Showing papers in "Journal of Atmospheric and Oceanic Technology in 2001"

Automated Analysis of Ocean Surface Directional Wave Spectra

Abstract: To facilitate investigations of surface wave processes in the open ocean, a wave spectral partitioning method with automated swell tracking and storm source identification capabilities has been developed. These tools collectively form the Wave Identification and Tracking System (WITS) and have been assembled entirely within the Matlab programming environment. A series of directional wave spectra, with supporting wind observations, is the only required input. Wave spectrum peaks representing specific wind sea and swell wave systems are extracted based on topographic minima, with wind sea peaks identified by wave age criteria. A swell tracking algorithm, combined with linear wave theory, provides a unique approach to storm source identification using the assimilated wave system statistics. The nature of the partitioned spectra allows the continuous, automated identification and tracking of multiple swell generation areas over space and time. Over a 6-day wave record in the Gulf of Alaska, 44 specif...

Comparison of Measurements of Atmospheric Wet Delay by Radiosonde, Water Vapor Radiometer, GPS, and VLBI

Abstract: The accuracy of the Global Positioning System (GPS) as an instrument for measuring the integrated water vapor content of the atmosphere has been evaluated by comparison with concurrent observations made over a 14-day period by radiosonde, microwave water vapor radiometer (WVR), and Very Long Baseline Interferometry (VLBI) The Vaisala RS-80 A-HUMICAP radiosondes required a correction to the relative humidity readings (provided by Vaisala) to account for packaging contamination; the WVR data required a correction in order to be consistent with the wet refractivity formulation of the VLBI, GPS, and radiosondes The best agreement of zenith wet delay (ZWD) among the collocated WVR, radiosondes, VLBI, and GPS was for minimum elevations of the GPS measurements below 10° After corrections were applied to the WVR and radiosonde measurements, WVR, GPS, and VLBI (with 5° minimum elevation angle cutoff) agreed within ∼6 mm of ZWD [1 mm of precipitable water vapor (PWV)] when the differences were averaged,

High-Resolution Doppler Lidar for Boundary Layer and Cloud Research

Abstract: The high-resolution Doppler lidar (HRDL) was developed to provide higher spatial, temporal, and velocity resolution and more reliable performance than was previously obtainable with CO2-laser-based technology. The improved performance is needed to support continued advancement of boundary layer simulation models and to facilitate high-resolution turbulent flux measurements. HRDL combines a unique, eye-safe, near-IR-wavelength, solid-state laser transmitter with advanced signal processing and a high-speed scanner to achieve 30-m range resolution and a velocity precision of ∼10 cm s−1 under a variety of marine and continental boundary layer conditions, depending on atmospheric and operating conditions. An attitude-compensating scanner has been developed to facilitate shipboard marine boundary layer observations. Vertical velocities, fine details of the wind profile near the surface, turbulence kinetic energy profiles, and momentum flux are measurable with HRDL. The system is also useful for cloud s...

Characterization and correction of relative humidity measurements from Vaisala RS80-A radiosondes at cold temperatures

Abstract: Radiosonde relative humidity (RH) measurements are known to be unreliable at cold temperatures. This study characterizes radiosonde RH measurements from Vaisala RS80-A thin-film capacitive sensors in the temperature range 0° to −70°C. Sources of measurement error are identified, and two approaches for correcting the errors are presented. The corrections given in this paper apply only to the Vaisala RS80-A sensor, although the RS80-H sensor is briefly discussed for comparison. A temperature-dependent correction factor is derived from statistical analysis of simultaneous RH measurements from RS80-A radiosondes and the NOAA cryogenic frostpoint hygrometer. The mean RS80-A measurement error is shown to be a dry bias that increases with decreasing temperature, and the multiplicative correction factor is about 1.3 at −35°C, 1.6 at −50°C, 2.0 at −60°C, and 2.4 at −70°C. The fractional uncertainty in the mean of corrected measurements, when large datasets are considered statistically, increases from 0.06...

