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At least 487 records · Page 27
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NASA NTRS1992

Hydrometeor discrimination in melting layer using multiparameter airborne radar measurement

Results from a multiparameter airborne radar/radiometer experiment (the Typhoon experiment) are presented. The experiment was conducted in the western Pacific with the NASA DC-8 aircraft, in which a dual-wavelength at X-band and Ka-band and dual-polarization at X-band radar was installed. The signatures of dBZ(X), dBZ(Ka), LDR (linear depolarization ratio) at X-band and DZ=dBZ(X)-dBZ(Ka) are discussed for the data obtained in the penetration of the typhoon Flo. With emphasis on discrimination of hydrometeor particles, some statistical features of the brightband in stratiform rain are discussed.

Kumagai, H.↗
NASA NTRS1992

Radar depolarization signatures of rain in cumulus clouds measured with a dual-frequency air-borne radar

Experimental data are presented on dual-wavelength scattering characteristics of subtropical rain observed during the convection and precipitation/electrification (CaPE) experiment. Linear depolarization ratios (LDRs) are calculated at both X-band and Ka-band channels. In stratiform rain, the cross-polarized return signal is generally below the noise level except within the melting layer where the maximum LDR at both frequencies is typically between -12 and -10 dB. In convective rain the range profiles of the LDR in X-band and those in Ka-band do not coincide. The X-band LDR values in convective rain typically remain smaller than -25 dB at high altitudes and take somewhat larger values of -25 to -15 dB at low altitudes. The Ka-band LDR values, in contrast, typically increase with radar range, sometimes reaching a level of -3 dB just before the cross-polarized signal falls below the noise level. Such high values of LDR in Ka-band are most likely attributable to multiple scattering effects.

Iguchi, Toshio↗
NASA NTRS1993

A study of the threshold method utilizing raingage data

The threshold method for estimation of area-average rain rate relies on determination of the fractional area where rain rate exceeds a preset level of intensity. Previous studies have shown that the optimal threshold level depends on the climatological rain-rate distribution (RRD). It has also been noted, however, that the climatological RRD may be composed of an aggregate of distributions, one for each of several distinctly different synoptic conditions, each having its own optimal threshold. In this study, the impact of RRD variations on the threshold method is shown in an analysis of 1-min rainrate data from a network of tipping-bucket gauges in Darwin, Australia. Data are analyzed for two distinct regimes: the premonsoon environment, having isolated intense thunderstorms, and the active monsoon rains, having organized convective cell clusters that generate large areas of stratiform rain. It is found that a threshold of 10 mm/h results in the same threshold coefficient for both regimes, suggesting an alternative definition of optimal threshold as that which is least sensitive to distribution variations. The observed behavior of the threshold coefficient is well simulated by assumption of lognormal distributions with different scale parameters and same shape parameters.

Short, David A.↗
NASA NTRS1990

Physical retrieval of precipitation water contents from Special Sensor Microwave/Imager (SSM/I) data. Part 1: A cloud ensemble/radiative parameterization for sensor response (report version)

The physical retrieval of geophysical parameters based upon remotely sensed data requires a sensor response model which relates the upwelling radiances that the sensor observes to the parameters to be retrieved. In the retrieval of precipitation water contents from satellite passive microwave observations, the sensor response model has two basic components. First, a description of the radiative transfer of microwaves through a precipitating atmosphere must be considered, because it is necessary to establish the physical relationship between precipitation water content and upwelling microwave brightness temperature. Also the spatial response of the satellite microwave sensor (or antenna pattern) must be included in the description of sensor response, since precipitation and the associated brightness temperature field can vary over a typical microwave sensor resolution footprint. A 'population' of convective cells, as well as stratiform clouds, are simulated using a computationally-efficient multi-cylinder cloud model. Ensembles of clouds selected at random from the population, distributed over a 25 km x 25 km model domain, serve as the basis for radiative transfer calculations of upwelling brightness temperatures at the SSM/I frequencies. Sensor spatial response is treated explicitly by convolving the upwelling brightness temperature by the domain-integrated SSM/I antenna patterns. The sensor response model is utilized in precipitation water content retrievals.

