Occurrence of polar stratospheric clouds at Kiruna

Polar stratospheric clouds (PSCs) are often observed in the Kiruna region in northern Sweden, east of the Scandinavian mountain range, during wintertime. PSC occurrence can be detected by ground-based optical instruments. Most of these require clear tropospheric weather. By applying the zenith-sky colour index technique, which works under most weather conditions, the data availability can be extended. The observations suggest that PSC events, especially of type II (water PSCs) may indeed more common than predicted by synoptic models, which is expected because of the frequent presence of mountain-induced leewaves. However, it will be of importance to increase the density of independent observations.     

Estimation of heterogeneous reaction rates for stratospheric trace gases with particular reference to the diffusional uptake of HCl and ClONO2 by polar stratospheric clouds

The stratosphere holds a variety of particulates like polar stratospheric clouds (PSCs) and sulphate aerosols which catalyse chemical reactions. These reactions cause changes in the composition of the stratosphere, including the redistribution of active chlorine which might lead to ozone destruction. As a result during recent years a lot of effort has been directed towards the quantification of the uptake of trace gases like ClONO₂, HCl, etc. into these particulates. However, it has been observed that many of the two and three dimensional models used in such studies are constrained by the lack of adequate rate constant data. This paper describes a theoretical approach to estimate the reaction rate constants for 23 gases on both types of polar stratospheric clouds (type I and II). It is found that for gases like N₂O₅, ClONO₂ and HCl, diffusional uptake is important and contributes significantly to the heterogeneous reaction rate. A complete Lennard-Jones calculation is used to accurately compute the trace gas diffusion coefficients.     

A new diagnostic of stratospheric polar vortices

We studied the main climatological features of the Arctic and Antarctic stratospheric vortices, using a new approach based on defining the vortex edge as the 50 hPa geostrophic streamline of maximum average velocity at each hemisphere. Given the use of NCAR-NCEP reanalysis data, it was thought advisable to limit the study to the periods 1958–2004 for the Northern Hemisphere (NH) and 1979–2004 for the Southern Hemisphere (SH). After describing the method and testing sample results with those from other approaches, we analysed the climatological means and trends of the four most distinctive characteristics of the vortices: average latitude, strength, area, and temperature. In general terms, our results confirm most of what is already known about the stratospheric vortices from previous studies that used different data and approaches. In addition, the new methodology provides some interesting new quantifications of the dominant wavenumber and its interannual variability, as well as the principal variability modes through an empirical orthogonal function analysis that was performed directly over the vortex trajectories. The main drawbacks of the methodology, such as noticeable problems characterising highly disturbed stratospheric structures as multiple or off-pole vortices, are also identified.     

Improvement of ERS-1 orbits using along-track accelerations from DORIS data on SPOT2

In long-arc precise orbit determinations of altimetric satellites such as ERS-1, large errors may occur from mismodelling of aerodynamic drag and solar radiation pressure. Such surface forces for non-spherical satellites require accurate modelling of the effective area and particle-surface interactions, but the dominant source of error is neutral air density as derived from thermospheric models for aerodynamic drag. Several techniques can be employed to alleviate air-drag mismodelling but all require the solution of additional parameters from the tracking data. However, for ERS-1 the sparsity of laser range data limits the application of such empirical techniques. To overcome this, use can be made of the dense DORIS Doppler tracking for SPOT2 which is in a similar orbit to ERS-1. A recent investigation by CNES examined the use of drag scale factors from SPOT2 to constrain the ERS-1 orbit. An improvement to that methodology is to consider along-track mismodelling as observed by timing errors in the Doppler data for each pass of SPOT2. The along-track correction to the acceleration as derived from SPOT2 can then be applied to ERS-1 orbits, solving for a scale factor to absorb systematic errors - particularly that arising from the 50 km altitude difference. Results are presented of the associated improvement in ERS-1 orbits as derived from concurrent SPOT2 arcs. It will be seen that the procedure not only improves the laser range fit, but more importantly, leads to more precise radial positioning as evident in the altimeter and crossover residuals.     

Potential applications of ERS-1 SAR data in near real-time

Summary Some important principles of remote sensing using synthetic aperture radar data (SAR data) from the first European Remote Sensing Satellite ERS-1 are explained. Examples illustrate the potential applications of satellite radar information. The possibility of using near-on-line processed remote information for operational purposes is discussed in particular.The satellite information used was received, processed and presented to the visitors of the exhibition which took place as part of the 1992 International Space Year (ISY) at the Bonn offices of the EEC. The BSH and DLR co-operated with DARA and TELEKOM. The intention was to familiarize a broader public with the potential of remote sensing and to stress the necessity of using space technology in combatting environmental problems.

