Showing papers in "Journal of Atmospheric and Oceanic Technology in 1989"

Precipitation retrieval over land and ocean with the SSM/I - Identification and characteristics of the scattering signal

Abstract: The subject of this study is the identification of precipitation in warm and cold land and ocean environments from the Defense Meteorological Satellite Program's (DMSP) Special Sensor Micmwave/Imager (SSM/I). The high sensitivity of the SSM/I 85.5 GHz channels to volume scattering by precipitation, especially ice above the freezing level, is the basis for this identification. This ice scattering process causes SSM/I 85.5 GHz brightness temperatures to occasionally fall below 100 K. It is demonstrated that the polarization diversity available at 85.5 GHz from the SSM/I allows discrimination between low brightness temperatures due to surface water bodies versus those due to precipitation. An 85.5 GHz polarization corrected temperature (PCT) is formulated to isolate the precipitation effect. A PCT threshold of 255 K is suggested for the delineation of precipitation. This threshold is shown to be lower than what would generally be expected from nonprecipitating cloud water alone, yet high enough to s...

On in situ 'calibration' of shipboard ADCPs

Abstract: Methods for in situ calibration of acoustic-Doppler current profilers (ADCPs) are considered for measurement of absolute current profiles from a moving ship. Errors are of two types: sensitivity and alignment. Least square error estimates are given for experimental determination of both factors, for use in the 'water track' or 'bottom tracking' mode. Errors in the estimation of either factor may lead to large errors in derived water velocities, although the major contributions of the two factors arise from different sources and are approximately orthogonal to one another.

Pulse Compression and Sea Level Tracking in Satellite Altimetry

Abstract: An account is given of the pulse-compression technique, in which a radar altimeter transmits a relatively long pulse and processes the returned signal in a way equivalent to transmitting a very short one and measuring the returned power in a sequence of range gates. The effective short pulse enhances the range resolution that would be obtained from the actual long pulse. Pulse compression and sea-level tracking are important to the overall error budget for altimetric estimates of sea level. Attention is presently given to the high degree of accuracy required for the NASA TOPEX altimeter scheduled for launch in mid-1992.

An Instantaneous CCN Spectrometer

Abstract: A thermal gradient diffusion cloud chamber with a supersaturation field that increases along the path of the flow of sample is used as a cloud condensation nucleus (CCN) spectrometer. The CCN spectrum is derived from the final droplet size distribution which has been determined to be related to the nucleus critical super-saturation based on routine calibration procedures using laboratory-produced monodisperse salt particles. The instrument is compared against a previously reported CCN counter on natural and laboratory-produced aerosol and against an absolute CN counter on the monodisperse aerosol. Preliminary results of atmospheric measurements are given. This device produces a CCN spectrum over the entire useful range of interest for cloud physics (0.01% to 1% supersaturation). More than 40 channels of resolution over this range are simultaneously provided. Data can be collected on a continuous basis for integration times as short as 2 seconds. The instrument has obtained data from four differen...

A Method for Calibrating Shipmounted Acoustic Doppler Profilers and the Limitations of Gyro Compasses

Abstract: Calibration of shipmounted acoustic Doppler profiles by a series of 90° turns during periods of GPS navigation provides estimates of misalignment angle φ and scaling factor A with standard deviations less than 0.2° and 0.3%. A varies by 1% with depth, and differs between bottom and water track modes, but φ is independent of these factors. Day to day variations in φ and A, however, are as large as 1° and 2%, the former because of long-period wander of the ship's gyro compass, the latter possibly because of variations in Doppler spectra in different conditions. The gyro compass also shows short-period bias of 2° after a 90° turn, with 0.5° bias persisting for over 20 minutes. All these errors indicate that the limit of current accuracy both along and athwartships is about 0.05 m s−1 for a ship speed of 5 m s−1.

Formalism for Comparing Rain Estimation Designs

Abstract: Space-time averages of rain rates are needed in several applications. Nevertheless, they are difficult to estimate because the methods invariably leave gaps in the measurements in space or time. A formalism is developed which makes use of the frequency-wavenumber spectrum of the rain field. The mean square error of the estimate is expressed as an integral over frequency and two-dimensional wavenumber of an integrand consisting of two factors, a design-dependent-filter multiplied by the space-time spectrum of the rain rate field. Such a formalism helps to separate the design issues from the peculiarities of rain rate random fields. Two cases are worked out in detail: a low orbiting satellite which takes cell-wide snapshots at discrete intervals and a network of raingages which are gappy in space but continuous in time.