The Marine-Atmospheric Emitted Radiance Interferometer: A High-Accuracy, Seagoing Infrared Spectroradiometer

Abstract: The Marine-Atmospheric Emitted Radiance Interferometer (M-AERI) is described, and some examples of the environmental variables that can be derived from its measurements and the types of research that these can support are briefly presented. The M-AERI is a robust, accurate, self-calibrating, seagoing Fourier-transform interferometric infrared spectroradiometer that is deployed on marine platforms to measure the emission spectra from the sea surface and marine atmosphere. The instrument works continuously under computer control and functions well under a very wide range of environmental conditions with a high rate of data return. Spectral measurements are made in the range of ∼3 to ∼18 μm wavelength and are calibrated using two internal, National Institute of Standards and Technology–traceable blackbody cavities. The environmental variables derived from the spectra include the surface skin temperature of the ocean, surface emissivity, near-surface air temperature, and profiles of temperature and h...

Testing a Procedure for Automatic Classification of Hydrometeor Types

Abstract: Examples of automatic interpretation of polarimetric measurements made with an algorithm that classifies precipitation, from an Oklahoma squall line and a Florida airmass storm are presented. Developed in this paper are sensitivity tests of this algorithm to various polarimetric variables. The tests are done subjectively by comparing the fields of hydrometeors obtained using the full set of available polarimetric variables with a diminished set whereby some variables have been left out. An objective way to test the sensitivity of the algorithm to variables and rank their utility is also devised. The test involves definition of a measure, which is the number of data points classified into a category using subsets of available variables. Ratios of various measures (similar to probabilities) define the percentage of occurrence of a class. By comparing these percentages for cases in which some variables are excluded to those whereby all are included, a relative merit can be assigned to the variables....

A Continuous-Flow Diffusion Chamber for Airborne Measurements of Ice Nuclei

Abstract: A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between ∼3- and 10-μm diameter in 3–10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than ∼0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational pro...

Long-Term Carbon Dioxide Fluxes from a Very Tall Tower in a Northern Forest: Flux Measurement Methodology

Abstract: Methodology for determining fluxes of CO 2 and H2O vapor with the eddy-covariance method using data from instruments on a 447-m tower in the forest of northern Wisconsin is addressed. The primary goal of this study is the validation of the methods used to determine the net ecosystem exchange of CO 2. Two-day least squares fits coupled with 30-day running averages limit calibration error of infrared gas analyzers for CO2 and H2O signals to 2%‐3%. Sonic anemometers are aligned with local streamlines by fitting a sine function to tilt and wind direction averages, and fitting a third-order polynomial to the residual. Lag times are determined by selecting the peak in lagged covariance with an error of 1.5%‐2% for CO2 and 1% for H2O vapor. Theory and a spectral fit method allow determination of the underestimation in CO2 flux ( ,5% daytime, ,12% nighttime) and H2O vapor flux ( ,21%), which is due to spectral degradation induced by long air-sampling tubes. Scale analysis finds 0.5-h flux averaging periods are sufficient to measure all flux scales at 30-m height, but 1 h is necessary at higher levels, and random errors in the flux measurements due to limited sampling of atmospheric turbulence are fairly large ( 15%‐20% for CO2 and 20%‐40% for H2O vapor at lower levels for a 1-h period).

Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation with a Correction for Thermal Offset Errors

Abstract: Diffuse-sky solar irradiance is an important quantity for radiation budget research, particularly as it relates to climate. Diffuse irradiance is one component of the total downwelling solar irradiance and contains information on the amount of downward-scattered, as opposed to directly transmitted, solar radiation. Additionally, the diffuse component is often required when calibrating total irradiance radiometers. A variety of pyranometers are commonly used to measure solar diffuse irradiance. An examination of some instruments for measuring diffuse irradiance using solar tracking shade disks is presented, along with an evaluation of the achieved accuracy. A data correction procedure that is intended to account for the offset caused by thermal IR exchange between the detector and filter domes in certain common diffuse pyranometers is developed and validated. The correction factor is derived from outputs of a collocated pyrgeometer that measures atmospheric infrared irradiance.

