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The Ross Sea region, including three main polynya areas in McMurdo Sound, Terra Nova Bay, and in front of the Ross Ice Shelf, has experienced a significant increase in sea ice extent in the first four decades of satellite observations. Here, we use Co-Registration of Optically Sensed Images and Correlation (COSI-Corr) to estimate 894 high-resolution sea ice motion fields of the Western Ross Sea in order to explore ice-atmosphere interactions based on sequential high-resolution Advanced Synthetic Aperture Radar (ASAR) images from the Envisat satellite acquired between 2002–2012. Validation of output motion vectors with manually drawn vectors for 24 image pairs show Pearson correlation coefficients of 0.92 ± 0.09 with a mean deviation in direction of −3.17 ± 6.48 degrees. The high-resolution vectors were also validated against the Environment and Climate Change Canada sea ice motion tracking algorithm, resulting in correlation coefficients of 0.84 ± 0.20 and the mean deviation in the direction of −0.04 ± 17.39 degrees. A total of 480 one-day separated velocity vector fields have been compared to an available NSIDC low-resolution sea ice motion vector product, showing much lower correlations and high directional differences. The high-resolution product is able to better identify short-term and spatial variations, whereas the low-resolution product underestimates the actual sea ice velocities by 47% in this important near-coastal region. The large-scale pattern of sea ice drift over the full time period is similar in both products. Improved image coverage is still desired to capture drift variations shorter than 24 h.

We compare the consistency between eight reanalyses: CERA20C, ERA5, ERA‐Interim, ERA20C, NCEP‐DOE, MERRA2, JRA55, and 20CRV2c. This comparison uses daily surface winds near Antarctica to classify synoptic patterns using the self‐organizing map technique. The relative frequency of occurrence (RFO) of these patterns are very similar during the satellite era in each reanalysis. The three most common patterns are the same in each reanalysis and changes between the reanalyses only display a 12% relative variation. Examination of the RFOs over time highlights that the CERA20C and 20CRV2c reanalyses display large changes previous to 1957. These changes are likely connected to model relaxation toward their climatology because of a lack of observational constraints. Primarily, we introduce the entropy coefficient (U) which quantifies the consistency between reanalyses in their representation of synoptic patterns. Examination of U shows current reanalyses (ERA5, ERA‐Interim, JRA55, and MERRA2) are highly consistent in the satellite era likely due to good observational coverage. However, centennial reanalyses (CERA20C, ERA20C, and 20CRV2c) show two upward step changes in consistency as measured by U at around 1957 and to a lesser extent 1979. Low values of U before 1957 suggest that centennial reanalyses are of limited use before this date, but may be useful after 1957 in this region. We also show that the entropy coefficient displays an inverse relationship with ensemble spread metrics of individual reanalyses. We conclude that the entropy coefficient provides a powerful quantification of the influence of changes in observation density on reanalysis quality in data sparse regions.

We use the 2B-GEOPROF-LIDAR R04 (2BGL4) and R05 (2BGL5) products and the 2B-CLDCLASS-LIDAR R04 (2BCL4) product, all generated by combining CloudSat radar and CALIPSO lidar satellite measurements with auxiliary data, to examine the vertical distribution of cloud occurrence around the Ross Ice Shelf (RIS) and Ross Sea region. We find that the 2BGL4 product, used in previous studies in this region, displays a discontinuity at 8.2 km which is not observable in the other products. This artefact appears to correspond to a change in the horizontal and vertical resolution of the CALIPSO dataset used above this level. We then use the 2BCL4 product to examine the vertical distribution of cloud occurrence, phase, and type over the RIS and Ross Sea. In particular we examine how synoptic conditions in the region, derived using a previously developed synoptic classification, impact the cloud environment and the contrasting response in the two regions. We observe large differences between the cloud occurrence as a function of altitude for synoptic regimes relative to those for seasonal variations. A stronger variation in the occurrence of clear skies and multi-layer cloud and in all cloud type occurrences over both the Ross Sea and RIS is associated more with synoptic type than seasonal composites. In addition, anomalies from the mean joint histogram of cloud top height against thickness display significant differences over the Ross Sea and RIS sectors as a function of synoptic regime, but are near identical over these two regions when a seasonal analysis is completed. However, the frequency of particular phases of cloud, notably mixed phase and water, is much more strongly modulated by seasonal than synoptic regime compositing, which suggests that temperature is still the most important control on cloud phase in the region.

