Showing papers in "Journal of Applied Meteorology in 1993"

A statistical analysis of mesoscale rainfall as a random cascade

Abstract: The paper considers the random cascade theory for spatial rainfall Particular attention was given to the following four areas: (1) the relationship of the random cascade theory of rainfall to the simple scaling and the hierarchical cluster-point-process theories, (2) the mathematical foundations for some of the formalisms commonly applied in the develpment of statistical cascade theory, (3) the empirical evidence for a random cascade theory of rainfall, and (4) the way of using data for making estimates of parameters and for making statistical inference within this theoretical framework An analysis of space-time rainfall data is presented Cascade simulations are carried out to provide a comparison with methods of analysis that are applied to the rainfall data

Developing Satellite-derived Estimates of Surface Moisture Status

Abstract: An evaluation is made of the remotely sensed surface temperature (Ts)/normalized difference vegetation index (NDVI) relationship in studies of the influence of biome type on the slope of Ts/NDVI, and of the automation of the process of defining the relationship so that the surface moisture status can be compared with Ts/NDVI at continental scales. The analysis is conducted using the NOAA AVHRR over a 300 x 300 km area in western Montana, as well as biweekly composite AVHRR data. A strong negative relationship is established between NDVI and Ts over all biome types.

Universal Multifractals: Theory and Observations for Rain and Clouds

Abstract: The standard model of atmospheric motions divides the atmosphere into distinct two- and three-dimensional isotropic turbulent regimes separated by a dimensional transition, the “mesoscale gap.” It is argued that the “gap” is fictional and that the atmosphere is scaling but anisotropic at all scales. According to this alternative unified scaling model, the dynamics are governed by anisotropic (differentially stratified and rotating) cascade processes yielding highly variable multifractal fields. Just as Gaussian random variables are associated with (linear) sums of random variables, these (nonlinear) multiplicative processes are generically associated with (special) universal multifractals in which many of the details of the dynamics are irrelevant. Although an attempt is made to outline these arguments in a widely accessible form, they are not new to this paper; they provide its context and motivation. The principal purpose of this paper is to test these ideas empirically. This is done using Land...

Effect of Amazon Smoke on Cloud Microphysics and Albedo-Analysis from Satellite Imagery

Abstract: The effect of smoke aerosol particles on the properties of low cumulus and stratocumulus clouds is studied on the basis of NOAA AVHRR images taken over the Brazilian Amazon Basin during the biomass burning season of 1987. The reflectance at a wavelength of 0.64 micron and the drop size, derived from the cloud reflectance at 3.75 microns, is studied for tens of thousands of clouds. It is shown that the presence of dense smoke can reduce the remotely sensed drop size of continental cloud drops from 15 to 9 microns. Due to both the high initial reflectance of clouds in the visible part of the spectrum and the presence of graphitic carbon, the average cloud reflectance at 0.6 micron is reduced from 0.71 to 0.68 for an increase in smoke optical thickness from 0.1 to 2.0. High concentration of aerosol particles is shown to cause a decrease in the cloud-drop size, and smoke is found to darken the bright Amazonian clouds.

Operational Cloud-Motion Winds from Meteosat Infrared Images

Abstract: The displacement of clouds in successive satellite images reflects the atmospheric circulation at various scales. The main application of the satellite-derived cloud-motion vectors is their use as winds in the data analysis for numerical weather prediction. At low latitudes in particular they constitute an indispensible data source for numerical weather prediction. This paper describes the operational method of deriving cloud-motion winds (CMW) from the IR image (10.5–12.5 µm) of the European geostationary Meteostat satellites. The method is automatic, that is, the cloud tracking uses cross correlation and the height assignment is based on satellite observed brightness temperature and a forecast temperature profile. Semitransparent clouds undergo a height correction based on radiative forward calculations and simultaneous radiance observations in both the IR and water vapor (5.7–7.1 µm) channel. Cloud-motion winds are subject to various quality checks that include manual quality control as the la...

The use of NOAA AVHRR data for assessment of the urban heat island effect

Abstract: The objective of the study was to evaluate the use of a satellite-derived vegetation index and surface temperature estimates for the assessment of the difference in urban and rural air temperature due to the urban heat island effect. The difference in the ND (normalized difference) index between urban and rural regions appears to be an indicator of the difference in surface properties (evaporation and heat storage capacity) between the two environments that are responsible for the urban heat island effect. The use of the approach proposed here may provide a globally consistent method for assessing this phenomenon.

