The nonresonant interference effects in the (3,3) channel proposed by Olsson are incorporated into a full amplitude that describes photopion production from a single nucleon. The reactions $p(\ensuremath{\gamma}, {\ensuremath{\pi}}^{+})n$, $n(\ensuremath{\gamma}, {\ensuremath{\pi}}^{\ensuremath{-}})p$, and $p(\ensuremath{\gamma}, {\ensuremath{\pi}}^{0})p$ are all well described by this amplitude. The charged pion production cross sections obtained with Olsson's effects are similar to those calculated by Blomqvist and Laget, but the new amplitude has the added feature of also describing neutral pion production. Asymmetry parameters calculated with the new and Blomqvist and Laget amplitudes are compared with experiment for the cases where either the incident photon, the target, or final state baryon are polarized; the new photopion production amplitude provides a good description of these experiments.NUCLEAR REACTIONS Photopion production amplitude, ($\ensuremath{\gamma}, \ensuremath{\pi}$) cross sections, polarization calculations.
Total Ozone Mapping Spectrometer (TOMS) data during the southern hemisphere spring are analyzed to obtain time‐mean and transient wave amplitudes for zonal wave numbers 1–10. It is found that at latitudes south of 50°S, the October total ozone is dominated by the wave number 1 time‐mean component, with largest amplitudes occurring in 1979 and 1982. The November wave number 1 time‐mean amplitudes are reduced from their October values; the largest reductions occur in 1979 and 1982. In contrast, at 40°–45°S the transient amplitudes are dominant. Phases angles for time‐mean wave number 1 are also presented and little year‐to‐year variation is noted south of 50°S. Also obtained with these data are traveling wave power and coherence‐squared spectra. The spectra for eastward traveling features having zonal wave numbers 1–3 are presented. All of these wave numbers exhibit eastward traveling features at high latitudes, with no evidence of westward traveling features.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
A large number of investigations are currently focused on low frequency oscillations (LFO) of the tropical atmosphere, with periods of one to two months. Recently, the existence of LFO in the stratosphere has been postulated by global circulation model results and also by observations based on satellite microwave and infrared data. However, the observational results are not well captured by the model calculations. The present work utilizes an independent set of satellite data (four years of data from the Total Ozone Mapping Spectrometer (TOMS), between latitudes 65N and 65S) as a check on the previous observational analyses. Evidence is found for the existence of 35‐50 day oscillations in the TOMS data over the Southeast Pacific and South Indian Oceans, corroborating the earlier observational report.
Abstract A mechanistic model simulation initialized on 14 September 2002, forced by 100-hPa geopotential heights from Met Office analyses, reproduced the dynamical features of the 2002 Antarctic major warming. The vortex split on ∼25 September; recovery after the warming, westward and equatorward tilting vortices, and strong baroclinic zones in temperature associated with a dipole pattern of upward and downward vertical velocities were all captured in the simulation. Model results and analyses show a pattern of strong upward wave propagation throughout the warming, with zonal wind deceleration throughout the stratosphere at high latitudes before the vortex split, continuing in the middle and upper stratosphere and spreading to lower latitudes after the split. Three-dimensional Eliassen–Palm fluxes show the largest upward and poleward wave propagation in the 0°–90°E sector prior to the vortex split (coincident with the location of strongest cyclogenesis at the model’s lower boundary), with an additional region of strong upward propagation developing near 180°–270°E. These characteristics are similar to those of Arctic wave-2 major warmings, except that during this warming, the vortex did not split below ∼600 K. The effects of poleward transport and mixing dominate modeled trace gas evolution through most of the mid- to high-latitude stratosphere, with a core region in the lower-stratospheric vortex where enhanced descent dominates and the vortex remains isolated. Strongly tilted vortices led to low-latitude air overlying vortex air, resulting in highly unusual trace gas profiles. Simulations driven with several meteorological datasets reproduced the major warming, but in others, stronger latitudinal gradients at high latitudes at the model boundary resulted in simulations without a complete vortex split in the midstratosphere. Numerous tests indicate very high sensitivity to the boundary fields, especially the wave-2 amplitude. Major warmings occurred for initial fields with stronger winds and larger vortices, but not smaller vortices, consistent with the initiation of wind deceleration by upward-propagating waves near the poleward edge of the region where wave 2 can propagate above the jet core. Thus, given the observed 100-hPa boundary forcing, stratospheric preconditioning is not needed to reproduce a major warming similar to that observed. The anomalously strong forcing in the lower stratosphere can be viewed as the primary direct cause of the major warming.