Despite warming trends in global temperatures, sea ice extent in the Southern Hemisphere has shown an increasing trend over recent decades. Wind-driven sea ice export from coastal polynyas is an important source of sea ice production. Areas of major polynyas in the Ross Sea, the region with largest increase in sea ice extent, have been suggested to produce a vast amount of the sea ice in the region. We investigate the impacts of strong wind events on the Ross Sea Polynyas and its sea ice concentration and possible consequences on sea ice production. We utilise Bootstrap sea ice concentration (SIC) measurements derived from satellite based, Special Sensor Microwave Imager (SSM/I) brightness temperatures. We compared these with surface winds and temperatures from automatic weather stations (AWS) of the University of Wisconsin-Madison Antarctic Meteorology Program. Our analysis focusses on the austral winter period defined as 1st April to 1st November in this study. Daily data were used to classified into characteristic regimes based on the percentiles of wind speed. For each regime, a composite of SIC anomaly was formed for the Ross Sea region. We found that persistent weak winds near the edge of the Ross Ice Shelf are generally associated with positive SIC anomalies in the Ross Sea Polynya (RSP). Conversely we found negative SIC anomalies in this area during persistent strong winds. By analysing sea ice motion vectors derived from SSM/I and SSMIS brightness temperatures, we find significant sea ice motion anomalies throughout the Ross Sea during strong wind events. These anomalies persist for several days after the strong wind event. Strong, negative correlations are found between SIC and AWS wind speed within the RSP indicating that strong winds cause significant advection of sea ice in the region. We were able to recreate these correlations using co-located ERA-Interim wind speeds. However when only days of a certain percentile based wind speed classification were used, the cross correlation functions produced by ERA-Interim wind speeds differed significantly from those produced using AWS wind speeds. The rapid decrease in SIC during a strong wind event is followed by a more gradual recovery in SIC. This increase occurs on a more gradual time scale than the average persistence of a strong wind event and the resulting sea ice motion anomalies, highlighting the production of new sea ice through thermodynamic processes. In the vicinity of Ross Island, ERA-Interim underestimates wind speeds by a factor of 1.7, which results in a significant misrepresentation of the impact of winds on polynya processes.

This study compares high-resolution output (1.1-km horizontal grid length) from twice-daily forecasts produced by the Antarctic Mesoscale Prediction System (AMPS) with a dense observational network east of Ross Island. Covering 10 000 km2, 15 SNOWWEB stations significantly increased the number of observation stations in the area to 19 during the 2014–15 austral summer. Collocated “virtual stations” created from AMPS output are combined with observations, producing a single dataset of zonal and meridional wind components used to train a self-organizing map (SOM). The resulting SOM is used to individually classify the observational and AMPS datasets, producing a time series of classifications for each dataset directly comparable to the other. Analysis of class composites shows two dominant weather patterns: low but directionally variable winds and high but directionally constant winds linked to the Ross Ice Shelf airstream (RAS). During RAS events the AMPS and SNOWWEB data displayed good temporal class alignment with good surface wind correlation. SOM analysis shows that AMPS did not accurately forecast surface-level wind speed or direction during light wind conditions where synoptic forcing was weak; however, it was able to forecast the low wind period occurrence accurately. Coggins’s regimes provide synoptic-scale context and show a reduced synoptic pressure gradient during these classes, increasing reliance on the ability of Polar WRF to resolve mesoscale dynamics. Available initialization data have insufficient resolution for the region’s complex topography, which likely impacts performance. The SOM analysis methods used are shown to be effective for model validation and are widely applicable.

The Amundsen Sea Low (ASL) is an area of climatologically low atmospheric pressure situated over the Southern Ocean. The depth and location of this feature have significant effects on winds, temperature, moisture transport, and sea ice in its vicinity. In this article, we quantify the modulating effect of this feature on winds over the Ross Sea and Ross Ice Shelf. We examine composites of surface winds sampled according to extrema in ASL depth, longitude, and latitude. We employ the output of a previously developed synoptic climatology to identify the explanatory synoptic-scale forcings. In autumn, winter, and spring (AWS) we find that the impact of the depth of the ASL is smaller than that of its location. The ASL moves eastward when it is deep, thereby reducing its influence on Ross Sea winds. When the ASL is northward, we find strongly enhanced southerly flows over the Ross Sea and Ice Shelf, forced by greater cyclonic activity in the north of the Ross Sea. In summer, we find increased cyclonic flow coinciding with a deeper ASL, despite the ASL being located in the Bellingshausen Sea at this time. The responses to the ASL longitude and latitude are profoundly different to those in AWS, suggesting that relationships are strongly dependent on the varying seasonal location of the low. We examine two metrics of the ASL depth and identify that the absolute mean sea level pressure (MSLP) has a more widespread response than that of the relative MSLP.

We provide the first characterization of diurnal and semidiurnal thermal tides in temperature from 30 to 110 km in the winter season (May through August) at McMurdo (77.8°S, 166.7°E), Antarctica. The observations were made with an Fe Boltzmann temperature lidar in 2011 and 2012. Over 330 h of winter data are compiled into a composite day of temperature perturbations that significantly reduce the incoherent wave effects while preserving the coherent tidal signatures. Both diurnal and semidiurnal tides have small amplitudes (less than 3 K) below 100 km with vertical wavelengths of ~29 and ~23 km, respectively. A new finding of this study is the fast growth of diurnal and semidiurnal tidal amplitudes above 100 km to at least 15 K near 110 km, exceeding that of the freely propagating tides originating from the lower atmosphere. Such fast growth exists for all Kp index cases and diurnal amplitude increases to 15–30 K at 110 km with larger Kp indices corresponding to larger tidal amplitudes and faster growth rates. The slopes of diurnal tidal phases become steeper above 100 km, and the tidal phases barely change with altitude from 100 to 106 km. The tidal growth behavior is reproduced in the Whole Atmosphere Model (WAM) with phases comparable to the observations but magnitudes significantly underestimated. WAM compares reasonably well with the observations below 100 km. The observed significant amplitude increases and phase structure changes suggest additional tidal sources near or above 100 km, which deserve future investigation.