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Zum Nutzungspotential von ERS-1-AAR-Daten in naher Echtzeit

Zusammenfassung Im vorliegenden Beitrag werden einige wichtige Grundlagen der Fernerkundung mit SAR-Daten des Ersten Europäischen Fernerkundungssatelliten ERS-1 erläutert und anhand einiger ausgewählter Beispiele Möglichkeiten der Nutzanwendung von Radar-Daten aufgezeigt. Dabei werden besonders die Möglichkeiten eines Nahe-Echtzeit-Einsatzes unter operationellen Gesichtspunkten aufgegriffen und diskutiert.Die verwendeten Daten wurden während einer Präsentation in der Vertretung der Kommission der Europäischen Gemeinschaft in Bonn anläßlich einer Veranstaltung zum Internationalen Weltraumjahr 1992 (ISY) von der BDDN-Fast-Delivery-Station des Bundesamts für Seeschiffahrt und Hydrographie (BSH) in Zusammenarbeit mit der Deutschen Forschungsanstalt für Luft- und Raumfahrt (DLR) und der TELEKOM aufgezeichnet, digital verarbeitet und den Besuchern der Ausstellung vorgestellt. Im Vordergrund stand, das Potential der Fernerkundung einer breiteren Öffentlichkeit zugänglich zu machen und die Notwendigkeit der Raumfahrt zur Erkundung unserer Umwelt zu bekräftigen.

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Le potentiel d'application des données ERS-1 SAR en utilisation presqueimmédiate

Résumé Des propriétés importantes du radar d'ouverture synthétique du premier satellite européen ERS-1 d'observation de la Terre sont présentées. Des exemples choisis illustrent les possibilités d'utilisation appliquée de l'information radar. La possibilité d'une exploitation en temps très légèrement différé sous des aspects opérationnels est démontrée.L'information du satellite ERS-1 a été reçue, traitée et archivée lors d'une présentation à la Commission de la Communauté Européene à Bonn. À l'occasion de la manifestation de l'année internationale de l'espace 1992 (ISY), avec le concours de l'Agence Maritime et Hydrographique Fédérale (BSH), en coopération avec le Centre Allemand de Recherches Aérospatiales (DLR) et les TELEKOM. Le but était de présenter au public les possibilités de la télédétection et de montrer la nécessité de l'usage de la technologie pour aider à résoudre les problèmes de l'environment.

     

Dynamical diagnosis of the breakup of the stratospheric polar vortex in the Northern Hemisphere

The research on climate change in polar regions, especially on the role of polar in the global climate system, has gain unprecedented level of interest. It has been the key scientific issue of the International Polar Year program (IPY, 2007―2008). In this paper, we dealt with the debate upon the breakup time of the stratospheric polar vortex in boreal spring. An observational study of the relation between strato- spheric polar vortex breakup and the extra-tropical circulation was performed. The mean breakup date―when the winter westerly at the core of polar jet turns to summer easterly―is about April 10. The breakup time has large interannual variation with a time span of about 2 months. It also has a long-term trend with the 1990s and 2000s witnessing more and more late breakups of polar vortex. Composite of wind speed at the core of polar jet for the extremely early and late breakup years shows that late years have two periods of westerly weakening while early breakup years have only one. The first weakening in the late years happens in middle January with wind speed dropping sharply from more than 40 m s?1 to about 15 m s?1. This is accompanied with anomalous activities of planetary waves in both strato- sphere and troposphere; while the second weakening in the late breaking years is mainly the results of diabatic heating with very weak wave activities. In early breakup years, the transition from westerly to easterly is rapid with wind speed dropping from more than 30 m s?1 to less than ?10 m s?1 within a month. This evolution is associated with a strong bidirectional dynamical coupling of the stratosphere and troposphere. The circulation anomalies at low troposphere are also analyzed in the extremely early and late breakup years. It shows that there are significant differences between the two kinds of extreme years in the geopotential height and temperature composite analysis, indicating the dynamical cou- pling of stratosphere and troposphere with the evolution of stratospheric polar vortex.     

