K. W. Hoppel

Intensity and phase statistics of multilook polarimetric and interferometric SAR imagery

Polarimetric and interferometric SAR data are frequently multilook processed for speckle reduction and data compression. The statistical characteristics of multilook data are quite different from those of single-look data. The authors investigate the statistics of their intensity and phase. Probability density function (PDFs) of the multilook phase difference, magnitude of complex product, and intensity and amplitude ratios between two components of the scattering matrix are derived, and expressed in closed forms. The PDFs depend on the complex correlation coefficient and the number of looks. Comparisons of these theoretically derived PDFs are made to measurements from NASA/JPL AIRSAR data. The results of this paper can be applied to feature classification using polarimetric SAR and to the estimation of decorrelation effects of the interferometric SAR. >

Observations of boreal forest fire smoke in the stratosphere by POAM III, SAGE II, and lidar in 1998

A substantial increase in stratospheric aerosol was recorded between May and October 1998 between 55° and 70°N. This phenomenon was recorded in the absence of reported volcanic eruptions with stratospheric impact potential. The POAM III and SAGE II instruments made numerous measurements of layers of enhanced aerosol extinction substantially higher than typical values 3 to 5 km above the tropopause. A comparison of these observations with lidar profiles, TOMS aerosol index data, and forest fire statistics reveals a strong link between stratospheric aerosol and forest fire smoke. Our analysis strongly suggests that smoke from boreal forest fires was lofted across the tropopause in substantial amounts in several episodes occurring in Canada and eastern Russia. Observations reveal a broad zonal increase in stratospheric aerosol that persisted for at least three months.

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Polar Ozone and Aerosol Measurement (POAM) II stratospheric NO2, 1993–1996

We describe, for the first time, measurements of stratospheric nitrogen dioxide (NO 2 ) by the Polar Ozone and Aerosol Measurement (POAM II) instrument. Measurements span October 1993 through mid-November 1996 and cover latitude ranges from 55° to 72°N and from 63 to 88°S. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAM II measurements for scientific investigations. Overall seasonal variations in both hemispheres are qualitatively consistent with standard photochemistry. In the austral late winter/early spring of 1994, however, anomalously high NO 2 mixing ratios were observed above 22 km. We conclude that these high NO 2 levels resulted from downward transport of NO x -enhanced air from the mesosphere or thermosphere inside the polar vortex. Enhanced NO 2 mixing ratios in 1994 were factors of ∼1.3 and 2.5 larger than at the corresponding times in 1995 and 1996. We conclude that POAM II observations of coincident, localized reductions in ozone of up to 40% were caused by the increased stratospheric NO via the standard catalytic NO cycle.

Principal components transformation of multifrequency polarimetric SAR imagery

A generalized principal components transform (PCT) that maximizes the signal-to-noise ratio (SNR) and that tailors to the multiplicative speckle noise characteristics of polarimetric SAR images is developed. An implementation procedure that accurately estimates the signal and the noise covariance matrices is established. The properties of the eigenvalues and eigenvectors are investigated, revealing that the eigenvectors are not orthogonal, but the principal component images are statistically uncorrelated. Both amplitude (or intensity) and phase difference images are included for the PCT computation. The NASA/JPL polarimetric SAR imagery of P, L, and C bands and quadpolarizations is used for illustration. The capabilities of this principal components transformation in information compression and speckle reduction makes automated image segmentation and better human interpretation possible. >

On the unexplained stratospheric ozone losses during cold Arctic Januaries

Using a combination of data from Match, POAM II, POAM III and MLS we show that the chemical loss rate of Arctic O3 during January of four cold winters (1992, 1995, 1996, and 2000) is consistently faster than can be accounted for by assuming complete activation of reactive chlorine and standard reaction kinetics. However, O3 loss rates measured during late February and early March 1996 are shown to be consistent with observations of ClO. The faster than expected O3 loss rates during January are shown to occur when air parcels are illuminated at high solar zenith angles (SZAs between ~85 and 94°), and to result in cumulative O3 loss of ~0.5 ppmv. The cause of the rapid January O3 loss is unclear, but may be related to a photolytic process at high SZA that is poorly represented by current photochemical models.

POAM III measurements of dehydration in the Antarctic lower stratosphere

We present measurements of stratospheric water vapor and aerosols in the Antarctic from the POAM III instrument during the period April through December 1998. The measured variations in water vapor enable us to study both descent in the vortex and the effect of dehydration that occurs in the lower stratosphere below ∼23 km when the temperature drops below the frost point in July. There is a temporal correlation between the dehydration that occurs in July and an increase in high aerosol optical depth events in the lower stratosphere, suggesting that these events are due to the presence of ice PSCs. When temperatures warm up there is some rehydration at the highest altitudes of the dehydrated region (∼20–23 km), probably resulting from descent within the vortex. At ∼12 km rehydration is probably the result of mixing in of air from outside the vortex. The temperature increase in October produces little rehydration at 17 km and no clear rehydration at 14 km, suggesting that the water has precipitated out of these layers.

An analysis of Polar Ozone and Aerosol Measurement (POAM) II Arctic polar stratospheric cloud observations, 1993–1996

The Polar Ozone and Aerosol Measurement (POAM) II instrument made numerous observations of polar stratospheric clouds (PSCs) in the northern hemisphere winters of 1993/1994 through 1995/1996. An updated POAM II PSC detection algorithm, described herein, is applied to POAM II 1060 nm aerosol extinction profiles to distinguish PSCs from noncloud measurements. The impact of the updated algorithm on previously published Antarctic PSC statistics is discussed. Operating sporadically in the 1993/1994 PSC season (defined as November through April), but continuously in the following two winters, POAM II made approximately 340 PSC profile measurements. We analyze the POAM II PSC observations with respect to polar vortex location, temperature, longitude, time, and altitude. Daily PSC probability (defined as the number of PSC profiles inside the polar vortex relative to the number of all profiles located inside the vortex) exceeds 60% during the most intense PSC episodes. There is considerable year-to-year variability in PSC probability, preferred location, and timing of onset and final appearance. The POAM II data also reveal interannual differences in the seasonal change of PSC altitude. Certain observations of opaque clouds are used to infer Type II PSCs. The pattern of observed PSCs is discussed with respect to recent studies of Arctic ozone loss.

Unsupervised estimation of speckle noise in radar images

Speckle in radar images has the characteristic of a multiplicative noise. In this article, two unsupervised methods are introduced to estimate the speckle noise statistics using the mean and the standard deviation of small image blocks (4 × 4, or 6 × 6 pixels). Since most radar images contain many small homogeneous areas, a scatter plot of the standard deviation versus the mean can reveal the characteristic of the noise. The blocks from inhomogeneous areas have higher values for the standard deviation, and they are scattered above the main cluster. They are considered as outliers, and should be excluded in the statistical estimation. These two methods are designed for obtaining a linear fit in the scatter plot by ignoring outliers. Several synthetic aperture radar(SAR) images are used for illustration.

Statistics of phase difference and product magnitude of multi-look processed Gaussian signals

The phase difference and the product of two complex Gaussian signals are important parameters in the study of interferometry and polarimetry. To reduce statistical variations, polarimetric and inte...