The surface wind regime of the Ross Ice Shelf (RIS) is dominated by northward outflow influenced by drainage from both East and West Antarctica and synoptic disturbances over the Ross Sea. To quantify the effects of the wind regime on the climate of the region, we produce a synoptic climatology of the RIS and Ross Sea via a k-means clustering technique applied to surface (10 m) winds from the ERA Interim reanalysis (1979–2011). We describe the generation of the climatology and discuss composites of surface winds, mean sea-level pressure and temperature, which show a high degree of internally consistent structure and are similar to previous synoptic climatologies. While some small-scale features are not fully resolved by the reanalysis, such as the channelling of katabatic flows, we identify a high degree of coherence between clustering results and available in situ data sources, suggesting that the ERA Interim represents the region well. We find that the surface wind field of the ice shelf is highly sensitive to the strength and position of cyclonic disturbances in the Ross Sea, the presence of which can aid outflow from the continental interior. Temperature displays a high level of sensitivity to circulation conditions due to the effects of steep temperature gradients surrounding the shelf and advective heat transport. Cold temperatures on the ice shelf coincide with drainage from East Antarctica, while warmer periods coincide with West Antarctic drainage; this is particularly clear during RIS air stream (RAS) events. The warmest periods occur during southward transport from the Ross Sea. The sensitivity of temperature to the origin of flows suggests circulation changes in the region are likely to significantly impact surface climate. Analysis of the frequency of varying synoptic events and their persistence confirms RAS is a very common and persistent feature of the region.

[1] This study presents the first coincident observation of inertia-gravity waves (IGWs) by lidar and radar in the Antarctic mesopause region. This is also the first known observation of two simultaneous IGWs at the same location. An Fe Boltzmann lidar at Arrival Heights (77.8°S, 166.7°E) provides high-resolution temperature data, and a co-located MF radar provides wind data. On 29 June 2011, coherent wave structures are observed in both the Fe lidar temperature and MF radar winds. Two dominant waves are determined from the temperature data with apparent periods of 7.7 ± 0.2 and 5.0 ± 0.1 h and vertical wavelengths of 22 ± 2 and 23 ± 2 km, respectively. The simultaneous measurements of temperature and wind allow the intrinsic wave properties to be derived from hodograph analyses unambiguously. The analysis shows that the longer-period wave propagates northward with an azimuth of θ = 11° ± 5° clockwise from north. This wave has a horizontal wavelength of λh = 2.2 ± 0.2 × 103 km and an intrinsic period of τI = 7.9 ± 0.3 h. The intrinsic horizontal phase speed (CIh) for this wave is 80 ± 4 m/s, while the horizontal and vertical group velocities (Cgh and Cgz) are 48 ± 3 m/s and 0.5 ± 0.1 m/s, respectively. The shorter-period wave has τI = 4.5 ± 0.3 h and θ = 100° ± 4° with λh = 1.1 ± 0.1 × 103 km and CIh = 68 ± 5 m/s. Its group velocities are Cgh = 58 ± 5 m/s and Cgz = 1.1 ± 0.1 m/s. Therefore, both waves propagate with very shallow elevation angles from the horizon (ϕ = 0.6° ± 0.1° and ϕ = 1.1° ± 0.1° for the longer- and shorter-period waves, respectively) but originate from different sources. Our analysis suggests that the longer-period IGW most likely originates from the stratosphere in a region of unbalanced flow.

This study describes SNOW-WEB, a distributed system of atmospheric sensors, which is cost-effective and can be efficiently deployed in Antarctica. The system supports traditional atmospheric sensors and has built-in redundancy as many units can be deployed in a relatively small area for a similar cost to one conventional weather station. Furthermore, each unit is equipped with wireless mesh-networking capabilities and so is able to share information with those units in its direct vicinity. This allows for the ferrying of collected information to a manned observation station and hence the ability to monitor data in real-time. GPS hardware installed on each unit also allows for high-resolution glacier or ice shelf tracking. As a testing study, eighteen such weather stations were deployed in the vicinity of Scott Base, Ross Island, Antarctica over the 2011/12 summer season. This paper reports on the successful development and deployment of the system, results from the testing period and challenges encountered during the experiment. Collected data is validated against automatic weather stations already operating in the region and an intercomparison is performed between SNOW-WEB data and forecast output from the Antarctic Mesoscale Prediction System. A high degree of agreement is found between data sources. We conclude that SNOW-WEB data is suitable for use in studies of mesoscale meteorology.