A Correction Method for Turbulence Measurements with a 3D Acoustic Doppler Velocity Profiler

Abstract: A method is proposed to reduce the noise contribution to mean turbulence parameters obtained by 3D acoustic Doppler velocity profiler measurements. It is based on a noise spectrum reconstruction from cross-spectra evaluations of two independent and simultaneous measurements of the same vertical velocity component over the whole water depth. The noise spectra and the noise variances are calculated and removed for the three fluctuating velocity components measured in turbulent, open-channel flow. The corrected turbulence spectra show a −5/3 slope over the whole inertial subrange delimited by the frequency band of the device, while the uncorrected turbulence spectra have flat high-frequency regions typical for noise effects. This method does not require any hypothesis on the flow characteristics nor does it depend on device-dependent parameters. The corrected profiles of turbulence intensities, turbulent kinetic energy, shear stress, and turbulent energy balance equation terms, such as production, t...

Ship-Based Sun Photometer Measurements Using Microtops Sun Photometers

Abstract: The use of hand-held Microtops II sun photometers (built by Solar Light Inc.) on ship platforms is discussed. Their calibration, filter stability, and temperature effects are also described. It is found that under rough conditions, the ship motion causes the largest error, which can result in a bias toward higher optical depths. In order to minimize this bias, a large number of sun photometer measurements (∼25) should be taken in a short period of time, and the higher values should be discarded. Under rough ocean conditions, it is also best to shorten the Microtops sun photometer sampling period (less than 5 s) and save only a single value (no averaging) and remove the high optical depths in postprocessing. It is found that the Microtops should be turned off frequently to correct for zero drift caused by temperature effects. Calibration is maintained by routine Langley plot calibrations at the Mauna Loa Observatory for each unit or through cross calibration.

The Direct Estimation of Near-Bottom Turbulent Fluxes in the Presence of Energetic Wave Motions

Abstract: Velocities produced by energetic waves can contaminate direct covariance estimates of near-bottom turbulent shear stress and turbulent heat flux. A new adaptive filtering technique is introduced to minimize the contribution of wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherence scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results from the oceanic test demonstrate that the technique succeeds at removing surface-wave contamination from shear stress and heat flux estimates using pairs of sensors separated in the vertical dimension by a distance of approximately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caused by internal motions that occur oc...

Profiling ALACEs and Other Advances in Autonomous Subsurface Floats

Abstract: Over the past decade more than 1200 autonomous floats have been deployed worldwide. In addition to velocity as marked by lateral movement, many of these floats measured quantities like profiles of temperature and salinity, temperature microstructure, and time series of vertical velocity. The authors' laboratory's implementation of profile measurements in what is called a Profiling Autonomous Lagrangian Circulation Explorer is described. Biofouling and degradation of antifouling coatings on the conductivity sensor both cause drifts that mean accurate salinity measurements will depend on corrections based on known temperature–salinity relations. A second generation autonomous float called the Sounding Oceanographic Lagrangian Observer (SOLO) has been developed to provide enhanced reliability and to provide complete two-way depth control. A dual hydraulic-pneumatic buoyancy system reduces the energy cost of vertical cycling and buoyancy generation at the surface. A SOLO Vertical Current Meter has be...

Remote Sensing of the Thermodynamic State of the Atmospheric Boundary Layer by Ground-Based Microwave Radiometry

Abstract: First results are presented of 18 months' experience with a microwave profiler that has been in operation in an unattended mode. Profiles of temperature and water vapor were retrieved without bias by a statistical regression method that was more accurate as opposed to a neural network approach, in particular for water vapor. Cloud liquid water was estimated by a neural network. The accuracy of the retrieved profiles estimated against quasisimultaneous radiosonde measurements are of comparable quality to that of the retrievals of ground-based Fourier transform infrared (FTIR) measurements. For temperature, the accuracy is about 0.6 K near the surface and less or equal to 1.6 K up to 7 km in summer and 4 km in winter. For water vapor, the corresponding values are 0.2–0.3 g m−3 near the surface and 0.8–1.0 g m−3 from 1- to 2-km altitude. The vertical resolution, however, is worse than that of FTIR measurements. Two case studies—a 1-week anticyclonic situation and 1 day with a cold front passage—demo...

