Airborne radar measurements of ocean wave spectra and wind speed during the grand banks ERS‐1 SAR wave experiment

Abstract

Measurements of ocean directional wave spectra, significant wave height, and wind speed over the Grand Banks of Newfoundland were made using the combined capabilities of the radar ocean wave spectrometer (ROWS) and scanning radar altimeter (SRA). The instruments were flown aboard the NASA P‐3A aircraft in support of the Grand Banks ERS‐1 Synthetic Aperture Radar (SAR) Wave Experiment. The NASA sensors use proven techniques, which differ greatly from SAR, for estimating the directional long‐wave spectrum; thus they provide a unique set of measurements for use in evaluating SAR performance. ROWS and SRA data are combined with spectra from the SAR aboard the Canadian Centre for Remote Sensing (CCRS) CV‐580 aircraft, the first‐generation Canadian Spectral Ocean Wave Model (CSOWM) hindcast, and other available in situ measurements to assess the ERS‐1 SAR's ability to correctly resolve wave field components along a 200‐ to 300‐km flight line for four separate satellite passes. Given the complex seas present on the Grand Banks, the complementary nature of viewing the sea spectrum from the perspectives of multiple sensors and a wave prediction model is apparent. The data intercomparisons show the ERS‐1 SAR to be meeting the expected goals for measuring swell, but the data also show evidence of this remote sensor's inability to detect the shorter waves travelling in the azimuth or along‐track direction. Example SAR spectra simulations are made using a non‐linear forward transform with ROWS measurements as input. Additionally, surface wind and wave height estimates made using the ROWS altimeter channel are presented. These data demonstrate the utility of operating the system in its new combined altimeter and spectrometer configuration.     

Analysis of marine radar image spectra collected during the grand banks ERS‐1 SAR wave experiment

Abstract

Remote measurements of wave spectra were made using MacLaren Plansearch's Marine Radar Wave Measuring System (Macrada^©) for the duration of the ERS‐1 Grand Banks Experiment (10–27 November 1991). The system consists of a Furuno X‐band marine navigation radar and a personal computer. The system was installed and operated onboard the BIO Research vessel CSS Hudson during the experiment. The radar system provides fully directional wave spectra in real time (within minutes of image acquisition) as well as archiving of the raw images for future analysis. During each of the satellite overpasses, excellent data were collected, covering a variety of wave conditions.

This paper describes the data analysis procedures and presents the results of the experiment. In addition, a sensitivity analysis is carried out to study: (1) sensitivity to the number of radar images processed on the resultant spectra, (2) effect of missing one or more images in a dataset, and (3) sensitivity to azimuth angle variation. Finally, the results are compared with corresponding directional wave spectra from other sensors (such as WAVEC Buoy and ERS‐1 SAR).     

Comparison of airborne electromagnetic ice thickness data with NOAA/AVHRR and ERS‐1/SAR images

Abstract

Snow‐plus‐ice thickness and surface‐ice roughness data collected by a helicopter‐towed sensor package was used to identify surface‐ice properties in March 1992 AVHRR and SAR images for the land‐fast and mobile pack ice off the northern coast of Newfoundland. The sensor package consisted of an electromagnetic induction sensor and laser profilometer. Observed snow depths and ice thicknesses verified that snow‐plus‐ice thickness over undeformed ice can be obtained to an accuracy of ±10 cm. Snow‐plus‐ice thickness and surface roughness data for flight sections covering several hundred kilometres indicated the change in pack ice properties seen in images from thin, smooth coastal ice and open water conditions to thick, rough consolidated offshore pack ice. Ice charts covering the same area showed similar variations in ice conditions based on AVHRR and fixed‐wing reconnaissance data. In the ERS‐1 SAR image, low backscattering coefficients were associated with large, smooth coastal floes interspersed with areas of high backscatter indicating the presence of waves in open water areas. Backscattering coefficients were higher in the rubble areas near the inshore edge of the pack ice than in the interior of the pack ice itself. Distinguishing ice types on the basis of tone alone in SAR imagery was found to be problematic; however in combination with other remotely sensed data such as AVHRR data, SAR data will become more useful in distinguishing ice types.     

Comparison and fusion of co‐occurrence,Gabor and MRF texture features for classification of SAR sea‐ice imagery

Abstract

Image texture interpretation is an important aspect of the computer‐assisted discrimination of Synthetic Aperture Radar (SAR) sea‐ice imagery. Co‐occurrence probabilities are the most common approach used to solve this problem. However, other texture feature extraction methods exist that have not been fully studied for their ability to interpret SAR sea‐ice imagery. Gabor filters and Markov random fields (MRF) are two such methods considered here. Classification and significance level testing shows that co‐occurrence probabilities classify the data with the highest accuracy, with Gaborfilters a close second. MRF results significantly lag Gabor and co‐occurrence results. However, the MRF features are uncorrelated with respect to co‐occurrence and Gabor features. The fused co‐occurrence/MRF feature set achieves higher performance. In addition, it is demonstrated that uniform quantization is a preferred quantization method compared to histogram equalization.     