Kennicutt, M.C. et. al.-- 17 pages, 3 figures, The Antarctic Roadmap Challenges (ARC) project identified critical requirements to deliver high priority Antarctic research in the 21st century. The ARC project addressed the challenges of enabling technologies, facilitating access, providing logistics and infrastructure, and capitalizing on international co-operation. Technological requirements include: i) innovative automated in situ observing systems, sensors and interoperable platforms (including power demands), ii) realistic and holistic numerical models, iii) enhanced remote sensing and sensors, iv) expanded sample collection and retrieval technologies, and v) greater cyber-infrastructure to process ‘big data’ collection, transmission and analyses while promoting data accessibility. These technologies must be widely available, performance and reliability must be improved and technologies used elsewhere must be applied to the Antarctic. Considerable Antarctic research is field-based, making access to vital geographical targets essential. Future research will require continent- and ocean-wide environmentally responsible access to coastal and interior Antarctica and the Southern Ocean. Year-round access is indispensable. The cost of future Antarctic science is great but there are opportunities for all to participate commensurate with national resources, expertise and interests. The scope of future Antarctic research will necessitate enhanced and inventive interdisciplinary and international collaborations. The full promise of Antarctic science will only be realized if nations act together, The authors recognize the financial support that made the Scan and ARC possible. The Council of Managers of National Antarctic Programs (COMNAP), the Tinker Foundation and the Scientific Committee on Antarctic Research (SCAR) provided the majority of the funding for this project including the costs of travel and participation of invited, non-COMNAP workshop attendees. In-kind support was provided by many COMNAP-Member national Antarctic programmes including Dirección Nacional del Antártico (DNA, Argentina), Australian Antarctic Division (AAD, Australia), Programa Antártico Brasileiro (PROANTAR, Brazil), Instituto Antártico Chileno (INACH, Chile), Polar Research Institute of China (PRIC, China), Instituto Antártico Ecuatoriano (INAE, Ecuador), Institut Polaire Français Paul Emile Victor (IPEV, France), Alfred Wegener Institute (AWI, Germany), National Institute of Polar Research (NIPR, Japan), Korea Polar Research Institute (KOPRI, Republic of Korea), Antarctica New Zealand (New Zealand), Arctic and Antarctic Research Institute (AARI, Russia), Spanish Polar Committee (CPE, Spain), British Antarctic Survey (BAS, UK), and the US National Science Foundation (NSF, USA)

We present an improved method for estimating accumulation and compaction rates of dry snow in Antarctica with ground penetrating radar (GPR). Using an estimate of the emitted waveform from direct measurements, we apply deterministic deconvolution via the Fourier domain to GPR data with a nominal frequency of 500 MHz. This reveals unambiguous reflection horizons which can be observed in repeat measurements made one year apart. At two measurement sites near Scott Base, Antarctica, we extrapolate point measurements of average accumulation from snow pits and firn cores to a larger area by identifying a dateable dust layer horizon in the radargrams. Over an 800 m × 800 m area on the McMurdo Ice Shelf (77°45´ S, 167°17´ E) the average accumulation is found to be 269 ± 9 kg m−2 a−1. The accumulation over an area of 400 m × 400 m on Ross Island (77°40´ S, 167°11´ E, 350 m a.s.l.) is found to be higher (404 ± 22 kg m−2 a−1) and shows increased variability related to undulating terrain. Compaction of snow between 2 m and 13 m depth is estimated at both sites by tracking several internal reflection horizons along the radar profiles and calculating the average change in separation of horizon pairs from one year to the next. The derived compaction rates range from 7 cm m−1 at a depth of 2 m, down to no measurable compaction at 13 m depth, and are similar to published values from point measurements.

Twelve years of horizontal wind data from the Scott Base MF radar and the Halley SuperDARN radar recorded between January 1996 and December 2007 are analysed to study the interannual variability of the migrating (S=2) and non-migrating (S=1) components of the semidiurnal tide around 78°S in the Antarctic upper mesosphere. Significant quasi-biennial modulation of the summer time S=1 component is observed. During early summer the amplitude of the component is up to 4 ms−1 stronger during the easterly phase of the equatorial stratospheric quasi-biennial oscillation (QBO) measured at 30 hPa. No statistically significant effect is seen in amplitude of the migrating component of the tide, or in the phase (time of maximum) of either component. These results are discussed in the light of previous observations of the interannual variability of the semidiurnal tide.