A comprehensive analysis of version 5 (V5) Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) ozone data using a Lagrangian transport (LT) model provides estimates of chemical ozone depletion for the 1991–1992 through 1997–1998 Arctic winters. These new estimates give a consistent, three‐dimensional picture of ozone loss during seven Arctic winters; previous Arctic ozone loss estimates from MLS were based on various earlier data versions and were done only for late winter and only for a subset of the years observed by MLS. We find large interannual variability in the amount, timing, and patterns of ozone depletion and in the degree to which chemical loss is masked by dynamical processes. Analyses of long‐lived trace gas data suggest that the LT model sometimes overestimates descent at levels above ∼520 K, so we have most confidence in the results at lower levels. When the vortex is shifted off the pole and the cold region is near the vortex edge (e.g., late winter 1993 and 1996), most rapid ozone depletion occurs near the vortex edge; when the vortex and cold region are pole‐centered (e.g., late winter 1994 and 1997), most ozone loss takes place in the vortex core. MLS observed the most severe ozone depletion in 1995–1996, with about 1.3 ppmv cumulative loss for the winter at 465 K by 3 March 1996; ∼1.0 ppmv cumulative loss is seen at 465 K by mid‐March 1993. Analyses of MLS data show significant ozone loss during January in most years, ranging from ∼0.3 to 0.6 ppmv at 465 K. A modified LT model used with the limited MLS data in 2000 gives rough estimates of ∼0.04 and 0.006–0.012 ppmv/day during 2–12 February and 12 February–29 March 2000, respectively, broadly consistent with other studies of the 1999–2000 winter. Estimates of depletion in MLS column ozone above 100 hPa are considerably smaller than other reported column loss estimates, primarily because many estimates include loss below 100 hPa and because MLS does not continuously observe the Arctic after early spring. Our results from analyses of MLS data confirm previous conclusions of broad overall agreement between many ozone loss estimates in the Arctic lower stratosphere near 450–480 K.
Using seven years of data from tha SAM 2 (Stratospheric Aerosol Measurement 2) and TOMS (Total Ozone Mapping Spectrometer) instruments, along with 70 mbar temperatures extracted from an NMC analysis, the effect of the austral spring polar stratospheric clouds (PSC) on the formation of total ozone miniholes is investigated. A total ozone minihole event is designated as the rapid decrease of more than 20 DU of total ozone over a time period of a day and a spatial extent of approximately 1000 by 1000 km. The severe decrease of total ozone during these minihole events could be explained in part by PSC being formed at altitudes of 10 to 24 km and preventing scattered UV radiation from ozone below the cloud from reaching the TOMS instrument. A result of the cloud's opaqueness is that the total ozone retrieval from TOMS data would underestimate the ozone column in the vicinity of the PSC. The approach to investigate the effect of PSC on total ozone was to use SAM 2 aerosol extinction values in conjunction with NMC stratospheric temperatures to determine if PSC are present during total ozone minihole events occurring during August and September, 1979 to 1986. The minihole events during these seven years were divided into two types: type 1, where the minihole region of 24 hour darkness from regions exposed to sunlight, and type 2, where the minihole occurred 5 to 10 degrees north of the terminator. The presence of PSC in a given region was ascertained by a maximum aerosol extinction greater than .006/km occurring with a temperature less than 189 K. It is found that PSC are consistently present with type 2 minihole events. This is contrasted with PSC rarely occurring in the same vicinity of type 2 miniholes. Also observed of that type 1 minihole events have minimum total ozone values which are on the average 3 to 10 DU smaller than type 2 miniholes. It can be concluded that care must be taken when trying to deduce a dynamical explanation of minihole events near the polar terminator and the role of PSC must be accounted for in type 1 minihole formation.
Stratospheric sudden warmings frequently influence temperatures and circulation in the Arctic winter stratosphere. A unique stratospheric warming in Nov 2000 was characterized by wave 1 amplification with little phase tilt with height, a large displacement of the vortex off the pole, a warm pool at high latitudes, and a modest polar temperature increase, all of which are characteristic of early winter “Canadian” warmings. Unlike most Canadian warmings, the Nov 2000 event led to a strong zonal mean wind reversal for ∼9 days in the mid and lower stratosphere. Wind reversals during Canadian warmings occurred only three times before in the last 23 years. Midstratospheric minimum temperatures continued to decrease during the warming, but lower stratospheric temperatures increased substantially. The Nov 2000 warming was unique in its timing, intensity and duration, and in its impact on the development of the polar vortex, especially in the lower stratosphere.
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