Doppler lidar measurement of profiles of turbulence and momentum flux

Abstract: A short-pulse CO2 Doppler lidar with 150-m range resolution measured vertical profiles of turbulence and momentum flux. Example measurements are reported of a daytime mixed layer with strong mechanical mixing caused by a wind speed of 15 m/sec, which exceeded the speed above the capping inversion. The lidar adapted an azimuth scanning technique previously demonstrated by radar. Scans alternating between two elevation angles allow determination of mean U-squared, V-squared, and W-squared. Expressions were derived to estimate the uncertainty in the turbulence parameters. A new processing method, partial Fourier decomposition, has less uncertainty than the filtering used earlier.

Modulation of the Spectra of Short Gravity Waves by Sea Surface Films: Slick Detection and Characterization with a Microwave Probe

Abstract: Organic sea surface films are often indicators of surface and subsurface processes that influence their formation and their spatial and temporal distribution. The films modify the physics of the sea surface and influence remotely sensed optical and microwave imagery. Natural sea surface films have been detected, charted, and characterized by measurements of the short-gravity spectra of wind waves during three experimental periods, one in the Sicilian Channel, and two in the Gulf of Maine. Wind wave spectra with and without organic films were measured in situ with a microwave probe. These spectra were analyzed to characterize the elasticity moduli of the wave modulating films using a recently developed theory. Theory and experiment are in good agreement. It has been demonstrated that the microwave probe can detect the presence of sea surface films and their visco-elastic properties by the measurement of their wave-modulating effects. The data collected by this system can possibly be further interp...

Two-Wavelength Method of Measuring Path-Averaged Turbulent Surface Heat Fluxes

Abstract: Because few geophysical surfaces are horizontally homogeneous, point measurements of the turbulent surface fluxes can be unrepresentative. Path-averaging techniques are therefore desirable. This paper presents a method that yields path-averaged measurements of the sensible and latent heat fluxes with a potential accuracy as good as that for eddy-correlation measurements. The method relies on electro-optical measurements of the refractive index structure parameter Cn2 at two wavelengths: one in the visible-to-mid-infrared region, where Cn2 depends largely on turbulent temperature fluctuations, and a second in the near-millimeter-to-radio region, where Cn2 depends more strongly on humidity fluctuations. A sensitivity analysis, the cornerstone of the study, provides quantitative guidelines for selecting wavelength pairs to use for the measurements. The sensitivity analysis also shows that the method is not uniformly accurate for all meteorological conditions; for limited ranges of the Bowen ratio, t...

Radiometric and Geometric Calibration of a Visible Spectral Electro-Optic “Fisheye” Camera Radiance Distribution System

Abstract: Camera systems which measure a complete hemispherical field (“fisheye” lens systems), can be applied to the measurement of the radiance, but accurate radiometric and geometric calibrations are required to obtain absolute radiance data. The calibration procedure applied to an instrument built for spectral and geometrical radiance distribution measurements in the visible wavelength region (450–650 nm), based on an electro-optic fisheye camera system, is described and validated through comparison with a mere standard radiometer.

Coincidence and Dead-Time Corrections for Particles Counters. Part II: High Concentration Measurements with an FSSP

Abstract: Meat of the procedures proposed in the past to correct the coincidence and dead time losses in droplet counters FSSP* or ASSP** are based on empirical formulae. A more rigorous analysis of the statistical behavior of these counters yields a better estimate of the high droplet concentration measurements and moreover, it becomes possible to explain the fluctuations of the operating characteristics of the probes (rejection criteria). A new method is then proposed to detect heterogeneities at scales smaller than the sampling scale and to estimate the concentration peak values in the sample, with the current parameters measured by the FSSP.

DIAL Tropospheric Ozone Measurement Using a Nd:YAG Laser and the Raman Shifting Technique

Abstract: The Differential Absorption Lidar (DIAL) offers a way to perform ozone measurements in the troposphere with the spatial and temporal resolutions needed for both short and long term studies. Only a few DIAL system have been used up to now for this purpose, relying mainly on tunable laser sources. In this paper, a new system using a Nd:YAG laser source and Raman shifting techniques to generate several wavelengths in the ultraviolet and the visible spectral range is proposed for use in a regional ozone network and in a small or medium size aircraft. This is system has been operated in a ground-based station in southern France for nighttime ozone measurements in the free troposphere. A comparison with balloon borne Brewer-Mast ozonesonde data is presented and measurement accuracy of these two instruments is discussed.

