Observations of the snowcover on sea ice in the Gulf of Bothnia

Abstract

During March 1988, detailed measurements of the physical properties and depth distributions of the snow cover on the sea ice in the Gulf of Bothnia were made as part of BEPERS-88 (Bothnian Experiment in Preparation for ERS-1). The observations included profiles of the density, crystal structure. salinity, temperature, and brine volume (at 1-2 cm depth intervals), and 1084 snow depth measurements. These data are presented along with discussions of the implications of the measured properties for the interpretation of SAR imagery, and the use of laser profilometry for remotely estimating ice surface roughnesses.     

The Effect of Posture on the Running Speed of Skiing

Abstract

These experiments were performed at the Syowa Station in Antarctica (69°00'S, 39° 35'E) where a straight ski course was constructed (length 100m. gradient 12°). The conditions were as follows; wind velocity: 0-0·5ms^?1, temperature: ? 25°C, snow temperature: ?29·5°C.snow density: 0·309 gcm^?3. Running speed was measured by a coil-magnet system. The subject with a small magnet fixed on one leg ran closely down past the coils placed every 5m along the course. Two healthy members of the Japanese Antarctic Research Expedition who were well-trained skiers acted as subjects. Running speed was measured in three different postural conditions (a) standing, (b) egg-shaped, (c) starting in the egg-shaped posture and then standing erect in the latter half of the course. Velocity was measured throughout the course and the following results were obtained: velocities at the final section of the coils, which were placed 65m from the start point, were as follows for each postural condition: Subject KW; (a) standing, 7·6ms^?1 (b) egg-shaped, 8·6ms^?1 (c) egg-shaped followed by standing erect, 8·2ms^?1. Subject B; (a) 7·8ms^?1, (b) 8·9ms^?1, (c) 8·4ms^?1. The friction between the ski and snow was calculated as about 0·13 for all conditions. It was concluded that postural difference may have a considerable and clear-cut effect on running speed even when the skiing speed is rather low.     

Fully polarimetric airborne SAR and ERS SAR observations of snow: implications for selection of ENVISAT ASAR modes

Snow cover has a substantial impact on processes involved in the interaction between atmosphere and surface, and the knowledge of snow parameters is important in both climatology and weather forecasting. With the upcoming launch of Advanced Synthetic Aperture Radar (ASAR) instruments on Envisat, enhanced snow-mapping capabilities are foreseen. In this paper fully polarimetric C- and L-band airborne SAR data, ERS SAR and auxiliary data from various snow conditions in mountainous areas are analysed in order to determine the optimum ASAR modes for snow monitoring. The data used in this study are from the Norwegian part of the snow and ice experiment within the European Multi-sensor Airborne Campaign (EMAC'95) acquired in the Kongsfjellet area, located in Norway, 66°?N, 14°?E. Fully polarimetric C- and L-band SAR data from ElectroMagnetic Institute SAR (EMISAR), an airborne instrument operated by the Danish Center for Remote Sensing (DCR), were acquired in March, May, and July 1995. In addition, several ERS SAR, airborne photos, field and auxiliary data were acquired.

A larger separation between wet snow and bare ground in EMISAR C-VV polarisation data was found at high incidence angle (55°) compared to lower incidence angle (45°). Cross-polarized observations from bare ground, dry and wet snow in the incidence angle range 35° to 65° are below the specified Envisat ASAR noise floor of –20–22 dB. The backscattering angular dependency for wet snow and bare ground derived from EMISAR C-VV and ERS SAR data corresponds well, and agrees to some extent with volume and surface scattering model results. The C-band is more sensitive to variation in snow properties than the L-band.     

The effect of changing environmental conditions on microwave signatures of forest ecosystems: preliminary results of the March 1988 Alaskan aircraft SAR experiment