A Combined Satellite Infrared and Passive Microwave Technique for Estimation of Small-Scale Rainfall

Abstract: There are numerous applications in climatology and hydrology where accurate information at scales smaller than the existing monthly/2.5° products would be invaluable. Here, a new microwave/infrared rainfall algorithm is introduced that combines satellite passive microwave (PMW) and infrared (IR) data to account for limitations in both data types. Rainfall estimates are produced at the high spatial resolution and temporal frequency of the IR data using rainfall information from the PMW data. An IRTb–rain rate relationship, variable in space and time, is derived from coincident observations of IRTb and PMW rain rate (accumulated over a calibration domain) using the probability matching method. The IRTb–rain rate relationship is then applied to IR imagery at full temporal resolution. MIRA estimates of rainfall are evaluated over a range of spatial and temporal scales. Over the global Tropics and subtropics, optimum IR thresholds and IRTb–rain rate relationships are highly variable, reflecting the co...

The Use of TRMM Precipitation Radar Observations in Determining Ground Radar Calibration Biases

Abstract: Since the successful launch of the Tropical Rainfall Measuring Mission (TRMM) satellite, measurements of a wide variety of precipitating systems have been obtained with unprecedented detail from the first space-based radar [precipitation radar (PR)]. In this research, a methodology is developed that matches coincident PR and ground-based volume scanning weather radar observations in a common earth parallel three-dimensional Cartesian grid. The data matching is performed in a way that minimizes uncertainties associated with the type of weather seen by the radars, grid resolution, and differences in radar sensitivities, sampling volumes, viewing angles, and radar frequencies. The authors present comparisons of reflectivity observations from the PR and several U.S. weather surveillance Doppler radars (WSR-88D) as well as research radars from the TRMM field campaigns in Kwajalein Atoll and the Large Biosphere Atmospheric (LBA) Experiment. Correlation values above 0.8 are determined between PR and ground radar matched data for levels above the zero isotherm. The reflectivity difference statistics derived from the matched data reveal radar systems with systematic differences ranging from 1 2t o27 dB. The authors argue that the main candidate for systematic differences exceeding 1 to 1.5 dB is the ground radar system calibration bias. To verify this argument, the authors used PR comparisons against well-calibrated ground-based systems, which showed systematic differences consistently less than 1.5 dB. Temporal analysis of the PR versus ground radar systematic differences reveals radar sites with up to 4.5dB bias changes within periods of two to six months. Similar evaluation of the PR systematic difference against stable ground radar systems shows bias fluctuations of less than 0.8 dB. It is also shown that bias adjustment derived from the methodology can have significant impact on the hydrologic applications of ground-based radar measurements. The proposed scheme can be a useful tool for the systematic monitoring of ground radar biases and the studying of its effect.

Autonomous Pinniped Environmental Samplers: Using Instrumented Animals as Oceanographic Data Collectors

Abstract: Data-recording tags applied to marine animals store data for later retrieval and can return valuable information on animal behavior and ecology, including habitat preference, physiology, and movement patterns, as well as environmental data. If properly instrumented, calibrated, and archived, data from these tags can add to the oceanographic datastream for parts of the ocean where data are sparse or lacking. Such data, from northern elephant seals instrumented with time‐temperature‐depth recorders (TTDR) and ARGOS platform terminal transmitters, is examined in this study. Northern elephant seals range widely over the northeastern Pacific on long foraging trips. The seals dive continuously on these trips to depths of 400‐600 m. Between March 1998 and March 1999, six female and three male elephant seals were tagged in central California and data were collected during subsequent foraging trips. Temperature and depth were measured and stored every 30 s and retrieved after the animals returned to the rookery months later. Portions of the track where both ARGOS and TTDR data were available from these nine animals averaged 4634 km over 67 days with 2.4 ARGOS positions per day. Mean dive duration was 20 min and mean dive depth was 428 m. A comparison of temperature profiles from seal TTDR with Global Temperature‐Salinity Profile Program (GTSPP) subsurface data showed very good agreement, as did surface temperatures to other sources of SST. Quality control of the data and entry into the World Ocean Database (WOD) is described. A total of 75 665 autonomous pinniped bathythermograph (APBT) profiles over the 41 702 km of seal trackline were added to the WOD. Biological autonomous sampling systems have immense potential to contribute oceanographic data in a costeffective manner. The northern elephant seal represents but one species covering portions of the northeast Pacific Ocean. Research programs presently exist on a variety of species, including southern elephant seals and other pinnipeds, tunas and billfish, sharks, seabirds, marine turtles, and whales. With improving technology, such tags will be applied to even more marine animals and the approach described here can be applied to other species to improve ocean data availability.