Olson, William S.↗
NASA NTRS1990

Forecast model applications of retrieved three dimensional liquid water fields

Forecasts are made for tropical storm Emily using heating rates derived from the SSM/I physical retrievals described in chapters 2 and 3. Average values of the latent heating rates from the convective and stratiform cloud simulations, used in the physical retrieval, are obtained for individual 1.1 km thick vertical layers. Then, the layer-mean latent heating rates are regressed against the slant path-integrated liquid and ice precipitation water contents to determine the best fit two parameter regression coefficients for each layer. The regression formulae and retrieved precipitation water contents are utilized to infer the vertical distribution of heating rates for forecast model applications. In the forecast model, diabatic temperature contributions are calculated and used in a diabatic initialization, or in a diabatic initialization combined with a diabatic forcing procedure. Our forecasts show that the time needed to spin-up precipitation processes in tropical storm Emily is greatly accelerated through the application of the data.

Raymond, William H.↗
NASA NTRS1993

Multiple scattering effects on the Linear Depolarization Ratio (LDR) measured during CaPE by a Ka-band air-borne radar

Air-borne radar measurements of thunderstorms were made as part of the CaPE (Convection and Precipitation/Electrification) experiment in Florida in July 1991. The radar has two channels, X-band (10 GHz) and Ka-band (34.5 GHz), and is capable of measuring cross-polarized returns as well as co-polarized returns. In stratiform rain, the cross-polarized components can be observed only at the bright band region and from the surface reflection. The linear depolarization ratios (LDR's) measured at X-band and Ka-band at the bright band are nearly equal. In convective rain, however, the LDR in Ka-band often exceeds the X-band LDR by several dB, and sometimes by more than 10 dB, reaching LDR values of up to -5 dB over heavy convective rain. For randomly oriented hydrometeors, such high LDR values cannot be explained by single scattering from non-spherical scattering particles alone. Because the LDR by single backscatter depends weakly on the wavelength, the difference between the Ka-band and X-band LDR's suggests that multiple scattering effects prevail in the Ka-band LDR. In order to test this inference, the magnitude of the cross-polarized component created by double scattering was calculated using the parameters of the airborne radar, which for both frequencies has beamwidths of 5.1 degrees and pulse widths of 0.5 microsecond. Uniform rain beyond the range of 3 km is assumed.

Iguchi, Toshio↗
NASA NTRS1993

Breaking Kelvin-Helmholtz waves and cloud-top entrainment as revealed by K-band Doppler radar

Radars have occasionally detected breaking Kelvin-Helmholtz (KH) waves under clear-air conditions in the atmospheric boundary layer and in the free troposphere. However, very few direct measurements of such waves within clouds have previously been reported and those have not clearly documented wave breaking. In this article, we present some of the most detailed and striking radar observations to date of breaking KH waves within clouds and at cloud top and discuss their relevance to the issue of cloud-top entrainment, which is believed to be important in convective and stratiform clouds. Aircraft observations reported by Stith suggest that vortex-like circulations near cloud top are an entrainment mechanism in cumuliform clouds. Laboratory and modeling studies have examined possibility that KH instability may be responsible for mixing at cloud top, but direct observations have not yet been presented. Preliminary analyses shown here may help fill this gap. The data presented in this paper were obtained during two field projects in 1991 that included observations from the NOAA Wave Propagation Laboratory's K-band Doppler radar (wavelength = 8.7 mm) and special rawinsonde ascents. The sensitivity (-30 dBZ at 10 km range), fine spatial resolution (375-m pulse length and 0.5 degrees beamwidth), velocity measurement precision (5-10 cm s-1), scanning capability, and relative immunity to ground clutter make it sensitive to non-precipitating and weakly precipitating clouds, and make it an excellent instrument to study gravity waves in clouds. In particular, the narrow beam width and short pulse length create scattering volumes that are cylinders 37.5 m long and 45 m (90 m) in diameter at 5 km (10 km) range. These characteristics allow the radar to resolve the detailed structure in breaking KH waves such as have been seen in photographic cloud images.

