T. J. Swissler

Abstract

Sightings of polar stratospheric clouds (PSC's) by the SAM II satellite system during the northern and southern winters of 1979 are reported. PSC's were observed in the Arctic stratosphere at altitudes between about 17 and 25 km during January 1979, with a single sighting in November 1978, and in the Antarctic stratosphere from June to October 1979 at altitudes from the tropopause up to about 23 km. The measured extinction coefficients at 1 μm wavelength were as much as two orders of magnitude greater than that of the background stratospheric aerosol. with peak extinctions up to 10−2 km−1. The PSC's were observed when stratospheric temperatures were very low with a high probability of observation when temperatures were colder than 190 K and a low probability when temperatures were warmer than 198 K. In the Antarctic, clouds were observed in more than 90% of the events in which the minimum temperature was 185 K or less, and were observed in fewer than 10% of the occasions when the temperature was greater than 196 K.

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G. S. Kent, E. R. Williams, P-H. Wang, M. P. McCormick, and K. M. Skeens

Abstract

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5–18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Niño-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the South American and Central African landmasses, where the percentage of cloud occurrence in the upper troposphere can exceed 75%. Cloud occurrence at all altitudes within the Tropics, over both land and ocean, increases with the underlying surface temperature at a rate of approximately 13%°C−1. Extrapolated threshold temperatures for the formation of cloud are about 5°C lower than those found from nadir viewing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. ENSO cycle changes in cloud occurrence are observed, not only over the Tropics but also over the subtropics, indicating a difference in the meridional Hadley circulation between ENSO warm and cold years. Sunrise–sunset cloud differences indicate that large-scale variations, whose form resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-tropospheric ice and its positive correlation with surface temperature on all timescales are generally consistent with the behavior of lightning and the global electrical circuit.

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M. P. McCormick, T. J. Swissler, W. P. Chu, and W. H. Fuller Jr.

Abstract

Liday observations of the stratosphere aerosol vertical distribution from October 1971 to July 1976 over midlatitude North America are presented. The results show the sudden increase in the stratospheric aerosol content after the eruption of the Volc´n de Fuego and its subsequent decline. The data are presented in terms of lidar scattering ratio profiles, vertically integrated aerosol backscattering, and rawinsonde temperature profiles. In the months immediately following the volcanic eruption, the lidar-derived aerosol structure is correlated with rawinsonde temperature structure showing the stratospheric temperature minimum occurring near the aerosol layer peak. Analysis of the time dependence of the integrated aerosol backscattering and the tropopause altitude indicates an approximate 0.9 correlation between aerosol loading and tropopause pressure. In addition, the integrated aerosol backscattering also showed some correlation with the minimum stratospheric temperature, i.e., a warmer stratospheric minimum is associated with a relatively higher aerosol loading.

The lidar backscatter data also show that rapid decay of the stratospheric aerosol occurred over the late winter to early spring period and that the summer to fall interval was quite stable. For both winter to summer periods of 1975 and 1976 in approximate 40% decrease in the total integrated aerosol backscattering was observed, while from January 1975 to January 1976 a 65% decrease occurred. For the 19-month period from January 1975 to July 1976 the exponential l/e decay time for the integrated aerosol backscattering was 11.6 months.

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H. M. Steele, Patrick Hamill, M. P. McCormick, and T. J. Swissler

Abstract

Measurements of the stratospheric aerosol by SAM II during the northern and southern winters of 1979 showed a pronounced increase in extinction on occasions when the temperature fell to a low value (below 200 K). In this paper we evaluate, from thermodynamic considerations, the correlation between extinction and temperature. As the temperature fails, the hygroscopic aerosols absorb water vapor from the atmosphere, growing as they do so. The effect of the temperature on the size distribution and composition of the aerosol is determined, and the optical extinction at 1 μm wavelength is calculated using Mie scattering theory. The theoretical predictions of the change in extinction with temperature and humidity am compared with the SAM II results at 100 mb, and the water vapor mixing ratio and aerosol number density are inferred from these results. A best fit of the theoretical curves to the SAM II data gives a water vapor content of 5–6 ppmv, and a total particle number density of 6–7 particles cm−3.

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P. B. Russell, J. M. Livingston, T. J. Swissler, M. P. McCormick, W. P. Chu, and T. J. Pepin

Abstract

We present a model of stratospheric aerosol optical properties (refractive index and relative size distribution) and their variability. The model's purposes are 1) providing flexible, efficient means for converting between different aerosol macroproperties (e.g., number or mass concentration, extinction or backscatter coefficient), and 2) quantifying the uncertainties in the conversion process. The latter purpose is achieved by including the results of a sensitivity analysis in the model output products.

The model has three layers, the boundaries of which are defined by tropopause height. Each layer includes a set of empirically based refractive indices and relative size distribution types. In contrast to previous models, this model allows for a range of sulfuric acid and ammonium sulfate refractive indices within the “inner stratospheric” layer (∼2 to 20 km above the tropopause, where the major peak in aerosol mixing ratio occurs). We show that nine different analytical types of size distribution previously recommended for this layer can be parameterized in terms of channel ratio—i.e., the relative size distribution indicator that has been extensively measured by dustsondes.

When so parameterized, all nine inner stratospheric function types give very similar results for the several conversion ratios of interest. This parameterization allows considerable saving of computer time while preserving the flexibility to handle certain types of size distribution change. We show that the inner stratospheric parameterization works because all nine inner stratospheric size distribution types are relatively narrow, and their optical integrals of interest are determined primarily by a size range that is well characterized by channel ratio.

Data from previous measurements made near the tropopause are used to demonstrate that, in that region, size distributions are broader than any of the inner stratospheric types, and that their optical integrals are strongly influenced by particles too large to be characterized by channel ratio. Hence, in the layer near the tropopause, conversion ratios can differ significantly from the inner stratospheric values; consequently, parameterization by channel ratios is not successful.

We develop methods for deriving vertical profiles of several conversion ratios and their uncertainties. We also demonstrate an application of the model: deriving profiles of number density and its uncertainty from satellite-measured profiles of extinction and its uncertainty. A companion paper applies the model to the task of validating satellite measurements of stratospheric aerosol extinction.

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