Abstract

In preparation for the first European Space Agency (ESA) Remote Sensing(ERS-I) mission,a series of multitemporal, multifrequency, multipolarization aircraft synthetic aperture radar (SAR) data sets were acquired over the Bonanza Creek Experimental Forest near Fairbanks, Alaska in March, 1988. P-, L- and C-band data were acquired with the NASA/JPL Airborne SAR on five differentdays over a period of two weeks. The airborne data were augmented with intensiveground calibration data as well as detailed, simultaneous in situ measurements of the geometric, dielectric and moisture properties of the snow and forest canopy. During the time period over which the SAR data were collected, the environmental conditions changed significantly; temperatures ranged from unseasonably warm (I to 9°C) to well below freezing (-8 to - 15°C), and the moisture content of the snow and trees changed from a liquid to a frozenstate. The SAR data clearly indicate the radar return is sensitive to these changing environmental factors and preliminary analysis of the L-band SAR data shows a 0·4 to 5·8dB increase (depending on polarization and canopy type) in the radar cross section of the forest stands under the warm conditions relative to the cold. These SAR observations are consistent with predictions from a theoretical scattering model. These preliminary results are presented to illustrate the opportunity afforded by the ERS-l SAR to monitor temporal changes in forest ecosystems.     

Radar estimates of soil moisture over the Konza Prairie

Abstract

Radar backscatter measurements were made as a part of the First International satellite land surface climatology project Field Experiment (FIFE) to estimate soil moisture for use by other investigators. The helicopter-mounted radar was flown along selected transects that coincided with soil moisture measurements. The radar operated at microwave frequencies of 5-3 and 9 6 GHz and at selected incidence angles between 0° and 60°. Vertical polarization was used for two days in June of 1987 and horizontal polarization was used for three days in July and October of 1987.

The scattering coefficient data from different days were grouped by frequency and antenna angles and then related to soil moisture along the flight paths using linear regression. A measure of linearity for the regression, R², ranged between 0·9 and 0·5. The larger coefficients were for X -band measurements made at large antenna incidence angles, and the smaller coefficients were for C-band measurements made: at incidence angles near vertical.     

C-band radar backscatter of Baltic sea ice: theoretical predictions compared with calibrated SAR measurements

Abstract

Airborne Synthetic Aperture Radar (SAR) data have been analysed together with in situ measurements of sea ice during the Bothnian Experiment in Preparation for ERS-1 (BEPERS) in March 1988. Based on the physical properties of the snow-covered level ice, a scattering model is used to predict the C-band like-polarization backscattering coefficient in an experiment area. Both the average backscattering coefficient and the SAR image texture were found to be in good agreement with the scattering model predictions. The backscatter signature of the level ice was found to be dominated by the ice surface r.m.s. height and autocorrelation function. These parameters were determined from profiles of the ice surface height, which were measured using a laser profiler device with sub-mm accuracy. The present model is expected to be accurate when the backscattering is dominated by scattering from the cm-scale snow or ice surface roughness.     

Advances in seasonal snow water equivalent (SWE) retrieval using in situ passive microwave measurements over first‐year sea ice

Dramatic changes have occurred in the Arctic over the past three decades in response to an accelerated warming that will have a significant impact on the world's climate. Snow accumulation (measured as snow water equivalent, SWE) over sea ice plays a key role in the changes observed due to its effect on the surface energy balance that dictates the timing of sea‐ice freeze‐up and decay. Increased awareness of the role of snow in the Arctic system has triggered numerous studies that have attempted to characterize snow accumulation from space since the early 1980s, but none has successfully quantified SWE on a seasonal basis.

This work presents the first seasonally valid SWE algorithm for first‐year sea ice based on in situ passive microwave radiometry. The in situ data were collected as a part of the Canadian Arctic Shelf Exchange Study (CASES) during the overwintering mission of the Canadian Coast Guard Ship (CCGS) Amundsen in 2003–2004. Previous work clearly demonstrated two different patterns of seasonal snow evolution, for which the algorithm presented in this paper accounts for. Our algorithm's results are valid for temperatures between ?5 and ?30°C and SWE in the range of 0–55 mm. Results show that the behaviour of the snow's thermophysical properties and brightness temperatures (T _b) is quite different in the winter cooling period compared with that in the warming period, where temperature gradient metamorphism begins at a SWE value of 33 mm. The SWE algorithm successfully models this change with a high degree of correlation.     

Passive microwave remote sensing of soil moisture

Results of radiometric measurements over bare soil obtained with a horizontally polarized microwave radiometer at 19·1 GHz frequency are presented. Radiometer measurements were made with incidence angles varying from 10 to 50°. Ground-truth acquisition of soil moisture in the 0–1 cm layer and of soil temperature near the surface was made concurrently with radiometer measurements. The measured brightness temperatures over a bare field are higher than those calculated from an emmissivity model.     