Martner, Brooks E.↗
NASA NTRS1993

Radiatively driven interactions between stratocumulus and synoptic waves

Quasi-geostrophic disturbances on a midlatitude beta-plane channel forced by radiative heating perturbations due to synoptic-scale variations of low-level stratiform cloud are considered. The longitudinal phase of the cloud is linked to that of the low-level streamfunction. Cloudiness is wavelike with cooling above cloudy areas that decreases exponentially with height. No perturbation cooling occurs above cloud-free areas. Two background flows are considered: one is constant and the other has a jet centered near the tropopause. Forced linear steady waves are found. Though infinitesimal amplitude disturbances are considered, the problem is nonlinear because of the coupling between cloud and winds. The resulting structures are sensitive to the phase shift between cloud and streamfunction with strongest coupling for cloud to the west of surface troughs. The waves have vertical scales on the order of the troposphere depth. The stationary structures capture the summertime pattern of stratocumulus off California and its linkage to the mid-Pacific ridge. When zonal-mean cloud cooling is allowed, the mean westerlies are strengthened in the northern half of the channel near the lower boundary. Synoptic-scale amplitudes respond to the mean cloud cooling by increasing (decreasing) just above the cloud in the northern (southern) part of the channel.

Clark, John H. E.↗
NASA NTRS1994

Multilevel cloud retrieval using multispectral HIRS and AVHRR data: Nighttime oceanic analysis

A multispectral, multiresolution (MSMR) method is developed for analyzing scenes of overlapping cloud layers. The MSMR method is applied to data from the NOAA 11 advanced very high resolution radiometer (AVHRR) and the high-resolution infrared radiometer sounder (HIRS-2). The data are from a nighttime oceanic scene in which a semitransparent cirrus veil overlays a large-scale stratus cloud. Low-cloud and clear-sky radiances are determined using a spatial coherence technique. Middle to upper level cloud pressures and radiances are estimated from HIRS-2 15 micrometer CO2 band radiometric data. The MSMR method improves the interpretation of a nighttime, oceanic scene containing thin cirrus over a large-scale stratiform cloud. If, for example, the same scene is analyzed using only the AVHRR 10.8 micrometer channel, the accompanying retrieved cloud heights are found to be between the cirrus and stratus cloud heights and are incorrectly identified as midlevel altostratus clouds. Theoretical radiative transfer model results for both water droplet spheres and randomly oriented hexagonal ice crystals are compared to observed AVHRR brightness temperature differences (BTD) between the 3.7- and 10.8 micrometer channels (BTD(sup 34)) and between the 10.8- and 12- micrometer channels (BTD(sup 45)) to distinguish among the effects of cloud optical depth, particle size, and phase for both single-layer clouds and overlapping two-layer clouds. Theoretical BTD calculations are used to estimate the range of effective particle sizes for eac h cloud layer. The data for the cirrus in the case study region near Bermuda are consistent with theoretical results for relatively small randomly oriented hexagonal ice crystals. The observed BTD(sup 34) and BTD(sup 45) values are lower for the cirrus above a lower-level cloud than for single-level cirrus with no underlying cloud. In certain cases the BTD analysis provides a way to distinguish between clouds composed of supercooled water droplets rather than ice particles. Analysis of nighttime data permits determination of stratus infrared optical depths smaller than 4.

Baum, Bryan A.↗
NASA NTRS1994

Microphysical growth state of ice particles and large-scale electrical structure of clouds

Cloud temperature, liquid water content, and vertical air velocity are all considered in evaluating the microphysical growth state of ice phase precipitation particles in the atmosphere. The large-scale observations taken together with in situ measurements indicated that the most prevalent growth condition for large ice particles in active convection is sublimation during riming, whereas the most prevalent growth condition in stratiform precipitation is vapor deposition. The large-scale electrical observations lend further support to the idea that particles warmed by riming into sublimation charge negatively and particles in vapor deposition charge positively in collisions with small ice particles.