Use of ERS-1 data for the extraction of flooded areas

The aim of this study, undertaken by Geoimage, was the setting up of a fast and precise location method of flooded areas over two sites in southern France. The use of satellite imagery seemed to be the appropriate tool for this study. Two types of flood had to be distinguished: (i) an oceanic flood, of long duration characteristic, and of low intensity on the Rhône Valley, (ii) a torrential flood, of short duration characteristic, but of high intensity, on the Var Valley. As we distributed of ERS-1 images over both sites, during the floods, we could test our methodology. A multitemporal approach using ERS-1 images in PRI mode, acquired before, during and after the flood, was set up. In the case of oceanic flood, the radar images characteristic answers, enabled us to extract and identify areas under water at each date of acquisition of the images. Therefore, if we distribute images at each step of the flood, its evolution can be precisely reconstituted (in terms of time and surface). In the case of torrential flood, it is more difficult to localize the flood with precision. This can be explained by the change of water surface, which has a large swell in this case. Radars are sensitive to these changes in the turbidity, an interaction occurs and thus the results were ‘turned off’. Nevertheless, simulation studies from other satellite data make possible the location of more or less strong hydrological risk accident areas. © 1997 John Wiley & Sons, Ltd.     

热带印度洋增暖对南极平流层极涡的影响

过去几十年,在全球变暖的大背景下,全球大部分海洋,特别是热带印度洋,显著增暖．同时,南极平流层极涡呈现发展加深的趋势．以前的模拟结果显示,臭氧耗损的辐射冷却效应是南极极涡加深的主导因子,但模拟的臭氧耗损单独引起的南极极涡加深比实际观测到的要强．这说明有其他因子参与影响了南极极涡的趋势变化,其作用是部分抵消臭氧耗损的影响．是否热带印度洋增暖是其中的因子之一,这个问题还不清楚．利用4个大气环流模式,通过给定理想的、与观测到的强度相当的热带印度洋增暖强迫,进行集合试验,研究了这一问题．结果表明：热带印度洋增暖有利于南半球春、夏季极地平流层增暖、南极极涡减弱,于是倾向于部分抵消臭氧耗损的辐射冷却效应．这一结果能部分解释以前的模拟发现～臭氧耗损单独导致的南极极涡加深比观测到的要强．鉴于平流层变暖不利于极地平流层冰晶云的形成、遂有利于臭氧恢复,现在的结果暗示：在全球变暖的大背景下,气候系统的内部动力调整过程将有利于南极臭氧洞的恢复．     

The influence of tropical Indian Ocean warming on the Southern Hemispheric stratospheric polar vortex

During the past decades, concurrent with global warming, most of global oceans, particularly the tropical Indian Ocean, have become warmer. Meanwhile, the Southern Hemispheric stratospheric polar vortex (SPV) exhibits a deepening trend. Although previous modeling studies reveal that radiative cooling effect of ozone depletion plays a dominant role in causing the deepening of SPV, the simulated ozone-depletion-induced SPV deepening is stronger than the observed. This suggests that there must be other factors canceling a fraction of the influence of the ozone depletion. Whether the tropical Indian Ocean warming (IOW) is such a factor is unclear. This issue is addressed by conducting ensemble atmospheric general circulation model (AGCM) experiments. And one idealized IOW with the amplitude as the observed is prescribed to force four AGCMs. The results show that the IOW tends to warm the southern polar stratosphere, and thus weakens SPV in austral spring to summer. Hence, it offsets a fraction of the effect of the ozone depletion. This implies that global warming will favor ozone recovery, since a warmer southern polar stratosphere is un-beneficial for the formation of polar stratospheric clouds (PSCs), which is a key factor to ozone depletion chemical reactions. Supported by National Natural Science Foundation of China (Grant Nos. 40775053 and 90711004), National Basic Research Program of China (Grant No. 2009CB421401), and Innovation Key Program of Chinese Academy of Sciences (Grant Nos. KZCXZ-YW-Q11-03, KZCZ2-YW-Q03-08)     

Coordinated optical and radar image measurements of noctilucent clouds and polar mesospheric summer echoes