Accuracy assessment of widely used orbit error approximations in satellite altimetry

Abstract: From simulations, the orbit error can be assumed to be a slowly varying sine wave with a predominant wavelength comparable to the Earth's circumference. Thus, one can derive analytically the error committed in representing the orbit error along a segment of the satellite ground track by a bias; by a bias and tilt (linear approximation); or by a bias, tilt, and curvature (quadratic approximation). The result clearly agrees with what is obvious intuitively, i.e., (1) the fit is better with more parameters, and (2) as the length of the segment increases, the approximation gets worse. But more importantly, it provides a quantitative basis to evaluate the accuracy of past results and, in the future, to select the best approximation according to the required precision and the efficiency of various approximations.

An Examination of Wind Profiler Signal Processing Algorithms

Abstract: A numerical model to simulate radar data is used for testing various estimators of the Doppler shift in Doppler radar echoes. The estimators are the pulse pair and poly-pulse pair algorithms in the correlation domain, a least-squares fitting to the spectral peak of the power spectra, and direct calculations of the moments from periodograms in the spectral domain. Two averaging schemes (a consensus average and a median filter) are also examined for data with poor signal-to-noise ratios. The data processing method used in Doppler radar wind profilers, which operate over a very wide range of signal to noise ratios, is examined in detail. It is shown that the direct moment calculation combined with a consensus averaging technique has the best overall performance for accuracy and the ability to use data with a very low signal-to-noise ratio.

Laboratory and wind tunnel evaluations of the Rosemount icing detector

Abstract: The Rosemount model 871FA ice detector was evaluated during a number of laboratory and wind tunnel studies. The purpose of this evaluation was to determine the sensitivity of the detector to a variety of icing conditions and to determine its utility as a ground-based or airborne liquid water content measuring device. The laboratory studies determined the ice mass sensitivity of a number of detectors and showed the variability among instruments. These studies also looked at the sensitivity of the detector to the position of mass accumulated on the detector's sensor. The wind tunnel studies determined the dynamic mass response of the detector and demonstrated that this detector can be a very sensitive, fast response, and relatively accurate liquid water content measuring instrument.

A Technique for Estimating Outgoing Longwave Radiation from HIRS Radiance Observations

Abstract: A new technique for estimating outgoing longwave radiation from observations on the NOAA operational satellites has been developed from a regression analysis of radiation model calculations. The technique consists of a weighted sum of radiance in but four intervals sensed by the High-resolution InfraRed Sounder (HIRS). The analysis shows that model outgoing flux may be reproduced to within ±2 W · m−2 rms, which is about a factor of 4 smaller than the rms error of the method used by NOAA to estimate flux from the AVHRR. The small errors suggest that the new technique holds the promise of eliminating the large systematic errors possible with the current NOAA technique. Additionally, the new technique often the possibility of directly relating flux changes to changes in atmospheric parameters.

The Spaced Antenna Technique for Radar Wind Profiling

Abstract: The Doppler method for radar wind profiling of the troposphere and stratosphere has been the standard technique used for operational and most research applications of the instrumentation An alternative approach for measuring winds, by making use of scattering and reflection from variations in the refractive index, is what is known as the spaced antenna technique This technique has received little attention so far, and few comparisons of the two techniques have been carried out, in spite of the fact that results to date indicate that there can be some advantages of the spaced antenna method over the Doppler method, especially when small-antenna and low-power VHF systems are being considered We describe the technique and summarize the diverse results that relate to a comparison between the Doppler and spaced antenna methods The expected advantages of the spaced antenna method for small radar systems are also described

Doppler Sonar and Surface Waves: Range and Resolution

Abstract: The performance limitations of an acoustic Doppler sonar system are explored and compared with anticipated requirements for the measurement of surface wave directional/frequency spectra. To obtain measurements to a range D requires a delay Δt between pings long enough for sound to propagate out to D and back: Δt(c/2) ≥ D. This defines a Nyquist frequency, ωN (radiances s−1). Linear dispersion relates this to a “matched wavenumber,” kN = ωN2/ g. Waves travelling obliquely and harmonics of longer waves appearing at ωN all have smaller wavenumbers, k ≤ kN; thus, kN; defines a maximum wavenumber requirement, or (equivalently) a matched range resolution, ΔR. From idealized surface wave spectra, the velocity resolution ΔV required to measure spectra out to (ωN, kN) can be estimated. For a given sonar “tone,” the error-product E = ΔRΔV is a constant, so velocity resolution and range resolution must be traded off. The error product decreases with increasing acoustic frequency f0 and number of tones. High...