This study examines the utility of the Empirical Mode Decomposition (EMD) time-series analysis technique to separate the horizontal wind field observed by the Scott Base MF radar (78° S, 167° E) into its constituent parts made up of the mean wind, gravity waves, tides, planetary waves and instrumental noise. Analysis suggests that EMD effectively separates the wind field into a set of Intrinsic Mode Functions (IMFs) which can be related to atmospheric waves with different temporal scales. The Intrinsic Mode Functions resultant from application of the EMD technique to Monte-Carlo simulations of white- and red-noise processes are compared to those obtained from the measurements and are shown to be significantly different statistically. Thus, application of the EMD technique to the MF radar horizontal wind data can be used to prove that this data contains information on internal gravity waves, tides and planetary wave motions. Examination also suggests that the EMD technique has the ability to highlight amplitude and frequency modulations in these signals. Closer examination of one of these regions of amplitude modulation associated with dominant periods close to 12 h is suggested to be related to a wave-wave interaction between the semi-diurnal tide and a planetary wave. Application of the Hilbert transform to the IMFs forms a Hilbert-Huang spectrum which provides a way of viewing the data in a similar manner to the analysis from a continuous wavelet transform. However, the fact that the basis function of EMD is data-driven and does not need to be selected a priori is a major advantage. In addition, the skeleton diagrams, produced from the results of the Hilbert-Huang spectrum, provide a method of presentation which allows quantitative information on the instantaneous period and amplitude squared to be displayed as a function of time. Thus, it provides a novel way to view frequency and amplitude-modulated wave phenomena and potentially non-linear interactions. It also has the significant advantage that the results obtained are more quantitative than those resultant from the continuous wavelet transform.

This paper combines four years of radar wind data from Halley and Scott Base in an attempt to resolve the zonal structure of the semi-diurnal tide over Antarctica and to determine the dominant component as a function of time. If only one odd and one even wavenumber component are present then it is possible to calculate the amplitudes and phases of those using data from two stations that are similar in latitude but separated in longitude, ideally by 180 ∘ . Assuming that the main components of the semi-diurnal tide at Scott Base and Halley latitudes are wavenumber 1 and 2, as has been suggested by other studies, we are able to determine the seasonal variation of both components. The wavenumber 1 component is often dominant in summer, in winter a mixture of wavenumber 1 and 2 is observed. In addition the longitude at which the maxima of the components coincide was determined and found to vary relatively little. Amplitude variations and the longitudes of maxima coincidence were used to investigate the generation mechanisms of the wavenumber 1 wave. Interaction between planetary waves with wavenumber 1 and the wavenumber 2 semi-diurnal tide was found to be a potential mechanism for the generation of the wavenumber 1 component of the tide, confirming theories and observations discussed in the literature. For periods where this explanation for the generation of the wavenumber 1 tide fails we suggest asymmetric heating through solar particle flux as an additional source.

Analysis of monthly mean wind velocities from a medium frequency (MF) radar at Scott Base (78°S, 167°E), Antarctica, are presented. The radar has been measuring winds in the upper mesosphere and lower thermosphere region since December 1982. Previous work related to this instrument focused on only small portions of the larger dataset. The present study examines changes in the full dataset with respect to phenomena associated with multi-year periods, namely the quasi-biennial oscillation (QBO) and the solar cycle, and long-term trends. No relationship between these phenomena and the mean winds observed are identified. A negative correlation between the amplitude of the semi-diurnal tide and solar activity is found, similar to other mesosphere and lower thermosphere observations. This correlation is also observed for the diurnal tide to a lesser extent. Other observations have usually indicated negative trends in tidal amplitudes, however, a positive trend is found in tidal amplitudes at Scott Base since 1987. Processes that could lead to explanations for the observed seasonal variations, solar cycle dependence and the amplitude trend of the tides are discussed.

A small number of studies have indicated that reductions in the signal strength of clear air returns can be observed at low altitudes in regions of precipitation. This study uses data from the NERC MST radar facility in Aberystwyth (52.4° N, 4.1° W) and co-located tipping bucket rain gauge data to determine whether this effect can be observed for all periods where high rainfall rates were observed at the ground. The period selected for examination includes all of the days where a peak rainfall rate of 6mm h-1 was exceeded in 2001. A statistical examination of VHF radar signal power during periods with and without surface rainfall suggests that the returned power is reduced by the presence of precipitating clouds. The corrected spectral width of the Doppler spectra is also significantly wider during periods of precipitation. The process which causes the decrease in the VHF signal power seems to be associated with a reduction in Fresnel reflection within precipitating clouds. This, in turn, may be due to a reduction of humidity gradients in clouds. UHF wind profiler data is also used to show that there is a relationship between enhanced UHF returns (signifying precipitation) and reduced VHF returns. To clarify the processes and effects observed we examine three case studies which show typical relationships between the VHF signal power and surface rainfall or enhanced UHF signal-to-noise ratios. The effect of precipitation on the signal processing scheme's derivation of signal power and spectral width is explored using individual Doppler spectra.