An Airborne Spectral Albedometer with Active Horizontal Stabilization

Abstract: An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- and upward irradiances between 400- and 1000-nm wavelength. It is installed on a small research aircraft (type Partenavia P68-B), but it can easily be mounted on other aircraft as well. The stabilization unit keeps the two radiation sensor heads (up- and downward looking) of the albedometer in a horizontal position during the flight with an accuracy of better than ±0.2° over a range of pitch and roll angles of ±6°. The system works properly for angular velocities up to 3° s-1 with a response time of the horizontal adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determined by laboratory and in-flight performance tests. As a result it is found that the new horizontal stabilization system ensures that misalignment-related uncertainties of the measured irradiances are ...

Rain Detection and Quality Control of SeaWinds

Abstract: A good assessment of the information content of scatterometer winds is particularly important in order to assimilate them in weather analysis. Besides retrieval problems in cases of a confused sea state, a particularly acute problem of Ku-band scatterometry is the sensitivity to rain. Elimination of poor quality data is therefore a prerequisite for the successful use of the National Aeronautics and Space Administration (NASA) Scatterometer (NSCAT) or QuikSCAT winds. Following the quality control for the European Remote-Sensing Satellite and NSCAT scatterometers performed at the Royal Netherlands Meteorological Institute, the authors further develop this methodology for QuikSCAT and define a quality indicator called the normalized residual (Rn). In order to characterize and validate the normalized residual, the authors use collocated Special Sensor Microwave Imager rain and European Centre for Medium-Range Weather Forecasts wind data. The results show indeed correlation between Rn and data quality...

Sampling Errors in Wind Fields Constructed from Single and Tandem Scatterometer Datasets

Abstract: Sampling patterns and sampling errors from various scatterometer datasets are examined. Four single and two tandem scatterometer mission scenarios are considered. The single scatterometer missions are ERS (with a single, narrow swath), NSCAT and ASCAT (dual swaths), and QuikSCAT (a single, broad swath obtained from the SeaWinds instrument). The two tandem scenarios are combinations of the broad-swath SeaWinds scatterometer with ASCAT and QuikSCAT. The dense, nearly uniform distribution of measurements within swaths, combined with the relatively sparse, nonuniform placement of the swaths themselves create complicated space–time sampling patterns. The temporal sampling of all of the missions is characterized by bursts of closely spaced samples separated by longer gaps and is highly variable in both latitude and longitude. Sampling errors are quantified by the expected squared bias of particular linear estimates of component winds. Modifications to a previous method that allow more efficient expecte...

Calculating Lagrangian Trajectories Using Time-Dependent Velocity Fields

Abstract: The authors report on the development of an efficient and accurate tool for computing Lagrangian trajectories using time-varying velocities. By linearly interpolating the velocities both in space and time one can obtain analytical expressions for the trajectory inside each model grid box. In combination with a numerical determination of the transit times through each grid box, these expressions allow for the implementation of expedient algorithms for offline analyses of large datasets resulting from general circulation models. The authors demonstrate the efficacy of this approach on a time-varying two-dimensional model gyre.

Laboratory Measurements of the Response of a PMS OAP-2DC

Abstract: Laboratory measurements of the response of the Particle Measuring Systems, Inc., 2DC probe have been conducted to characterize counting and sizing errors of the probe for spherical particles. Measurements of the shadow threshold intensity of a Meteorological Service of Canada (MSC) 2DC probe varied from approximately 30% to 51%, depending on the photodiode, and averaged 46% for the central 16 photodiodes. Depth-of-field and sizing measurements are quite sensitive to this threshold, which is nominally considered as 50% for the 2DC probe. Response times also varied significantly, from 0.44 to 0.90 ms. Measurements of the depth of field for known particle sizes at low velocity agreed well with published calculations at zero velocity. For particles smaller than 100 mm, the depth of field decreased significantly with increasing airspeed due to the nonzero response time of the sensing photodiodes. The average particle size also decreased with increasing airspeed but did so in such a manner as to counteract oversizing due to out-of-focus images. At 100 m s 21, the average measured sizing error of a 100-mm particle was close to negligible, rising to approximately 5% at 500 mm. The application of measured depth-of-field values and sizing calibrations at specific sizes to improve 2DC size distribution accuracy is nontrivial because measurement errors cause particles to be redistributed to other sizes in a complicated manner. However, when hypothetical true particle distributions were redistributed according to a distortion matrix approximated by the results of this study, the average error of uncorrected size distributions measured by the MSC 2DC probe, expressed as a sizing error, was found to be 610% for particles larger than 125 mm. Although these results are not strictly transferable to other 2DC probes, the methods described can be used to derive similar results for other probes.