Williams, Earle↗
NASA NTRS1994

A double-moment multiple-phase four-class bulk ice scheme, Part 1: Description

A detailed ice-phase bulk microphysical scheme has been developed for simulating the hydrometeor distributions of convective and stratiform precipitation in different large-scale environmental conditions. The proposed scheme involves 90 distinct microphysical processes, which predict the mixing ratios and the number concentrations of small ice crystals, snow, graupel, and frozen drops/hail, as well as the mixing ratios of liquid water on wet precipitation ice (snow, graupel, frozen drops). The number of adjustable coefficients has been significantly reduced in comparison with other bulk schemes. Additional improvements have been made to the parameterization in the following areas: (1) representing small ice crystals with nonzero terminal fall velocities and dispersive size distributions, (2) accurate and computationally efficient calculations of precipitation collection processes, (3) reformulating the collection equation to prevent unrealistically large accretion rates, (4) more realistic conversion by riming between different classes of precipitation ice, (5) preventing unrealistically large rates of raindrop freezing and freezing of liquid water on ice, (6) detailed treatment of various rime-splintering ice multiplication mechanisms, (7) a simple representation of the Hobbs-Rangno ice enhancement process, (8) aggregation of small ice crystals and snow, and (9) allowing explicit competition between cloud water condensation and ice deposition rates rather than using saturation adjustment techniques. For the purposes of conserving the higher moments of the particle distributions, preserving the spectral widths (or slopes) of the particle spectra is shown to be more important than strict conservation of particle number concentration when parameterizing changes in ice-particle number concentrations due to melting, vapor transfer processes (sublimation of dry ice, evaporation from wet ice), and conversion between different hydrometeor species. The microphysical scheme is incorporated into a nonhydrostatic cloud model in Part 2 of this study. The model performed well in simulating the radar and microphysical structures of a midlatitude-continental squall lines and a tropical-maritime squall system with minimal tuning of the parameterization, even though the vertical profiles of radar reflectivity differed substantially between these storms.

Ferrier, Brad Shoenberg↗
NASA NTRS1994

Cirrus cloud retrieval using infrared sounding data: Multilevel cloud errors

In this study we perform an error analysis for cloud-top pressure retrieval using the High-Resolution Infrared Radiometric Sounder (HIRS/2) 15-microns CO2 channels for the two-layer case of transmissive cirrus overlying an overcast, opaque stratiform cloud. This analysis includes standard deviation and bias error due to instrument noise and the presence of two cloud layers, the lower of which is opaque. Instantaneous cloud pressure retrieval errors are determined for a range of cloud amounts (0.1-1.0) and cloud-top pressures (850-250 mb). Large cloud-top pressure retrieval errors are found to occur when a lower opaque layer is present underneath an upper transmissive cloud layer in the satellite field of view (FOV). Errors tend to increase with decreasing upper-cloud effective cloud amount and with decreasing cloud height (increasing pressure). Errors in retrieved upper-cloud pressure result in corresponding errors in derived effective cloud amount. For the case in which a HIRS FOV has two distinct cloud layers, the difference between the retrieved and actual cloud-top pressure is positive in all cases, meaning that the retrieved upper-cloud height is lower than the actual upper-cloud height. In addition, errors in retrieved cloud pressure are found to depend upon the lapse rate between the low-level cloud top and the surface. We examined which sounder channel combinations would minimize the total errors in derived cirrus cloud height caused by instrument noise and by the presence of a lower-level cloud. We find that while the sounding channels that peak between 700 and 1000 mb minimize random errors, the sounding channels that peak at 300-500 mb minimize bias errors. For a cloud climatology, the bias errors are most critical.

Baum, Bryan A.↗
NASA NTRS1994

A statistical analysis of the dependency of closure assumptions in cumulus parameterization on the horizontal resolution