General Probability-matched Relations between Radar Reflectivity and Rain Rate

Abstract: An improved method for transforming radar-observed reflectivities Ze into rain rate R is presented. The method is based on a formulation of a Ze-R function constrained such that (1) the radar-retrieved pdf of R and all of its moments are identical to those determined from the gauges over a sufficiently large domain, and (2) the fraction of the time that it is raining above a low but still has an accurately measurable rain intensity is identical for both the radar and for simultaneous measurements of collocated gauges on average. Data measured by a 1.65-deg beamwidth C-band radar and 22 gauges located in the vicinity of Darwin, Australia, are used. The resultant Ze-R functions show a strong range dependence, especially for the rain regimes characterized by strong reflectivity gradients and substantial attenuation. The application of these novel Ze-R functions to the radar data produces excellent matches to the gauge measurements without any systematic bias.

Effects of the Large-Scale Flow on Characteristic Features of the Sea Breeze

Abstract: A two-dimensional nonlinear numerical model has been used to examine the effects of the ambient wind on the development of characteristic features of the sea breeze. The specific features that were examined were the maximum shoreward velocity component (both total and as a perturbation from the large-scale flow) anywhere in the simulation domain, the inland penetration of the sea breeze, the depth of the inflow layer at the coastline, the maximum vertical velocity anywhere in the domain, the maximum potential-temperature gradient anywhere in the domain, and the surface heat flux. Thirty-one simulators were performed, with large-scale geostrophic winds ranging from onshore (i.e., supporting) flow of 15 m s−1 to offshore (i.e., opposing) flow of 15 m s−1, at 1 m s−1 intervals. The results indicated that the sea-breeze perturbation was suppressed for onshore large-scale flow of a few meters per second or more. In contrast, a sea breeze was produced for opposing large-scale flow as strong as 11 m s−1...

Estimation of Monthly Rainfall over Japan and Surrounding Waters from a Combination of Low-Orbit Microwave and Geosynchronous IR Data

Abstract: A method is described for estimating the mean monthly rainfall data for climate studies by combining the geosynchronous IR and low-orbit microwave data. The microwave technique uses the brightness temperature at 37 and 86 GHz from the Special Sensor Microwave/Imager instrument on board the DMSP satellite to define raining areas over water and land, and the 86-GHz scattering signal to assign rain rate based on cloud model-microwave calculations. The IR estimates are initially computed separately, using hourly data from the Japanese Geostationary Meteorological Satellite. Results show that, in areas where the microwave technique performs well, the combined microwave-IR monthly total estimates have better error statistics than either the microwave of the IR techniques individually.

The Relationship between Overlying Synoptic-Scale Flows and Winds within a Valley

Abstract: The relationship between winds above and within the Tennessee Valley is investigated climatologically and with an atmospheric numerical model. For the climatological analyses, winds above the valley were determined by interpolation from four surrounding rawinsonde stations, while winds within the valley were measured on four 100-m towers. Tennessee Valley winds are generally weak and bidirectional, oriented along the valley's axis. The valley wind direction depends strongly on the component of the synoptic-scale pressure gradient that is superimposed along the valley's axis at ridge-top level, with winds blowing along the valley's axis from high toward low pressure. This relationship between winds above and within the valley can result in countercurrents similar to those observed in the Rhine Valley. While winds in the Tennessee Valley are driven primarily by this pressure-driven channeling mechanism, downward momentum transport can cause afternoon winds within the valley to approach the wind dir...

Global Fields of Soil Moisture and Land Surface Evapotranspiration Derived from Observed Precipitation and Surface Air Temperature.

Abstract: The global fields of normal monthly soil moisture and land surface evapotranspiration are derived with a simple water budget model that has precipitation and potential evapotranspiration as inputs. The precipitation is observed and the potential evapotranspiration is derived from the observed surface air temperature with the empirical regression equation of Thornthwaite (1954). It is shown that at locations where the net surface radiation flux has been measured, the potential evapotranspiration given by the Thornthwaite equation is in good agreement with those obtained with the radiation-based formulations of Priestley and Taylor (1972), Penman (1948), and Budyko (1956-1974), and this provides the justification for the use of the Thornthwaite equation. After deriving the global fields of soil moisture and evapotranspiration, the assumption is made that the potential evapotranspiration given by the Thornthwaite equation and by the Priestley-Taylor equation will everywhere be about the same; the inverse of the Priestley-Taylor equation is used to obtain the normal monthly global fields of net surface radiation flux minus ground heat storage. This and the derived evapotranspiration are then used in the equation for energy conservation at the surface of the earth to obtain the global fields of normal monthly sensible heat flux from the land surface to the atmosphere.

Warm-Rain Initiation: An Overview of Microphysical Mechanisms.