Field Calibration of Mixed-Layer Drifters

Abstract: A set of field experiments was conducted to determine the water-following characteristics of mixed-layer drifters with “holey-sock” drogues. Through the use of a drifting current meter array, direct estimates of slip velocity (or the difference between the velocity of the drifter and that of the water surrounding the drogue) were obtained with precision of better than ± 1 cm s−1. The range of slip velocities was 1 to 4 cm s−1, with the orientation of the slip principally in the downwind direction. The results are consistent with a simple model for slip induced by a wind-drift current, assuming a static balance of drag forces.

A Micrometeorological Facility for Eddy Flux Measurements of CO2 and H2O

Abstract: A ground-based eddy flux system which is used to measure the CO2 and H2O exchange between the atmosphere and the biosphere is described Fluctuations of CO2, H2O, air temperature and atmospheric turbulence are sampled at frequencies up to 50 Hz A microcomputer-based data acquisition system is used to calculate in real time, using standard hardware, the raw and corrected flux of CO2, latent and sensible heat and momentum Flux density data taking into account all known sources of energy exchange in a maize crop are presented as an example of the strategy employed to control the quality of the data in measuring eddy fluxes under field conditions and to demonstrate the sources of errors and the degree of accuracy which we can obtain CO2 and H2O flux density measurements with modern equipment

Temporal Sampling Requirements for Surface Drifting Buoys in the Tropical Pacific

Abstract: Drifting buoy data from the eastern tropical Pacific Ocean are used to evaluate the degradation of sea surface temperature and current information incurred by reducing the number of transmissions from drifting buoys using the ARGOS system for position finding. Buoy locations are interpolated at uniform time intervals using an optimum interpolation method known as Kriging, which provides also an estimate of the rms position error. It is found that the published standard for surface current measurement for the TOGA Program (5 cm s−1) can be met with transmissions on one day of three in the Southern Hemisphere. Due to stronger mesoscale variability in the Northern Hemisphere the standard would be jeopardized by reducing transmissions even to one day of two. The standard for observation of sea surface temperature (0.1°C) can be met in either hemisphere with transmissions on one day of four. The Lagrangian decorrelation times for the Northern Hemisphere region of the eastern tropical Pacific are estim...

Use of Holography for Airborne Cloud Physics Measurements

Abstract: The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well...

Meteorological Data Needs for Modeling Air Quality Uncertainties

Abstract: A probabilistic framework for incorporating uncertainty in air quality models is described. The quantitative dependence of the uncertainty in calculated air quality concentrations on the uncertainty in the input meteorological data is illustrated using a simple Second-order Closure integrated Model Plume in combination with the EPRI Plume Model Validation and Development Data Set. Evaluation of the model results demonstrate that even though individual hourly samples cannot be deterministically predicted downwind of a powerplant stack, statistical representations of the observed cumulative distribution of the sample values are quite predictable. We discuss the data needed to improve the definition of the range of meteorological uncertainty within an ensemble of flows defined by given meteorological data, and thus provide for improvements in predictability models of the type illustrated. We argue that attempts to collect the data needed to define more precisely the variance within the ensemble of c...

Sampling Characteristics of Satellite Orbits

Abstract: The irregular space-time sampling of any finite region by an orbiting satellite raises difficult questions as to which frequencies and wavenumbers can be determined and which will alias into others. Conventional sampling theorems must be extended to account for both irregular data distributions and observational noise - the sampling irregularity making the system much more susceptible to noise than in regularly sampled cases. The problem is formulated here in terms of least-squares and applied to spacecraft in 10-day and 17-day repeating orbits. The 'diamond-pattern' laid down spatially in such repeating orbits means that either repeat period adequately samples the spatial variables, but the slow overall temporal coverage in the 17-day pattern leads to much greater uncertainty than in the shorter repeat cycle. The result is not definitive and it is not concluded that a 10-day orbit repeat is the most appropriate one. A major conclusion, however, is that different orbital choices have potentially quite different sampling characteristics which need to be analyzed in terms of the spectral characteristics of the moving sea surface.