The effects of view zenith angle on the relationship between three spectral vegetation indices (NDVI, SAVI, GEMI) and forest biophysical characteristics is examined. The work uses both simulated reflectance data, derived from a Monte Carlo ray-tracing model, and multiangle Advanced Solid-state Array Spectrometer (ASAS) data. Factorial analysis of variance was applied to the simulated reflectance data set in order to quantify the view angle dependence of the relative sensitivity of an index to independent variations in forest cover, crown leaf-area-index (LAI), stand structure, and background reflectance. Overall, it was found that off-nadir viewing improved the performance of the indices for discriminating cover and LAI, when compared with nadir viewing. The potential of the indices for discriminating forest cover was low either because of strong sensitivity to the other stand characteristics, or because of nonlinear relationships with cover. At low covers, GEMI performed best in the forward-scatter direction, while GEMI had poor sensitivity in the nadir case. The potential for discriminating crown LAI was good; both SAVI and GEMI were found to be sensitive to crown LAI in the backscatter direction. In the plane perpendicular to the solar principal plane, off-nadir viewing gave improved sensitivity to cover (NDVI) and to LAI (GEMI). The results indicate that Sun–sensor geometry may be exploited to improve discrimination of forest characteristics using spectral indices, and further work is needed to test these findings with satellite data. The results are discussed in the context of off-nadir viewing sensors such as the ATSR-2.

This work presents an investigation of the forest information content of spectral vegetation indices including the SR, NDVI, PVI, SAVI, TSAVI, and GEMI. The results were derived from two separate reflectance models including a geometric–optical and a Monte Carlo ray-tracing model, assuming nadir viewing. The effects of the atmosphere on the indices were also examined, using the 6S atmospheric code, and a short section is devoted to an analysis of Landsat Thematic Mapper data. Overall, the indices were not linear with respect to forest cover and, in many situations, an index did not have a unique value for a given cover. The general form of the indices’ responses to changes in cover could be explained by shadowing effects, linked to the heterogeneous nature of forest. The indices were also significantly affected by perturbations including solar zenith angle, background reflectance, stand structure, and leaf-area index. The relative importance of these perturbations depended on both the cover and on the particular spectral index tested. Existing indices should be selected according to the characteristics of the site under investigation, since no single index performed well over all covers. At low covers, GEMI performed best, where a decrease in the value of GEMI corresponded to an increase in crown cover. There was some evidence that the soil-adjusted indices (PVI, SAVI, TSAVI, and GEMI) partially reduced background reflectance effects in the data, although the reductions were not always associated with increased sensitivity to crown cover. At high covers, SAVI and TSAVI were found to perform best, since these indices had large dynamic ranges and small susceptibility to atmospheric perturbations. Further work is required to develop spectral indices specifically for conifer forests.

Lidar observations of Rayleigh backscatter and temperature profiles measured by ozonesondes have been used to investigate gravity waves in the upper and lower stratosphere, respectively, over Aberystwyth (52.4°N, 4.1°W). Both data sets have been used to investigate the vertical wavenumber spectrum of the wave field at high wavenumbers. Similar analytic techniques applied to each data set enable direct comparison of the spectra. The possibility of laminar structures generated by differential advection contaminating gravity-wave fields deduced from temperature and/or density measurement is discussed and the behaviour of the wave-field mean potential energy reveals a seasonal cycle throughout the stratosphere with a late winter maximum and a summer minimum.

[1] A seasonal analysis of the relationship between mesoscale orographic gravity wave activity and polar stratospheric cloud (PSC) composition occurrence around the whole of Antarctica is presented. Gravity wave variances are derived from temperature measurements made with the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System Radio Occultation (GPS-RO) satellites. Data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite are used to determine the PSC composition class distribution and spatial volume. The results show intermittent large wave activity above the Antarctic Peninsula which is coincident with large volumes of H2O ice PSCs. These ice PSC volumes advect downstream, where increases in nitric acid trihydrate (NAT) PSC volumes occur, supporting the mountain wave seeding hypothesis. During winter 2007 in the latitude range 60°S–70°S, near the edge of the vortex and where temperatures are close to PSC formation thresholds, 30% of all PSCs are attributable to orographic gravity waves. In the separate composition classes, around 50% of both H2O ice PSCs and a high NAT number density liquid–NAT mixture class of PSCs are due to these waves. While we show that planetary waves are the major determinant of PSC presence at temperatures close to the NAT formation threshold, we also demonstrate the important role of mesoscale, intermittent orographic gravity wave activity in accounting for the composition and distribution of PSCs around Antarctica.

[1] This study utilizes COSMIC satellite and lidar observations to examine the spatial and temporal variability of stratospheric temperature at a number of scales. The geographic variation of the RMS temperature difference between pairs of COSMIC profiles shows a strong correspondence to previous climatologies of gravity wave activity. In addition, the second-order structure functions we form can be directly related to the horizontal wave number power spectrum. These structure functions for different seasons and altitudes display a close correspondence to previous studies which examined the form of the horizontal wave number power spectra. Our analysis suggests that the wavefield may be particularly affected by changes in the zonal wind between 15 and 25 km and that the wind reversal between tropospheric westerlies and stratospheric easterlies in summer strongly contributes to critical-level filtering. Inspection also shows that longer horizontal wavelength waves are preferentially removed in this region. At low altitudes, the variability related to gravity waves shows a remarkably similar pattern as a function of horizontal separation in both hemispheres but is quite different at higher altitudes. Such contrast implies that seasonal variability at higher altitudes may be dominated by changes in propagation conditions in the lower stratosphere. Examination of temperature variability as a function of spatial and temporal separation indicates that gravity wave activity dominates stratospheric temperature variability, and this has impacts on validation study site selection. For example, validation exercises in the summer hemisphere stratosphere are likely to be less affected by geophysical variability than those in the winter hemisphere.