Novel coincident 3-D radar, lidar and optical image measurements of dynamical structures in polar mesosphere summer echoes (PMSE) and noctilucent clouds (NLC) are presented. Common volume mesospheric measurements were made over central Alaska using the new Poker Flat Incoherent Scatter Radar (PFISR), a co-located Rayleigh lidar and remote, two-station digital image observations, enabling the first detailed investigation of the horizontal and vertical structures of NLC and PMSE. Coincident measurements were made of an unusual NLC display recorded on 10–11 August 2007, characterized by a broad luminous band that contained several prominent wave forms. Concurrent lidar and image measurements established the presence of NLC within the radar volume from ～09:00 UT (01:00 LT), when the solar depression angle was 10.4°, until dawn. Strong but intermittent PMSE were detected by PFISR, with distinct patchy structures that exhibited a similar southward motion as the NLC. Detailed comparison of the 3-D PMSE structures and the NLC lidar and image data have revealed striking similarities when account was taken of the NLC layer altitude, suggesting a direct link between their small-scale spatial signatures (within the current resolution of the radar measurements). At the same time, the lidar detected a sustained increase in the backscatter signal, while the imagers revealed the development of copious short horizontal wavelength (4.9 km) billow waves. We conclude that strong wind shears associated with the Kelvin–Helmholtz billow instabilities played a key role in the development of a neutral turbulence layer in close proximity to the NLC layer resulting in the strong but intermittent PMSE detected at 450 MHz on this occasion.     

COSMIC掩星数据在平流层的温度特性验证

针对掩星技术的反演精度在上大气层(一般在35 km以上)的验证会受到限制,本文选用2007年1月、4月、7月和10月COSMIC掩星观测的月平均温度数据,与MERRA再分析场和ECMWF再分析场月平均温度数据进行比较,以不同的方式进行对比,主要对20～50 km COSMIC掩星温度资料进行精度和可用性的检验.结果显示...     

Assimilation of ERS-1 altimeter data in a tidal model of the continental shelf

Summary In this paper ERS-1 altimeter data are used for the calibration of a tidal model of the European Continental Shelf. The data assimilation procedure to estimate the uncertain parameters in the model is based on the adjoint method. Here the estimation problem is formulated as a large scale optimization problem, that is solved with a gradient based optimization method. The gradient is determined efficiently by using the solution of the adjoint problem. The data assimilation procedure is applied by using the ERS-1 data with and without other measurement information from water level stations.

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Assimilation von ERS-1 Altimeter Daten in einem Gezeitenmodell des Kontinentalschelfs

Zusammenfassung In dieser Arbeit werden ERS-1 Altimeter Daten zum Kalibrieren eines Gezeitenmodells des Europäischen Kontinentalschelfs benutzt. Das Verfahren der Datenassimilation zur Bestimmung der unbekannten Modellparameter beruht auf der adjungierten Methode. Die Aufgabe ist ein großskaliges Optimierungsproblem, das mit einem Gradientenverfahren gelöst wird. Zur Gradientenbestimmung benutzt man die Lösung des adjungierten Problems. Neben den Daten von ERS-1 werden wahlweise noch Beobachtungen von Küstenpegeln assimiliert.

     

Responses of polar mesospheric cloud brightness to stratospheric gravity waves at the South Pole and Rothera,Antarctica

We present the first observational proof that polar mesospheric cloud (PMC) brightness responds to stratospheric gravity waves (GWs) differently at different latitudes by analyzing the Fe Boltzmann lidar data collected from the South Pole and Rothera (67.5°S, 68.0°W), Antarctica. Stratospheric GW strength is characterized by the root-mean-square (RMS) relative density perturbation in the 30–45 km region and PMC brightness is represented by the total backscatter coefficient (TBC) in austral summer from November to February. The linear correlation coefficient (LCC) between GW strength and PMC brightness is found to be +0.09 with a 42% confidence level at the South Pole and ?0.49 with a 98% confidence level at Rothera. If a PMC case potentially affected by a space shuttle exhaust plume is removed from the Rothera dataset, the negative correlation coefficient and confidence level increase to ?0.61 and 99%, respectively. The Rothera negative correlation increases when shorter-period waves are included while no change is observed in the South Pole correlation. Therefore, observations show statistically that Rothera PMC brightness is negatively correlated with the stratospheric GW strength but no significant correlation exists at the South Pole. A positive correlation of +0.74 with a confidence level of 99.98% is found within a distinct subset of the South Pole data but the rest of the dataset exhibits a random distribution, possibly indicating different populations of ice particles at the South Pole. Our data show that these two locations have similar GW strength and spectrum in the 30–45 km region during summer. The different responses of PMC brightness to GW perturbations are likely caused by the latitudinal differences in background temperatures in the ice crystal growth region between the PMC altitude and the mesopause. At Rothera, where temperatures in this region are relatively warm and supersaturations are not as large, GW-induced temperature perturbations can drive subsaturation in the warm phase. Thus, GWs can destroy growing ice crystals or limit their growth, leading to negative correlation at Rothera. Because the South Pole temperatures in the mesopause region are much colder, GW-perturbed temperature may never be above the frost point and have less of an impact on crystal growth and PMC brightness. The observed phenomena and proposed mechanisms above need to be understood and verified through future modeling of GW effects on PMC microphysics and ray modeling of GW propagation over the South Pole and Rothera.     