Simulated data from the UCLA cumulus ensemble model are used to investigate the quasi-universal validity of closure assumptions used in existing cumulus parameterizations. A closure assumption is quasi-universally valid if it is sensitive neither to convective cloud regimes nor to horizontal resolutions of large-scale/mesoscale models. The dependency of three types of closure assumptions, as classified by Arakawa and Chen, on the horizontal resolution is addressed in this study. Type I is the constraint on the coupling of the time tendencies of large-scale temperature and water vapor mixing ratio. Type II is the constraint on the coupling of cumulus heating and cumulus drying. Type III is a direct constraint on the intensity of a cumulus ensemble. The macroscopic behavior of simulated cumulus convection is first compared with the observed behavior in view of Type I and Type II closure assumptions using 'quick-look' and canonical correlation analyses. It is found that they are statistically similar to each other. The three types of closure assumptions are further examined with simulated data averaged over selected subdomain sizes ranging from 64 to 512 km. It is found that the dependency of Type I and Type II closure assumptions on the horizontal resolution is very weak and that Type III closure assumption is somewhat dependent upon the horizontal resolution. The influences of convective and mesoscale processes on the closure assumptions are also addressed by comparing the structures of canonical components with the corresponding vertical profiles in the convective and stratiform regions of cumulus ensembles analyzed directly from simulated data. The implication of these results for cumulus parameterization is discussed.

Xu, Kuan-Man↗
NASA NTRS1995

Microwave and infrared simulations of an intense convective system and comparison with aircraft observations

A three-dimensional cloud model, radiative transfer model-based simulation system is tested and validated against the aircraft-based radiance observations of an intense convective system in southeastern Virginia on 29 June 1986 during the Cooperative Huntsville Meteorological Experiment. NASA's ER-2, a high-altitude research aircraft with a complement of radiometers operating at 11-micrometer infrared channel and 18-, 37-, 92-, and 183-GHz microwave channels provided data for this study. The cloud model successfully simulated the cloud system with regard to aircraft- and radar-observed cloud-top heights and diameters and with regard to radar-observed reflectivity structure. For the simulation time found to correspond best with the aircraft- and radar-observed structure, brightness temperatures T(sub b) are simulated and compared with observations for all the microwave frequencies along with the 11-micrometer infrared channel. Radiance calculations at the various frequencies correspond well with the aircraft observations in the areas of deep convection. The clustering of 37-147-GHz T(sub b) observations and the isolation of the 18-GHz values over the convective cores are well simulated by the model. The radiative transfer model, in general, is able to simulate the observations reasonably well from 18 GHz through 174 GHz within all convective areas of the cloud system. When the aircraft-observed 18- and 37-GHz, and 90- and 174-GHz T(sub b) are plotted against each other, the relationships have a gradual difference in the slope due to the differences in the ice particle size in the convective and more stratiform areas of the cloud. The model is able to capture these differences observed by the aircraft. Brightness temperature-rain rate relationships compare reasonably well with the aircraft observations in terms of the slope of the relationship. The model calculations are also extended to select high-frequency channels at 220, 340, and 400 GHz to simulate the Millimeter-wave Imaging Radiometer aircraft instrument to be flown in the near future. All three of these frequencies are able to discriminate the convective and anvil portions of the system, providing useful information similar to that from the frequencies below 183 GHz but with potentially enhanced spatial resolution from a satellite platform. In thin clouds, the dominant effect of water vapor is seen at 174, 340, and 400 GHz. In thick cloudy areas, the scattering effect is dominant at 90 and 220 GHz, while the overlaying water vapor can attenuate at 174, 340, and 400 GHz. All frequencies (90-400 GHz) show strong signatures in the core.

Prasad, N.↗
NASA NTRS1994

The evolution of cloud-to-ground lightning within a portion of the 10-11 June 1985 squall line

The character of cloud-to-ground lightning is examined during the life cycle of a distinct mesoscale segment of the 10-11 June 1985 mesoscale convective system (MCS). Three phases of lightning activity are identified and related to both the radar-observed structure of the convection and to the severe weather produced by the MCS. Positive strikes to ground are dominant when the MCS is first developing. Negative strikes then dominate during a period of intense leading-line convective activity with high storm tops. Finally, a period of relatively frequent positive strikes within the trailing stratiform region occurs during the demise of the MCS. This last phase begins after the vertical extent of the leading convective line decreases rapidly and markedly, with moderate intensity echoes (i.e., 30-40 dBZ) occurring mostly below the freezing level. The first period of frequent positive flashes results from the lightning associated with a single severe thunderstorm in southwest Kansas; however, a second severe storm occurs nearby and produces mainly negative strikes. An extended period of strong surface winds does not appear to have any direct relationship with the observed character of the lightning activity.