Abstract: Rain triggering mechanisms are evaluated in three microphysical steps: droplet activation on cloud condensation nuclei, droplet growth by condensation, and droplet growth by coalescence. Although considerable progress has been made since the pioneering work of Squires, crucial questions in each of the above steps remain unresolved: Under what conditions do giant particles trigger rain by acting as coalescence nuclei? What is the contribution of stochastic condensation to the growth of large droplets in regions of entrainment? What are the collection efficiencies for droplet sizes critical to the onset of colaescence growth? Such questions cannot be answered without better observations. Aircraft instruments are becoming available with the potential to measure the very largest particles and cloud droplets at the concentration of raindrops. Recent advances in sampling and analysis techniques have extended observations of cloud microstructure to smaller scales, providing new insight on the growth of ...

A Review of Thunderstorm Electrification Processes

Abstract: Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer o...

Entrainment in Cumulus Clouds

Abstract: Entrainment of dry air into cumulus clouds influences the development of the clouds in a major way. The many aspects of the entrainment process are examined in this paper by critically reviewing the literature from the time when investigations began. It is an interesting time in the evolution of the study of cumulus clouds with the advent of different models and several new instruments. Traditional entraining plume and thermal models that received considerable attention during the early years are being replaced by episodic-type mixing models. Recent observations of the source of entrained air are in part responsible for the new thinking, but the ideas really originate with the suggestion made by Squires more than 40 years age that vertical rather than horizontal mixing causes the dilution of cumuli by the process of penetrative downdrafts.

Wavelet Analysis of Coherent Structures at the Atmosphere-Forest Interface.

Abstract: Wavelet analysis was applied to turbulence data for temperature and vertical velocity within and above a deciduous forest. This method appears to provide an objective technique for examining thermal and flow fields associated with coherent structures occurring near the forest. The two-dimensional unfolding in time and scale by the wavelet transform illustrates discrete warm and cool centers associated with organized updrafts and downdrafts, which have similar patterns but different magnitudes at different heights. Wavelet variances computed for temperature and velocity at different heights appear to have local maximum values corresponding to certain time scales, which are self-consistent and useful for objective determination of the principal time scale of the structures. Within the canopy, the principal time scales of the structures determined by this technique are 56–60 s and 40–44 s for the temperature and vertical velocity fields, respectively. These time scales are close to those determined ...

Foundations for statistical-physical precipitation retrieval from passive microwave satellite measurements. II: Emission-source and generalized weighting-function properties of a time-dependent cloud-radiation model

Abstract: The foremost physical aspects of microphysical-radiative interactions that control the formation and modulation of frequency-dependent passive microwave TB over a continental cold-rain precipitation system are explained within the framework of the essential ingredients of a physically based precipitation-retrieval algorithm. The analysis is based on a modeling study in which a 3D cloud model has provided the objective basis for generating an extensive set of microphysical profiles that describe numerous precipitation features in the course of the evolution of a continental hail storm. A summary of the various components of the algorithm, as well as the surface rain rates, is given. The algorithm employs the cloud model to provide a consistent and objectively generated source of detailed microphysical information as the underpinnings to an inversion-based perturbative retrieval scheme.

A Comparison of Several Techniques to Assign Heights to Cloud Tracers

Abstract: Experimental results are presented which suggest that the water-vapor technique of radiance measurement is a viable alternative to the CO2 technique for inferring the height of semitransparent cloud elements. Future environmental satellites will rely on H2O-derived cloud-height assignments in the wind-field determinations with the next operational geostationary satellite. On a given day, the heights from the H2O and CO2 approaches compare to within 60-110 hPa rms.

Weather Radar Performance at Long RangeSimulated and Observed

Abstract: A large set of high-vertical-resolution reflectivity profiles was used to simulate the performance of radars in the United Kingdom weather radar network. In particular, limitations in the estimation of surface precipitation due to incomplete beam filling and variability in the reflectivity profile were investigated. Marked seasonal variations in range performance were found and detection failures were shown to make a large contribution to accumulation underestimation at long range. Although some current methods of correcting radar data for the effect of range may be capable of reducing mean biases, the ratio between radar measured precipitation rate and the rate at ground level is subject to large uncertainty at long range due to variability in reflectivity profile shape on the scale of individual 5-km pixels. The results of the simulation experiments were validated by comparison with monthly averages of data from two radars in the operational network. It was concluded that long-term averages of ...

Influence of the Agulhas Current on Summer Rainfall along the Southeast Coast of South Africa

Abstract: Summer rainfall on the southeast coast of Africa is shown to be influenced by the proximity and temperature of the adjacent warm Agulhas Current. Student's t-test correlations between coastal rainfall and local sea surface temperature (r = +0.88), shelf width (r = −0.92), and offshore distance to the thermal core (r = −0.73) all exceed significance at the 99% confidence limit. Summer rainfall patterns indicate the existence of a large-scale gradient from northeast to southwest, in the same direction as that of the SST. Alongshore variations in the rainfall gradient appear related to the distance between the coast and the warm current at the shelf edge, and to orographic effects. It is thought that a quasi-stationary mesoscale circulation system over the Agulhas Current interests with transient weather systems to produce alongshore changes in the rainfall gradient. Additionally, the modification of onshore moving air by surface heat fluxes gives rise to the close relationships between summer rainf...