The longitudinal variability of the semidiurnal tide in the Antarctic upper mesosphere is investigated by comparison of observations from two radars at approximately opposite sides of Antarctica. Under the assumption that the tide is composed of an S = 2 (migrating) and S = 1 (westward-propagating, non-migrating) component only, the relative phases of the components are shown to vary with season such that the waves are typically in constructive interference during the winter (summer) months at longitudes around 0 degrees E (180 degrees E). We show that this has profound effects on the seasonal behaviour of the semidiurnal tide around 78 degrees S dependent on the longitude, and that no single-station observations at this latitude can be considered representative of a "zonal mean". The superposition of these two waves is used to interpret differences in previously-published ground-based climatologies of the tide. Citation: Hibbins, R. E., O. J. Marsh, A. J. McDonald, and M. J. Jarvis (2010), A new perspective on the longitudinal variability of the semidiurnal tide, Geophys. Res. Lett., 37, L14804, doi:10.1029/2010GL044015.

Overland flow is an important component of peatland hydrology. Hydrological models of peatlands are being developed that require estimates of flow velocity and its controls. However, surprisingly little is known about overland flow velocities in peatlands. Some peatlands have also been drained using open ditches, and these need to be incorporated into flow models. This paper presents field data on the velocity of overland flow and drain flow in upland peatlands. The relationships between flow velocity, vegetation cover, slope, and water depth are explored. Sphagnum provided a significantly greater effective hydraulic roughness to overland flow than peatland grasses. In all cases, a significant break in process occurred for flows with water depths of around 1 cm so that there were two components of the roughness curve. This is consistent with partial submergence theory for very shallow flows where resistance increases with depth as the soil surface first becomes fully submerged. While each surface cover type should be considered separately, the results also suggest that a first-order estimate of Darcy-Weisbach roughness and mean velocity can be based on a single parameter for each surface cover. This paper presents an empirical overland flow velocity forecasting model that can be applied to peatlands. The model combines the partially submerged component for flows with water depths below 1 cm with the fully submerged component for flows with depths up to 5 cm, which are representative of the depths of flows that occur across peatlands.

[1] The seasonal, geographical, and altitudinal dependence of gravity wave activity in the lower stratosphere over Antarctica is presented. Gravity wave activity is estimated by calculating potential energy, Ep, from radio occultation profiles obtained by the Challenging Minisatellite Payload/Global Positioning System (CHAMP/GPS) experiment. Significant seasonal variation of wave activity is observed. Smaller wave activity in summer is attributed to waves with small phase velocities experiencing critical level filtering. At other times of the year, when wave activity is large, less filtering occurs and the strong background wind is likely to cause Doppler shifting of waves to longer vertical wavelengths, which can reach larger amplitudes before saturating. Relationships between gravity wave activity and geographic location indicate that topography is a strong source for wave activity especially over the Antarctic Peninsula. However, wind rotation in this area was found to reduce the wave energy in summer at certain altitudes. A strong enhancement of wave energy is observed at the edge of the polar vortex. Again, reduced critical level filtering and Doppler shifting in the area of the jet are likely to be major causes for this finding. Possible limitations to this study due to observational filtering are discussed.

[1] A function that approximates atmospheric tidal behavior in the polar regions is described. This function is fitted to multistation radar measurements of wind in the mesosphere and lower thermosphere with the aim of obtaining a latitude-longitude-height description of the variation of tides over the whole Antarctic continent. Archival wind data sets are combined with present-day ones to fill the spatial distribution of the observations and to reduce the potential effects of spatial aliasing. Multiple years are combined through the compilation of monthly station composite days, yielding results for each month of the year. Despite potential problems associated with year-to-year variations in the tidal phase, a useful climatology of Antarctic zonal and meridional tidal wind components is compiled. The results of the fits reproduce the major features of the high-latitude tidal wind field: the dominance of the semidiurnal migrating mode in the winter months and the presence of a semidiurnal zonal wave number one component in the summer months. It is also found that the summer semidiurnal tide contains a zonal wave number zero component.