Ground‐based measurements of ozone and NO2 during MANTRA 1998 using a Zenith‐sky spectrometer

Abstract

A portable ground‐based instrument has been constructed for the automated measurement of vertical column abundances of a number of gases pertinent to stratospheric ozone chemistry. The instrumentation is described in this paper and results are presented from the first set of field measurements, made during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) 1998 field campaign at Vanscoy, Saskatchewan, Canada. Zenith‐sky spectra in the near ultraviolet and visible wavelength regions were recorded for a period of seven days, prior to and following the launch of the MANTRA balloon on 24 August 1998. The spectra were then analysed using the differential optical absorption spectroscopy (DOAS) technique in conjunction with a radiative transfer model to determine vertical column amounts of ozone and NO₂. Ozone measurements compared favourably with concurrent observations by ozonesondes, a Brewer spectrophotometer, and satellite instruments. Vertical NO₂ columns were in broad agreement with those determined by the Global Ozone Monitoring Experiment (GOME) satellite instrument.     

C‐band SAR backscatter characteristics of Arctic sea and land ice during winter

Abstract

Synthetic Aperture Radar (SAR) data has become an important tool for studies of polar regions, due to high spatial resolution even during the polar night and under cloudy skies. We have studied the temporal variation of sea and land ice backscatter of twenty‐four SAR images from the European Remote Sensing satellite (ERS‐1) covering an area in Lady Ann Strait and Jones Sound, Nunavut, from January to March 1992. The presence of fast ice in Jones Sound and glaciers and ice caps on the surrounding islands provides an ideal setting for temporal backscatter studies of ice surfaces. Sample regions for eight different ice types were selected and the temporal backscatter variation was studied. The observed backscatter values for each ice type characterize the radar signatures of the ice surfaces. This time series of twenty‐four SAR images over a 3‐month period provides new insights into the degree of temporal variability of each surface. Ice caps exhibit the highest backscatter value of ‐3.9 dB with high temporal variability. Valley glacier ice backscatter values decrease with decreasing altitude, and are temporally the most stable, with standard deviations of 0.08–0.10 dB over the 90‐day period.

First‐year ice and lead ice show a negative trend in backscatter values in time and a positive correlation of up to 0.59 with air temperature over the 90‐day period. For first‐year ice and lead ice, episodes of large temperature fluctuations (±12°C) are associated with rapid changes in backscatter values (±2 dB). We attribute the backscatter increase to a temperature‐induced increase in brine volume at the base of the snow pack. Multi‐year ice, conglomerate ice and shore ice are relatively stable over the 3‐month period, with a backscatter variation of only a few dBs. An observed lag time of up to three days between backscatter increase/decrease and air temperature can be attributed to the insulation effect of the snow cover over sea ice. The net range of the backscatter values observed on the most temporally stable surface, valley glacier ice, of about 0.30 dB indicates that the ERS‐1 SAR instrument exceeds the 1 dB calibration accuracy specified for the Alaska SAR Facility processor for the three winter months.     

A revised method for determining the direct and diffuse components of the total short‐wave radiation

A relationship, derived by Liu and Jordan (1960), under which the total short‐wave radiation may readily be subdivided into its direct and diffuse components is shown to vary both spatially and seasonally. This variability is attributed to changes in the importance of the multiple reflection of short‐wave radiation between the earth's surface and atmosphere. A revised relationship, which incorporates the influence of this process, is shown to have applicability at a large number of Canadian locations.     

Evaluating regional atmospheric water vapour estimates derived from GPS and short‐range forecasts of the Canadian Global Environmental Multiscale model in southern Alberta

Abstract

Integrated atmospheric moisture has been derived from a network of Global Positioning System (GPS) receivers established in southern Alberta. GPS receivers and post‐processing techniques provide the ability to estimate integrated precipitable water vapour (PWV) at temporal and spatial scales not usually available using conventional observational techniques and without costly expendables. GPS‐derived PWV was evaluated during the Alberta GPS Atmospheric Moisture Evaluation (A‐GAME) using nearby radiosonde observations from the Airdrie, Olds‐Didsbury and Sundre airports during field campaigns in the summers of 2003 and 2004. For the 2004 A‐GAME period, the regional (15 km) Global Environmental Multiscale model (GEM)‐modelled PWV was compared to the GPS derived PWV using a distance weighting approach. GEM model performance was assessed with regards to prognosis time (from 0 to 9 hours), grid cell elevation, location and the presence of storms in the study region. The results show that there is good agreement between radiosonde‐derived PWV and PWV derived from nearby GPS sites with correlations (r²) ranging from 0.76 to 0.84; the GPS‐derived PWV showed a small dry bias averaging 0.6 mm. When compared to GPS‐derived PWV, GEM model performance was found to be favourable out to the hour‐3 prognosis with an overall correlation (r²) of 0.63. Performance decreased with increasing prognosis time and as a result of the presence of storm activity in the study region but did not decrease with increasing grid cell elevation.     