Nielsen, K. E.↗
NASA NTRS1995

Diagnosis of hydrometeor profiles from area-mean vertical-velocity data

A simple one-dimensional microphysical retrieval model is developed for estimating vertical profiles of liquid and frozen hydrometeor mixing ratios from observed vertical profiles of area-mean vertical velocity in regions of convective and/or stratiform precipitation. The mean vertical-velocity profiles can be obtained from Doppler radar (single and dual) or other means. The one-dimensional results are shown to be in good agreement with two-dimensional microphysical fields from a previous study. Sensitivity tests are performed.

Braun, Scott A.↗
NASA NTRS1993

A regional rainfall climatology over Mexico and the southwest United States derived from passive microwave and geosynchronous infrared data

A three-year climatology of satellite-estimated rainfall for the warm season for the southwest United States and Mexico has been derived from data from the Special Sensor Microwave Imager (SSM/I). The microwave data have been stratified by month (June, July, August), year (1988, 1989, 1990), and time of day (morning and evening orbits). A rain algorithm was employed that relates 86-GHz brightness temperatures to rain rate using a coupled cloud-radiative transfer model. Results identify an early evening maximum in rainfall along the western slope of the Sierra Madre Occidental during all three months. A prominent morning rainfall maximum was found off the western Mexican coast near Mazatlan in July and August. Substantial differences between morning and evening estimates were noted. To the extent that three years constitute a climatology, results of interannual variability are presented. Results are compared and contrasted to high-resolution (8 km, hourly) infrared cloud climatologies, which consist of the frequency of occurrence of cloud colder than -38 C and -58 C. This comparison has broad implications for the estimation of rainfall by simple (cloud threshold) techniques. By sampling the infrared data to approximate the time and space resolution of the microwave, we produce ratios (or adjustment factors) by which we can adjust the infrared rain estimation schemes. This produces a combined microwave/infrared rain algorithm for monthly rainfall. Using a limited set of raingage data as ground truth, an improvement (lower bias and root-mean-square error) was demonstrated by this combined technique when compared to either method alone. The diurnal variability of convection during July 1990 was examined using hourly rain estimates from the Geostationary Operational Environmental Satellite (GOES) precipitation index and the convective stratiform technique, revealing a maximum in estimated rainfall from 1800 to 2100 local time. It is in this time period when the SSM/I evening orbit occurs. A high-resolution topographic database was available to aid in interpreting the influence of topography on the rainfall patterns.

Negri, Andrew J.↗
NASA NTRS1996

Correction of WindScat Scatterometric Measurements by Combining with AMSR Radiometric Data

The Seawinds scatterometer on the advanced Earth observing satellite-2 (ADEOS-2) will determine surface wind vectors by measuring the radar cross section. Multiple measurements will be made at different points in a wind-vector cell. When dense clouds and rain are present, the signal will be attenuated, thereby giving erroneous results for the wind. This report describes algorithms to use with the advanced mechanically scanned radiometer (AMSR) scanning radiometer on ADEOS-2 to correct for the attenuation. One can determine attenuation from a radiometer measurement based on the excess brightness temperature measured. This is the difference between the total measured brightness temperature and the contribution from surface emission. A major problem that the algorithm must address is determining the surface contribution. Two basic approaches were developed for this, one using the scattering coefficient measured along with the brightness temperature, and the other using the brightness temperature alone. For both methods, best results will occur if the wind from the preceding wind-vector cell can be used as an input to the algorithm. In the method based on the scattering coefficient, we need the wind direction from the preceding cell. In the method using brightness temperature alone, we need the wind speed from the preceding cell. If neither is available, the algorithm can work, but the corrections will be less accurate. Both correction methods require iterative solutions. Simulations show that the algorithms make significant improvements in the measured scattering coefficient and thus is the retrieved wind vector. For stratiform rains, the errors without correction can be quite large, so the correction makes a major improvement. For systems of separated convective cells, the initial error is smaller and the correction, although about the same percentage, has a smaller effect.

Song, S.↗