A Comparison of Sensible and Latent Heat Flux Calculations Using the Bowen Ratio and Aerodynamic Methods

Abstract: An analysis technique is outlined that calculates the sensible and latent heat fluxes by the Bowen ratio and aerodynamic methods, using profile measurements at any number of heights. Field measurements at two sites near Churchill, Manitoba, comprising over 4000 hours of data from June through September, are used to assess the relative accuracy of the two methods. If hourly data are eliminated when the Bowen ratio method gives the wrong sign for the fluxes, or when the Bowen ratio is close to −1, the two methods agree very well. The results also indicate that the aerodynamic method can provide acceptable results with only a single measurement of wind speed and a good estimate of the surface roughness length. The relative error associated with the use of only a single anemometer is small for unstable conditions, and only becomes significant under very stable conditions when the fluxes themselves are small. The overall comparison of the two methods yields a mean bias error of less than 10 W m−2, and...

Identifying and Screening Filter Skew and Noise Bias in Acoustic Doppler Current Profiler Measurements

Abstract: Acoustic Doppler current profiler (ADCP) velocity measurements an subject to bias due to the effect of the signal processing filters on the spectrum of the Doppler-shifted signal and on the noise. Bias will occur when the filter is not centered on the signal. Numerical models of the received signal and the processing filter used in RD Instruments profilers show that biases on the order of 10 cm s−1 can occur in the lower half of the current profile in regions of high current shear. Errors tend to increase with the width of the acoustic beam and with the speed of the ship, and decrease with the pulse length. These biases are identified in ADCP velocity measurements made in the high shear of the equatorial undercurrent. We suggest criteria for editing existing ADCP data to remove excessive bias, and we recommend changes in profiler parameters which should greatly reduce the bias in future datasets.

Errors in the Determination of Wind Speed by Doppler Radar

Abstract: A numerical model to simulate radar data is used for testing various estimators of the Doppler shift in Doppler radar echoes. Five estimators for the Doppler shift are considered: the pulse pair and poly-pulse pair algorithms in the correlation domain, least squares fitting to the power spectra in linear and logarithmic coordinates, and a matched filter in the spectral domain. An experiment with real data, to test the algorithms further and to assess the importance of small-scale wind fluctuations on radar performance, shows that geophysical limitations on the accuracy of the wind estimates are the dominant factor for observations with good signal-to-noise ratio.

Coincidence and Dead-Time Corrections for Particle Counters. Part I: A General Mathematical Formalism

Abstract: Counting loss due to coincidences limit the efficiency of the retriggerable particle counters. When the particle rate increases, the counted rate rapidly reaches a maximum so that it becomes impossible to precisely estimate the actual value. On the contrary, the activity time of the probe increases continuously and is a more efficient parameter for high concentration measurements. Exact relationships between these parameters and the actual particle rate are given here for particles randomly dispersed according to a Poisson distribution. Relations between activity and counted particle rate are also useful for the evaluation of the heterogeneities in the spatial distribution of the particles.

Efficient Processing of Alternately Polarized Radar Signals

Abstract: Processing of simultaneous measurements of differential polarization parameters (differential reflectivity, ZDR, and differential phase shift, ϕDP) and Doppler spectral moments is discussed. It is shown how ZDR and ϕDP modulate the Doppler signal and what effects they have on the autocovariance at lag 1 if a sequence of alternately polarized fields (linear horizontal, H, and vertical, V) is transmitted. A scheme to overcome these effects is proposed and demonstrated on recorded time series data from a radar with polarization diversity.

Evaluation of liquid water measuring instruments in cold clouds sampled during FIRE

Abstract: Airborne liquid water content (LWC) measurements were conducted with an icing detector and a forward-scattering spectrometer probe during 10 flights into cold clouds, as part of the First ISCCP Research Experiment (FIRE). The LWC measurements thus obtained compare favorably with those from the hot-wire probes in the range where LWC is above the detection limits of the latter; the hot-wire probes have detection thresholds about one order of magnitude higher than is possible with the icing detector and spectrometer probe. FIRE experiment data indicate that LWC should be taken into consideration in cloud studies at temperatures down to at least 35 C.