Comparisons of Rain Rate and Reflectivity Factor Derived from the TRMM Precipitation Radar and the WSR-88D over the Melbourne, Florida, Site

Abstract: Validating the results from the spaceborne Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) requires comparisons with well-calibrated ground-based radar measurements. At altitudes near the storm top, where effects of PR signal attenuation are small, the data are used to check the relative calibration accuracy of the radars. Near the surface, where attenuation effects at the PR frequency of 13.8 GHz can be significant, the data provide an assessment of the performance of the PR attenuation correction algorithm. The ground-based data are taken from the Doppler Weather Surveillance (WSR-88D) radar located at Melbourne, Florida. In 1998, 24 overpasses of the TRMM satellite over the Melbourne site occurred during times when significant precipitation was present in the overlap region of the PR and WSR-88D. Resampling the ground-based and spaceborne datasets to a common grid provides a means by which the radar reflectivity factors (dBZ) can be compared at different heights and for dif...

Use of the 3D Radon Transform to Examine the Properties of Oceanic Rossby Waves

Abstract: One of the most successful applications of satellite-borne radar altimeter data over the oceans in recent years has been the extraction of information about long-wavelength baroclinic Rossby (or planetary) waves, which play a significant role in ocean circulation and climate dynamics These waves cross ocean basins from east to west at speeds of few centimetres per second at mid-latitudes The cross-basin propagation time may therefore be several months or even years and an accurate estimation of the speed of the waves is important We review the methods for obtaining information on Rossby wave velocity from altimetry data, particularly the two-dimensional Radon transform Unfortunately the use of longitude-time plots, although it allows the estimation of the zonal phase speeds, does not give any information on the speed vector when the propagation of the waves is not purely zonal (east-west) We show how the two-dimensional Radon Transform can be generalised to three dimensions, enabling not only the true propagation velocity component to be determined, but also the direction of the waves and thus any deviation from the pure-westward case As examples of the application of this extended technique, we show maps of direction, speed and energy of Rossby waves in the North Atlantic Ocean

Recovery of Near-Surface Velocity from Undrogued Drifters

Abstract: The authors have quality controlled six global datasets of drifting buoy data, made comparisons of 15-m drogued and undrogued buoy observations, and developed a 2D linear regression model of the difference between drogued and undrogued drifter velocity as a function of wind. The data were acquired from 2334 Surface Velocity Program (SVP) drifters, including 1845 SVP drifters after they lost their drogues; 704 AN/WSQ-6 Navy drifter buoys; and 503 First Global GARP Experiment (FGGE) drifter buoys. Meridional and zonal surface wind velocity components from the global synoptic FNMOC model, the global synoptic ECMWF model, and the global synoptic NCEP model were interpolated to naval AN/WSQ-6, WOCE–TOGA buoy, or FGGE buoy positions and date/times in the datasets. Two-day mean buoy drift velocities and positions were computed: 122 101 SVP drifter mean velocities before they lost their drogues and 58 201 SVP drifter mean velocities after they lost their drogues, 21 799 Navy drifter mean velocities, and ...