Detection of tropospheric and stratospheric aerosol layers by optical radar (Lidar)

Summary The National Center for Atmospheric Research Lidar Sustem is briefly described. Data will be presented showing stratification and temporal variations of optical backscattering due to atmospheric aerosol up to a height of 40 kilometers. Short lived (less than 4 seconds) layers with thicknesses and spacing of 30 meters are routinely observed over the entire range of observation. A well-defined layer of temporal stability of several hours is very often found at the tropopause. The so-called 20 kilometer sulphate layer is found to be subject to variations in altitude, thickness, and assuming a non-varying size distribution and composition, in concentration. Significant and quasistable layers exist between 25 and 40 kilometers including one at the 36 kilometer leve which has been postulated by preliminary calculations of Mateer to be responsible for the blue and white bands observed and photographed by astronauts.Paper published by Journal of Geophysical Research75 (1970), 3123–3132.     

Seasonal timing of stratospheric final warming associated with the intensity of stratospheric sudden warming in preceding winter

The association of seasonal timing of stratospheric final warming events(SFWs) in spring and the occurrence of major and minor stratospheric sudden warming events(SSWs) in midwinter were investigated through statistical analysis, parallel comparison, and composite analysis, based on the NCEP-NCAR reanalysis dataset covering 1958–2012. It was found that the intensity and occurrence of winter SSW events can largely affect the timing of spring SFWs. Specifically, the SFW onset dates tend to be later(earlier) after the occurrence(absence) of winter major SSWs. However, the occurrence or absence of minor SSWs does not change the frequency of early and late SFWs. A parallel comparison of the temporal evolution of the anomalous circulation and planetary-waves between major SSW and minor SSW winters indicates that the stratospheric polar vortex(polar jet) will keep being anomalously stronger 30 days after major SSW onset. And the associated significant negative Eliassen-Palm(EP) flux anomalies can persist for as long as 45 days after major SSW events. In contrast, the circulation anomalies around the occurrence of minor SSW events can last only a few days. To further verify the possible influence of the occurrence of major SSWs on the seasonal timing of SFWs, composite analysis was performed respectively for the 21 major-SSW years, 15 minor-SSW years, and the 15 non-SSW years. Generally, planetary-wave activity in the extratropical stratosphere tends to be stronger(weaker) and the westerly polar jet is anomalously weaker(stronger) in major-SSW(non-SSW) winters. But in the following spring, the planetary-wave activity is weaker(stronger) accompanied with an anomalously stronger(weaker) stratospheric polar vortex. In spring after minor-SSW years, however, the stratospheric polar vortex and the westerly polar jet exhibit a state close to climatology with relatively gentle variations.     

The role of the SO2 oxidation for the background stratospheric sulfate layer in the light of new reaction rate data

Presently available data on the reaction of SO₂ with OH radicals (OH + SO₂ + \(M\xrightarrow[{k_1 }]{}\) HSO₃ +M) are critically reviewed in light of recent stratospheric sulfur budget calculations. These calculations impose that the net oxidation ratek of SO₂ within the stratosphere should fall within the range 10^?7≤k≤10^?9, if the SO₂ oxidation model for the stratospheric sulfate layer is assumed to be correct. The effective reaction rate constantk ₁ ^* =k ₁[M] at the stratospheric temperature is estimated as $$k_1^* = \frac{{(8.2 \pm 2.2) \times 10^{ - 13} \times [M]}}{{(0.79 \mp 0.34) \times 10^{ - 13} + [M]}}cm^3 /molecules sec$$ where [M] refers to the total number density (molecules/cm³). Using the above limiting values ofk ₁ ^* , and the estimated OH density concentrations, the net oxidation rate is calculated as 3.6×10^?7≤k≤1.3×10^?8 at 17 km altitude. This indicates that the upper limit of thesek values exceeds the tolerable range imposed by the model by a factor of about four. Obviously the uncertainty of thek ₁ ^* values and of the OH concentrations in the stratosphere is still too large to make definite conclusions on the validity of the SO₂ model.     