Development of a winter snow water equivalent algorithm using in situ passive microwave radiometry over snow-covered first-year sea ice

A snow water equivalent (SWE) algorithm has been developed for thin and thick snow using both in situ microwave measurements and snow thermophysical properties, collected over landfast snow covered first-year sea ice during the Canadian Arctic Shelf Exchange Study (CASES) overwintering mission from December 2003 to May 2004. Results showed that the behavior of brightness temperatures (T_bs) in thin snow covers was very different from those in a thick snowpack. Microwave SWE retrievals using the combination of T_b 19 GHz and air temperature (multiple regression) over thick snow are quite accurate, and showed very good agreement with the physical data (R² = 0.94) especially during the cooling period (i.e., from freeze up to the minimum air temperature recorded) where the snow is dry and cold. Thin snow SWE predictions also showed fairly good agreement with field data (R² = 0.70) during the cold season. The differences between retrieved and in situ SWE for both thin and thick snow cover are mainly attributable to the variations in air temperature, snow wetness and spatial heterogeneity in snow thickness.     

On the accuracy of the global positioning system—A test using a hand-held receiver

Abstract

This study aims to evaluate the accuracy of location (x, y and z) using a Magellan GPS NAV 1000 receiver. The position of a known reference point was compared with 50 fix points measured in two-dimensional (2D) mode (x, y) and with 50 fix points measured in three-dimensional (3D) mode (x, y and z).

The mean errors and the root mean square errors (RMSE) in the Euclidian distances between the reference point and the measured fix points were 17·9?m and 20·0?m respectively for the 2D measurements, and 25·7?m and 34·8?m respectively for the 3D measurements. The mean error in elevation was 0·2?m and the corresponding RMSE was 39·1?m. The study indicates a significant difference in the total error (Euclidian distance) between points obtained with different signal qualities and different geometric qualities. The error in the Euclidian distance never exceeded 44·0?m using 2D mode, which suggests that the Magellan GPS NAV 1000 receiver should be a useful supportive tool for many remote sensing applications.     

Investigation of C- and X-band backscattering signatures of Baltic Sea ice

Backscattering signatures of various Baltic Sea ice types and open water leads were measured with the helicopter-borne C- and X-band Helsinki University of Technology scatterometer (HUTSCAT) during six ice research campaigns in 1992–1997. The measurements were conducted at incidence angles of 23° and 45°. The HUTSCAT data were assigned by video imagery into various surface type categories. The ground data provided further classification of the HUTSCAT data into different snow wetness categories (dry, moist and wet snow). Various basic statistical parameters of backscattering signature data were used to study discrimination of open water leads and various ice types. The effect of various physical parameters (e.g. polarization, frequency, snow condition) on the surface type discrimination was investigated. The results from the data analysis can be used to help the development of sea ice classification algorithms for space-borne SAR data (e.g. Radarsat and Envisat). According to the results from the maximum likelihood classification it is not possible to reliably distinguish various surface types in the SAR images only by their backscatter intensity. In general, the best ice type discrimination accuracy is achieved with C-band VH-polarization σ° at an incidence angle of 45°.     

A new relationship between radar cross-section and ocean surface wind speed using ERS-t scatterometer and buoy measurements

The ERS–I spacecraft scatterometer, C-band VV polarization, acquired radar cross-section measurements over the global oceans during 1992 and 1993. We investigate the cross-section dependence on mean wind speed U using collocated buoys within ±25km of the scatterometer cells. These collocated measurements result in over 75000 matches in two diITerent oceanic regions. The buoys measure hourly mean wind speeds from 0·2–10 mS ¹ and 0·2–18ms ^-1 in the equatorial Pacific Ocean and at mid^-latitudes off the North American coasts, respectively. We present experimental evidence for a new and compact exponential model dependence on wind speed. The previously used power–law form inadequately characterizes the cross-section measurements based on a single index over a large wind speed range. The cross-sectional slope varies from about zero dB/ms^-1 at high wind speeds U=18ms ^-1 and small incidence angles 0=20° to about 1·3dB/ms ^-1 at low wind speeds U=3ms ^-1 and large incidence angles, 0=55°. The CMOD4 model significantly underestimates the radar cross section measurements for U≤3ms ^-1 whereas the exponential model exhibits less bias.     