Use of a High-Resolution Model to Analyze the Mapping Capabilities of Multiple-Altimeter Missions

Abstract: The contribution of merging multiple-satellite altimeter missions to the mapping of sea level is analyzed from a North Atlantic high-resolution (1/10°) numerical simulation. The model is known to represent the mesoscale variability quite well and offers a unique opportunity for assessing the mapping capability of multiple-altimeter missions. Several existing or planned orbits [TOPEX/Poseidon (T/P), Jason-1, ERS-1/2–ENVISAT, GEOSAT-GFO] are analyzed, and Jason-1 and T/P orbits are assumed to be interleaved. The model sea level anomaly fields are first subsampled along T/P, ERS, GFO, and Jason-1 tracks and a random noise of 3-cm rms is added to the simulated altimeter data. A suboptimal mapping method is then used to reconstruct the 2D sea level anomaly from alongtrack data and the reconstructed fields are compared with the reference model fields. Comparisons are performed in the North Atlantic and over a complete year. These results confirm the main conclusions of the Le Traon and Dibarboure study...

Determining Turbulent Kinetic Energy Dissipation from Batchelor Curve Fitting

Abstract: An algorithm is presented for obtaining the rate of turbulent kinetic energy dissipation by fitting the theoretical Batchelor spectrum to the temperature gradient spectrum at high wavenumbers. The algorithm is relatively robust in selecting the turbulent Batchelor component from temperature gradient spectra, which have finestructure, internal wave, and noise contributions. The theoretical curve is fitted using an error function that takes into account many of the characteristics of the Batchelor spectrum. Overall, the use of the algorithm to determine dissipation of the turbulent kinetic energy is considerably more time efficient than manual methods. Some limits on the accuracy of the method are also discussed.

Rainfall Estimation from Polarimetric Radar Measurements: Composite Algorithms Immune to Variability in Raindrop Shape–Size Relation

Abstract: Polarization diversity radar measurements such as reflectivity factor, differential reflectivity, and differential propagation phase are extensively used in rainfall estimation. Algorithms to estimate rainfall from polarimetric radar measurements are based on a model for the raindrop shape as a function of drop diameter. Most of the current algorithms use an equilibrium shape‐size model for raindrops. Variation of the prevailing mean raindrop shapes from an assumed model has a direct impact on the accuracy of radar rainfall estimates. This paper develops composite algorithms to estimate rainfall from polarimetric radar data without an a priori assumption about the specific form of mean raindrop shape‐size model such as equilibrium shape model. The accuracy of rainfall estimates is evaluated in the presence of random measurement errors as well as systematic bias errors. The composite algorithms, independent of a prespecified raindrop shape model, were applied to radar parameters simulated from disdrometer data collected over 3 months, and the corresponding rainfall estimates were found to be in good agreement with disdrometer estimates. The composite algorithms were also tested with Colorado State University CHILL radar observations of the 28 July 1997 Fort Collins (Colorado) flood event. The storm total precipitation estimates based on the composite algorithms developed in this paper were in much better agreement with rain gauge estimates in comparison with conventional algorithms.

An Operational Method for Separating Wind Sea and Swell from Ocean Wave Spectra

Abstract: Coexistence of wind sea generated locally and swell radiated from distant storms often results in double-peaked or multiple-peaked spectra. Identification and separation of the wave energies of wind sea and swell provide a more realistic description of the sea state, which is of great importance to scientific and engineering applications. This paper describes a method based on the peak frequency of a newly defined steepness function to separate the wave energies of wind sea and swell from the omnidirectional wave spectra. This steepness method does not rely on the availability of the information of wind velocities and wave directions and can be easily implemented for operational uses. Verification results using directional wave data collected from buoys in the Gulf of Mexico and offshore California are presented.

Listening to Raindrops from Underwater: An Acoustic Disdrometer

Abstract: Different sized raindrops splashing on a water surface produce sound underwater that is distinctive and can be used to measure the drop size distribution in the rain. Five acoustically significant raindrop sizes are described. An inversion of the underwater sound to measure the drop size distribution in the rain is described and demonstrated. Limitations to the inversion include problems associated with the relative loudness of the largest drops (diameter over 3.5 mm), the relative quietness of the medium drops (diameter 1.2–2.0 mm), and the influence of wind to suppress the signal from the otherwise remarkably loud small drops (diameter 0.8–1.2 mm). Various measures of rainfall, including rainfall rate, equivalent radar reflectivity, median drop size, and other integrated moments of the drop size distribution are measured acoustically and used to examine rainfall research issues. The relationship between equivalent reflectivity and rainfall rate, the Z–R diagram, is partitioned acoustically show...