High spatial resolution 3-D SAR imagery was recorded by the UK' s Natural Environment Research Council GB-SAR Microwave Measurement Facility at the University of She c eld. X-band VV polarisation measurements were made using a near-e eld monostatic imaging system inside an anechoic chamber. The measurement process employs vector network analyser techniques to sample backscattered signalsover a 2-D aperture, allowing a 3-D reconstruction of a target. This technique is used to provide a detailed 3-D map of the spatial scattering behaviour of a small Colorado Blue Spruce tree ( Picea pungens glauca ). The images produced are at a su c ciently high spatial resolution ( ~5cm) that individual plant components can be discerned. An ability to select any volume pixel from within the target allows features in the microwave reconstruction to be readily associated with structures in the tree. The scattering behaviour associ- ated with the uppermost set of branches shows it to be dominated by scattering from the branch tips.

Previous studies have indicated that VHF clear-air radar return strengths are reduced during periods of precipitation. This study aims to examine whether the type of precipitation, stratiform and convective precipitation types are identified, has any impact on the relationships previously observed and to examine the possible mechanisms which produce this phenomenon. This study uses a combination of UHF and VHF wind-profiler data to define periods associated with stratiform and convective precipitation. This identification is achieved using an algorithm which examines the range squared corrected signal to noise ratio of the UHF returns for a bright band signature for stratiform precipitation. Regions associated with convective rainfall have been defined by identifying regions of enhanced range corrected signal to noise ratio that do not display a bright band structure and that are relatively uniform until a region above the melting layer. This study uses a total of 68days, which incorporated significant periods of surface rainfall, between 31August 2000 and 28February 2002 inclusive from Aberystwyth (52.4° N, 4.1° W). Examination suggests that both precipitation types produce similar magnitude reductions in VHF signal power on average. However, the frequency of occurrence of statistically significant reductions in VHF signal power are very different. In the altitude range 2-4 km stratiform precipitation is related to VHF signal suppression approximately 50% of the time while in convective precipitation suppression is observed only 27% of the time. This statistical result suggests that evaporation, which occurs more often in stratiform precipitation, is important in reducing the small-scale irregularities in humidity and thereby the radio refractive index. A detailed case study presented also suggests that evaporation reducing small-scale irregularities in humidity may contribute to the observed VHF signal suppression.

[1] The signature of mid-latitude convection observed by a 46.5 MHz wind-profiling radar at Aberystwyth, UK (52.4°N, 4.0°W), is demonstrated by way of a case study. A key feature of such radars is their ability to measure the vertical air velocity directly, even under conditions of precipitation. Higher frequency radars, which have more typically been used for such studies, measure the net effect of hydrometeor terminal velocities and the air motion under such conditions. The radar is capable of observing both updrafts and downdrafts, with peak updrafts of the order of 10 m s−1. The signature of convection additionally consists of enhanced values of the radar return spectral width, which cannot be interpreted in terms of turbulence intensity, and reduced reliability of the corresponding radar-derived horizontal wind components. When convection reaches the uppermost troposphere, the radar return signal power can be anomalously large leading to the radar-derived tropopause altitude being overestimated. The presence of convection is confirmed by thermal infrared satellite imagery of high-topped clouds. Use is also made of radiosonde and UHF wind-profiler data.

Radar measurements at Aberystwyth (52.4° N, 4.1° W) of winds at tropospheric and lower stratospheric heights are shown for 12-13 March 1994 in a region of highly curved flow, downstream of the jet maximum. The perturbations of horizontal velocity have comparable amplitudes in the troposphere and lower stratosphere with downward and upward phase propagation, respectively, in these two height regions. The sense of rotation with increasing height in hodographs of horizontal perturbation velocity derived for hourly intervals show downwards propagation of energy in the troposphere and upward propagation in the lower stratosphere with vertical wavelengths of 1.7 to 2.3 km. The results indicate inertia-gravity waves propagating in a direction similar to that of the jet stream but at smaller velocities. Some of the features observed contrast with those of previous observations of inertia-gravity waves propagating transverse to the jet stream. The interpretation of the hodographs to derive wave parameters has taken account of the vertical shear of the background wind transverse to the direction of wave propagation.Key words. Meteorology and atmospheric dynamics (mesoscale meteorology; middle atmosphere dynamics; waves and tides)

A model is developed which treats the floodplain as a storage area for flows in excess of the channel capacity. The volume of overspill is distributed over the floodplain restricted only by the major division of the floodplain and the topography. An example of the application of the model is given for a section of the floodplain of the River Nith in SW Scotland. Flood area prediction for a 5-yr. return interval flood is shown to be 85% accurate. It is argued that storage models are more appropriate and potentially more useful than channel extension models.

The conditions prior to and the site and results of a debris flow are described. Baseline data collected after this extreme event are outlined. Similarities between antecedent conditions of this flow and others reported in the literature are noted. From the field evidence comment is made on the mobile nature of the flow which may be analogous to spontaneous liquefaction in sands.

International audience; • Current-era GCMs overestimate sea spray concentrations relative to measurements in the Ross Sea • Better constraints for sea spray flux were found by tuning wind-speed based parameterizations to these observations • Variations in sea surface temperature did not explain further variability within the temperature range studied