Variability peculiarities of distribution of transparency and suspended matter content in the northwestern part of the Black Sea in spring caused by large-scale processes in the ocean–atmosphere system

The display is considered of global processes in the ocean-atmosphere system in the variability of hydrophysical and hydrobiological fields of the northwestern part of the Black Sea in spring period of 1978–1995. It is demonstrated that the variability of North Atlantic and Southern oscillations in winter-spring period affects the spring hydrometeorological conditions in catchment areas of European rivers of the Black Sea basin causing the variability of runoff volumes of these rivers and the scales of spreading river waters at the northwestern shelf. Hydrological and hydrobiological characteristics of shelf waters varying in the process influence the formation of distribution of suspended matter content and transparency.     

A decade of cloud‐to‐ground lightning in Canada: 1999–2008. Part 1: Flash density and occurrence

Abstract

Flash density and occurrence features for more than 23.5 million cloud‐to‐ground (CG) lightning flashes detected by the Canadian Lightning Detection Network (CLDN) from 1999 to 2008 are analyzed on 20 × 20 km equal area squares over Canada. This study was done to update an analysis performed in 2002 with just three years of data. Flashes were detected throughout the year, and distinct geographic differences in flash density and lightning occurrence were observed. The shape and locations of large scale patterns of lightning occurrence remained almost the same, although some details were different. Flash density maxima occurred at the same locations as found previously: the Swan Hills and Foothills of Alberta, southeastern Saskatchewan, southwestern Manitoba and southwestern Ontario. A region of greater lightning occurrence but relatively low flash density south of Nova Scotia occurred at the same location as reported previously. New areas of higher flash density occurred along the US border with northwestern Ontario and southern Quebec. These appear to be northward extensions of higher flash density seen in the previous study. The greatest average CG flash density was 2.8 flash km^?2 y^?1 in southwestern Ontario, where the greatest single‐year flash density (10.3 flash km^?2 y^?1) also occurred. Prominent flash density minima occurred east of the Continental Divide in Alberta and over the Niagara Escarpment in southern Ontario.

Lightning activity is seen to be highly influenced by the length of the season, proximity to cold water bodies and elevation. The diurnal heating and cooling cycle exerted the main control over lightning occurrence over most land areas; however, storm translation and transient dynamic features complicated the time pattern of lightning production. A large portion of the southern Prairie Provinces experienced more than 50% of flashes between 22:30 and 10:30 local solar time. The duration of lightning over a 20 × 20 km square at most locations in Canada is 5–10 h y^?1, although the duration exceeded 15 h y^?1 over extreme southwestern Ontario. Lightning occurred on 15–30 days each year, on average, over most of the interior of the country. The greatest number of days with lightning in a single year was 47 in the Alberta foothills and 50 in southwestern Ontario. Beginning and ending dates of the lightning season show that the season length decreases from north to south; however, there are considerable east‐west differences between regions. The season is nearly year‐round in the Pacific coastal region, southern Nova Scotia, southern Newfoundland and offshore.     

Comment on “Kinetics of the reactions of Cl atoms with 2-buten-1-ol, 2-methyl-2-propen-1-ol, and 3-methyl-2-buten-1-ol as a function of temperature” by Rodriguez et al. (J. Atmos. Chem. (2008) 59:187–197)

The Arrhenius expressions and the data plotted in Figure 2 of Rodriguez et al. 2008 give rate coefficients of approximately 2?×?10^-8 cm³ molecule^-1 s^-1 at 255 K. Such values are approximately two orders of magnitude larger than expected from simple collision theory (Finlayson-Pitts and Pitts 1986). The rate coefficients reported at sub-ambient temperatures are substantially greater than the gas kinetic limit and are not physically plausible. The rate coefficients reported by Rodriguez et al. imply a long range attraction between the reactants which is not reasonable for reaction of neutral species such as chlorine atoms and unsaturated alcohols. We also note that the pre-exponential A factors (10^-23-10^-20) and activation energies (?15 kcal mol^-1) are not physically plausible. We conclude that there are large systematic errors in the study by Rodriguez et al. (Atmos Chem 59:187–197, 2008).