Radar backscatter measurements over saline ice

Abstract

During the 1984 and 1985 winter seasons, radar backscatter measurements were performed on artificial sea ice at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) at Hanover, New Hampshire. Radar data were collected at selected frequencies in the 4-17 GHz region for incidence angles from 0 to 60^° with like and cross polarizations. These measurements were performed on smooth, rough, bare and snow-covered saline ice and open water. Backscattering from ice increased with its thickness until the ice was about 1 cm thick and then decreased gradually with further growth. Rough ice and snow-covered ice gave similar returns at 13-6 GHz, but the scattering coefficients of snow-covered ice were lower than that of rough ice at 9-6 GHz. Depolarized scattering from smooth, thin ice and water were much lower than from rough ice and snow-covered ice.     

Derivation of a threshold function for the Advanced Very High Resolution Radiometer 3.75 µm channel and its application in automatic cloud discrimination over snow/ice surfaces

The distinct contrast between the reflectance of solar radiation in Advanced Very High Resolution Radiometer (AVHRR) channel 3 (3.75?µm) by clouds and by bright surfaces provides an effective means of cloud discrimination over snow/ice surfaces. A threshold function for the top-of-atmosphere (TOA) albedo in channel 3 (r ₃) is derived and used to develop an improved method for cloud discrimination over snow/ice surfaces that makes explicit use of TOA r ₃. Corrections for radiance anisotropy and temperature effects are required to derive accurate values of r ₃ from satellite measurements and to utilize the threshold function. It has been used to retrieve cloud cover fractions from National Oceanic and Atmospheric Administration (NOAA)-14 AVHRR data over the Arctic Ocean and over the North Slope of Alaska (NSA) Atmospheric Radiation Measurement (ARM) site in Barrow, Alaska. The retrieved cloud fractions are in good agreement with SHEBA (Surface HEat Budget of the Arctic Ocean) surface visual observations and with NSA cloud radar and lidar observations, respectively. This method can be utilized to improve cloud discrimination over snow/ice surfaces for any satellite sensor with a channel near 3.7?µm.     

Observed and modelled effects of ice lens formation on passive microwave brightness temperatures over snow covered tundra

Immediately before an April 2007 snow survey and passive microwave radiometer field campaign in the Northwest Territories, Canada, a rain-on-snow event deposited a thin (~ 3 mm) continuous layer of ice on the surface of the snowpack. At eight sites the brightness temperature (T_b) of the undisturbed snow pack was measured with a multi-frequency dual polarization (6.9, 19, 37, and 89 GHz) ground based radiometer system. The ice lens was then carefully removed and the T_bs were measured again. The individual V-pol channels and the 37 V − 19 V difference were largely unaffected by the presence of the ice lens, exhibiting a systematic shift of about 3 K. In comparison, the ice lens had a considerable effect on the H-pol T_b at all frequencies, with a mean difference (ice lens present − ice lens removed) of − 9 K (± 5.3 K) at 6.9 GHz, − 40 K (± 11.3 K) at 19 GHz, − 33 K (± 7.6 K) at 37 GHz, and − 19 K (± 8.0 K) at 89 GHz. The effect of the ice lens on H-pol measurements was also observed with spaceborne data from the Advanced Microwave Scanning Radiometer (AMSR-E) satellite data.Simulations of T_b were produced for each site using a new two layer formulation of the Helsinki University of Technology (HUT) snow emission model. The ice lens was used as the top layer and the underlying snowpack considered as a homogenous second layer. The agreement between observations and simulations was variable, with agreement strongest at 19 GHz. A comparison with simulations produced using the Microwave Emission Model of Layered Snowpacks (MEMLS) suggests HUT model uncertainty is related not to the ice lens, but to difficulties in simulating emission from deep snow. Overall, the observations and simulations suggest H-pol measurements are capable of detecting new ice layers across the tundra snowpack, while V-pol measurements are more appropriate for snow water equivalent (SWE) retrievals due to their relative insensitivity to ice layers.     