Further development in theory/data closure of the photoelectron-driven polar wind and day-night transition of the outflow

Recent in situ observations have revealed novel features in the polar wind. Measurements between 5000 and 9000 km altitude by the Akebono satellite indicate that both H⁺ and O⁺ ions can have remarkably higher outflow velocities in the sunlit region than on the nightside. Electrons also display an asymmetric behavior: the dayside difference in energy spread, greater for upward-moving than downward-moving electrons, is absent on the nightside. Here, we discuss the further development of a theory by Tam et al. that can explain most of these observed peculiar properties by properly taking into account the global, kinetic, collisional effects of the sunlit photoelectrons, on the background polar wind and the electric field. Quantitative comparisons of the calculated results with actual data will be described. In addition, transition from the daytime photoelectron-driven polar wind to the night-time polar wind will be discussed.     

Assessment of the mapping capabilities of ERS-1 SAR data for flood mapping: a case study in Germany

GAF examined, under contract to DARA (German Space Agency), the applicability of ERS-1 SAR data for flood mapping under operational conditions. The flood event investigated was the flooding in the Rhine valley in winter 1993–1994. In order to carry out an examination close to the end-user needs, the specific user requests concerning information about flood events were identified. The mapping accuracy in view of the flood extent and the flood level, the production of flood maps as well as the demonstration of the runoff turned out to be the most interesting points. The specific user information needs were considered in the project objectives to define the applicability as well as the deficits of ERS-1 data concerning an operational use for flood mapping. After a detailed analysis of the time aspects of the traditional mapping method and a satellite data analysis, a visual interpretation as well as an automatic classification were applied, including various filter steps to derive the flood boundary. As a result, the visual interpretation proved to be the more accurate method. Crucial domains for both the visual interpretation and the automatic classification turned out to be settlements, forests and bushes as well as regions with layover and foreshortening effects. The comparison between the flood level derived from satellite data and the flood level registered by the water authority boards brought a height difference which ranged between 0·5 and 2·0 m. The relatively coarse resolution and problems with correct interpretation of the flood line proved to be the reason for this difference. In general the results are convenient, but in relation to field measurements of the water level they are too inaccurate. A cost and benefit analysis as well as a proposal for an operational GIS system using ERS-1 SAR data are still under investigation. © 1997 John Wiley & Sons, Ltd.     

Detection of crevasses over polar ice shelves using Satellite Laser Altimeter

Ice shelf breakups account for most mass loss from the Antarctic Ice Sheet as the consequence of the propagation of crevasses(or rift)in response to stress.Thus there is a pressing need for detecting crevasses’location and depth,to understand the mechanism of calving processes.This paper presents a method of crevasse detection using the ICESat-1/GLAS data.A case study was taken at the Amery Ice Shelf of Antarctica to verify the accuracy of geo-location and depth of crevasses detected.Moreover,based on the limited crevasse points,we developed a method to detect the peak stress points which can be used to track the location of the crack tips and to identify the possible high-risk area where an ice shelf begins to break up.The spatial and temporal distribution of crevasse depth and the spatial distribution of peak stress points of the Amery Ice Shelf were analyzed through 132 tracks in 16 campaign periods of ICESat-1/GLAS between 2003 and 2008.The results showed that the depth of the detected crevasse points ranged from 2 to 31.7 m,which were above the sea level;the crevasse that advected downstream to the front edge of an ice shelf has little possibility to directly result in breakups because the crevasse depth did not show any increasing trend over time;the local stress concentration is distributed mainly in the suture zones on the ice shelves.     

Sensitivity experiments with a quasi-geostrophic model of stratospheric sudden warming

We describe a series of sensitivity experiments with a quasi-geostrophic model of the interaction of stationary planetary waves with the mean zonal flow in the stratosphere and mesosphere. The model is of the Matsuno type, which neglects wave-wave interaction and includes only a single zonal harmonic of the planetary wave spectrum in each simulation. We employed the model to investigate the source of the double-layer structure previously obtained by several authors for the stratospheric sudden warming with wavenumber one. Our results suggest that this characteristic of the model-produced warming is a property only of models without damping. When reasonable dissipation is included in the model, the double-layer structure disappears. This implies the importance of the drag parameterization in properly simulating warming events and, since the actual drag very probably is effected by breaking internal waves, it suggests that future analysis should include a specific representation of this effect. We also investigated the dependence of stratospheric warming on the structure of the zonal wind field. Our analyses show in particular that substantial reduction of the height of the polar night jet mitigates strongly against the occurrence of a sudden warming event.