Ruth Branch

Observations of rain-induced near-surface salinity anomalies

Vertical salinity gradients in the top few meters of the ocean surface can exist due to the freshwater input from rain. If present, surface gradients complicate comparing salinity measured at depths of a few meters to salinities retrieved using L-band microwave radiometers such as SMOS and Aquarius. Therefore, understanding the spatial scales and the frequency of occurrence of these vertical gradients and the conditions under which they form will be important in understanding sea surface salinity maps provided by microwave radiometers. Salinity gradients in the near-surface ocean were measured using a towed profiler that profiled salinity in the top 2 m of the ocean with a minimum measurement depth of 0.1 m. In addition, an Underway Salinity Profiling System was installed on the R/V Thomas G. Thompson. This measured near-surface salinity at depths of 1 and 2 m. Both the towed profiler and the underway system found the occurrence of negative salinity anomalies (i.e., salinity decreasing toward the surface) was correlated with the presence of rain. The magnitude of the anomaly (i.e., the difference between salinity at 0.1 m and the salinity at 0.26 m) was proportional to the cube of the rain rate for rain rate, R, greater than 6 mm h−1. From this, for R > 15–22 mm h−1, depending on the areal extent of the salinity anomalies, rain can cause scene-averaged salinity offsets that are as large as the accuracy goal for Aquarius of 0.1‰.

Integrated Ocean Skin and Bulk Temperature Measurements Using the Calibrated Infrared In Situ Measurement System (CIRIMS) and Through-Hull Ports

Abstract The design and performance of a shipboard-integrated system for underway skin and bulk temperature is presented. The system consists of the Calibrated Infrared In situ Measurement System (CIRIMS) and through-hull temperature sensors. The CIRIMS is an autonomous shipboard radiometer system that measures the sea surface skin temperature Tskin for validation of satellite-derived sea surface temperature products. General design considerations for shipboard radiometer systems are discussed and the philosophy behind the CIRIMS design is presented. Unique features of the design include a constant temperature housing to stabilize instrument drift, a two-point dynamic calibration procedure, separate sky- and sea-viewing radiometers for simultaneous measurements, and the ability to use an infrared transparent window for environmental protection. Laboratory testing and field deployments are used to establish an estimated error budget, which includes instrumentation and environmental uncertainties. The combi...

Comparisons of Shipboard Infrared Sea Surface Skin Temperature Measurements from the CIRIMS and the M-AERI

Abstract Extensive comparisons are made of the infrared sea surface skin temperature Tskin measured by the Calibrated Infrared In situ Measurement System (CIRIMS) and the Marine-Atmospheric Emitted Radiance Interferometer (M-AERI). Data were collected from four separate deployments on the NOAA research vessel (R/V) Ronald H. Brown and the U.S. Coast Guard (USCG) Polar Sea over a wide range of latitudes and environmental conditions. The deployment time totaled roughly 6 months over a 4-yr period and resulted in over 7000 comparison values. The mean offset between the two instruments showed that CIRIMS consistently measured a lower temperature than the M-AERI, but by less than 0.10°C. This mean offset was found to be dependent upon sky condition, wind speed, and ship roll, which implies the offset is likely due to uncertainty in the emissivity. The CIRIMS Tskin was recomputed using two alterative emissivity values, one based on emissivity measured by the M-AERI and the other based on a wind-speed-dependent ...

Thermal Infrared Multipath Reflection from Breaking Waves Observed at Large Incidence Angles

The infrared signature of breaking waves at large incidence angles was investigated using laboratory experiments and a radiometric model. Infrared imagery of the water surface at incidence angles greater than 70 ° shows multipath reflections for both spilling and plunging waves generated using a programmable wave maker. For the spilling breakers, the multipath signature was initially distinct from the breaking wave front roller signature but then merged to create a single large bright distributed target. For the plunging breakers, the roller and multipath signatures overlapped from the inception of breaking. The radiance of the multipath reflection was higher than the surrounding water for simulated cold sky conditions and lower for a simulated warm sky. A specular double-reflection model successfully predicted the presence of multipath reflection but the magnitude was sensitive to small uncertainties in geometry, wave slope, and input temperatures. The results show that multipath reflection from breaking waves is characteristic of large incidence angle infrared measurements and increases the area and magnitude of the infrared signature of breaking waves compared to the background.

Relating Microwave Modulation to Microbreaking Observed in Infrared Imagery

Microwave modulation by swell waves and its relation to microbreaking waves were investigated in an ocean experiment. Simultaneous collocated microwave and infrared (IR) measurements of wind waves and swell on the ocean were made. The normalized radar cross section sigma0 and the skin temperature T skin were both modulated by the swell, but with differing phases. In general, sigma0 maxima occurred on the front face, whereas T skin maxima occurred on the rear face of the swell. Infrared imagery has shown that swell-induced microbreaking occurred at or near the swell crest and that the resulting warm wakes occurred on the rear face of the wave. When tilt and range modulations are taken into account, the location of microbreaking also accounts for the maximum of sigma0 occurring on the front face of the swell. Thus, microbreaking waves generated near the crest of low-amplitude swell can produce microwave and IR signatures with the observed phase. The relationship between microwave and IR signals was further emphasized by comparing microwave Doppler spectra with simultaneous IR and visible images of the sea surface from the same location. When small and microscale breaking waves were present, Doppler spectra exhibited characteristics that are similar to those from whitecaps, having peaks with large Doppler offsets and polarization ratios near unity. When no microbreakers were present, Doppler offsets and polarization ratios were much smaller in accordance with a composite surface scattering theory.

Infrared Signatures of Microbreaking Wave Modulation

Infrared (IR) imagery of microbreaking waves in the ocean and laboratory showed modulation of breaking by swell and paddle-generated waves, respectively. Skin temperature also was modulated by the long waves, with the maxima occurring on the rear face of the long waves in both the laboratory and the field. The IR imagery from the ocean and laboratory showed that long-wave-induced microbreaking occurred at or near the long wave crest, and the resulting warm wakes occurred on the rear face. Thus, microbreaking waves generated near the crest of low-amplitude long waves can produce modulation with the maxima on the rear face. This mechanism was shown to be responsible for modulation of the measured in the laboratory and also likely contributed to the modulation observed in the field.

Remote Measurements of Tides and River Slope Using an Airborne Lidar Instrument

AbstractTides and river slope are fundamental characteristics of estuaries, but they are usually undersampled due to deficiencies in the spatial coverage of water level measurements. This study aims to address this issue by investigating the use of airborne lidar measurements to study tidal statistics and river slope in the Columbia River estuary. Eight plane transects over a 12-h period yield at least eight independent measurements of water level at 2.5-km increments over a 65-km stretch of the estuary. These data are fit to a sinusoidal curve and the results are compared to seven in situ gauges. In situ– and lidar-based tide curves agree to within a root-mean-square error of 0.21 m, and the lidar-based river slope estimate of 1.8 × 10−5 agrees well with the in situ–based estimate of 1.4 × 10−5 (4 mm km−1 difference). Lidar-based amplitude and phase estimates are within 10% and 8°, respectively, of their in situ counterparts throughout most of the estuary. Error analysis suggests that increased measureme...