Relationship between pulse rate and energy expenditure during graded work at different temperatures

Abstract

Pulse rate and energy expenditure were measured on fifteen male subjects who had been given ergometer cycling of ‘O’ and 32·6 W to 97·8 W in three temperatures of 22°C, 30°C, and 37°C. The variation in pulse rate of each graded work period was compared with the variation in energy expenditure for each environmental temperature. The pulse rate was significantly increased with the rise of environmental temperature (p << 0·01) whereas the energy expenditure (kJ min^?1) was consistently or effectively decreased with higher graded work. However, a correlation was obtained between the pulse rate and energy expenditure during graded work up to a limit of 150 beats min^?1, 160 beats min^?1, and 170 beats min^?1 in temperatures of 22°C, 30°C, and 37°C respectively. The results were analysed by computing the analysis of variance and regression equation evaluated for each temperature, indicating that independent regression lines having two components, one above 95 and other below 95 beats min^?1 were required in each set of temperature. The percentage error between observed and predicted values (pooled) or energy expenditure for the two ranges of pulse rates varied from 0·3 to 11·5 and from 0·5 to 9·5 respectively in the three temperatures.     

Comparison of two different cooling systems in alleviating thermal and physiological strain during prolonged exercise in the heat

Abstract

This study compared the efficacy of an ice vest comprising of water (WATER) or a water-carbon (CARBON) emulsion on thermophysiological responses to strenuous exercise in the heat. Twelve male cyclists completed three 50-minute constant workload trials (55% of peak power output, ambient temperature 30.4?±?0.6°C) with WATER, CARBON, and without ice vest (CONTROL), respectively. The increase in core body temperature (Tcore) was lower in WATER at 40 (?0.49?±?0.34 °C) and 50?minutes (?0.48?±?0.48 °C) and in CARBON at 30 (?0.41?±?0.48 °C), 40 (?0.54?±?0.51 °C), and 50?minutes (?0.67?±?0.62 °C) as compared to CONTROL (p?<?0.05, ES > 0.8). While heart rate and blood lactate kinetics did not differ between the conditions, statistical main effects in favour of both WATER and CARBON were found for thermal sensation (condition p?<?0.001 and interaction p?<?0.01) and rating of perceived exertion (condition p?<?0.05). Per-cooling with CARBON and WATER similarly reduced Tcore but not physiological strain during prolonged exercise in the heat.

Practitioner Summary: Exercise in the heat is characterised by increases in thermophysiological strain. Both per-cooling with a novel carbon-based and a conventional water-based ice vest were shown to reduce core temperature significantly. However, due to its lower mass, the carbon-based system may be recommended especially for weight-bearing sports.     

Cooling of motorcyclists in various clothing during winter in Britain

Abstract

Skin and ear-canal temperatures of seven volunteer motorcyclists have been measured during control periods and during rides of up to 161km at air temperatures below 10°C. While wearing their own clothing in air temperatures between 2·6 and 10 C the riders showed changes in heat storage of - 438 to - 1611 Wm^?2. The average of the three lowest temperatures (°C) recorded from selected sites from different subjects (and the means of the laboratory control values from all seven subjects + standard errors) were: foot, 14·7 (30·6 ± 0·93); shin, 21·4 (32·2± 0·40); thigh, 17·8 (31·8± 0·36); abdomen and chest, 25·3(34·7 ± 0·26); forearm, 28·8 (33·8±0·19; hand, 15·5 (30·8±0·58); ear canal, 36·1 (37·2 ± 0·7). The rate at which riders' feet cooled was not closely related to the rates their bodies cooled. Even when there was no body cooling, their feet cooled by at least 3°C hour^?1, while riding. The thickness of motorcycle clothing and its wind-proofing are both important in preventing cooling. Zip fasteners need special protection beyond that needed merely to keep out rain.     

Sun-stimulated chlorophyll fluorescence 2: Impact of atmospheric properties

Abstract

The Sun-stimulated chlorophyll fluorescence is a small but significant property of phytoplankton which can be detected using remote-sensing techniques. Besides the influence of oceanic properties, chlorophyll fluorescence is masked by atmospheric extinction. While an increase in chlorophyll concentration of 1 mg/m³ causes an increase in the upwelling radiances of about 0·03Wm^?2sr^?1 μm^?1 just above the water surface and due to the chlorophyll fluorescence, the upward radiances measured at λ_F = 685nm and at the top of the atmosphere ranges from 8 to 20Wm^?2sr^?1 μm^?1 for realistic atmospheric turbidity variations and a solar zenith distance of Θ_s = 50·7°. Additionally, the fluorescence, peaking at λ_F = 685nm with a half-width of about 10 nm, is reduced by the absorption of O₂ and H₂O. However, the fluorescence signal is nearly unaffected, when wavelengths λ≥686nm are exluded and a spectral interval of Δλ_F = 5nm is used for the radiance measurements.