Polarimetric signatures in the stratiform region of a mesoscale convective system

Abstract: Four polarimetric measurands were collected in the stratiform region of a mesoscale convective system. The four are the reflectivity factor, the differential reflectivity, the correlation coefficient between orthogonal copolar echoes, and the differential propagation constant. Most striking is a signature of large aggregates (about 10 mm in size) seen in the differential phase through the melting layer. Another significant feature is an abrupt notch in the correlation coefficient that occurs towards the bottom of the bright band. Aircraft observations and a one-dimensional cloud model are used to explain some polarimetric measurements and to infer the presence of aggregates, graupel, and supercooled cloud water in the stratiform region. These unique observations and model data provide inferences concerning the presence of graupel and the growth of large aggregates in the melting layer.

A Method for Determining Cirrus Cloud Particle Sizes Using Lidar and Radar Backscatter Technique

Abstract: A method to determine cirrus cloud effective radii remotely using lidar and radar backscatter data is presented. The difference in backscattered returns from instruments widely separated in wavelength holds information on the characteristic sizes of the scatterers. The method compares theoretically expected backscatter coefficients to observed backscatter returns from NOAA's 3.2-cm and 8.6-mm radars and the 10.6-µm lidar. Measurements were taken during a two-phase cloud experiment held in northeastern Colorado from 6 September to 5 October 1989 and 15 February to 31 March 1991. It was found that the particle sizes estimated from the lidar-radar method agree closely with in situ aircraft measurements. Case studies are presented to demonstrate the method and the potential for multiwavelength remote sensing of cirrus cloud radiative properties.

Cloud Condensation Nuclei

Abstract: The state of knowledge of the particles upon which liquid droplets condense to form atmospheric water clouds is presented. The realization of cloud condensation nuclei (CCN) as a distinct aerosol subset originated with the cloud microphysical measurements and theoretical insights of Patrick Squires 40 years ago. He helped originate and continue the development of CCN counters and made significant CCN measurements for more than 25 years. Recognition of the importance of CCN has expanded from warm-rain efficiency to aerosol scavenging, cloud radiative properties and other topics. In spite of a promising beginning and much encouragement over the years, CCN knowledge has increased minimally. Significant uncertainties about global climate change cannot be reduced without expansion of the knowledge base of CCN.

Sequential Assimilation of Soil Moisture from Atmospheric Low-Level Parameters. Part I: Sensitivity and Calibration Studies

Abstract: This paper and its companion report on the development of a sequential assimilation technique based upon optimum interpolation in order to initialize soil moisture in atmospheric models. A previous study by Mahfouf has demonstrated that it is possible to estimate soil moisture from the evolution of atmospheric temperature and relative humidity near the surface. The main purpose of this paper is to examine more precisely the dependence of atmospheric low-level parameters upon soil moisture and how this dependence is affected by various factors (soil characteristics, vegetation type, low-level wind). The sensitivity of atmospheric parameters to soil moisture is expressed as the statistical quantities of the optimal interpolation. The importance of observation errors, which define the relevance of the atmospheric parameters for the assimilation procedure, is also investigated. An analytical formulation of the optimal interpolation coefficients is proposed. Finally, the usefulness and limitations of ...

A Study of Sampling-Variability Effects in Raindrop Size Observations

Abstract: Because of the randomness associated with sampling from a population of raindrops, variations in the data reflect some undetermined mixture of sampling variability and inhomogeneity in the precipitation. Better understanding of the effects of sampling variability can aid in interpreting drop size observations. This study begins with a Monte Carlo simulation of the sampling process and then evaluates the resulting estimates of the characteristics of the underlying drop population. The characteristics considered include the liquid water concentration and the reflectivity factor; the maximum particle size in each sample is also determined. The results show that skewness in the sampling distributions when the samples are small (which is the usual case in practice) produces a propensity to underestimate all of the characteristic quantities. In particular, the distribution of the sample maximum drop sizes suggests that it may be futile to try to infer an upper truncation point for the size distribution...

Flow Boundaries in Random-Flight Dispersion Models: Enforcing the Well-Mixed Condition

Abstract: Lagrangian stochastic (LS) dispersion models often use trajectory reflection to limit the domain accessible to a particle. It is shown how the well-mixed condition (Thomson) can he expressed in the Chapman-Kolmogorov equation for a discrete-time LS model to provide a test for the validity of a reflection algorithm. By that means it is shown that the usual algorithm (perfect reflection) is exactly consistent with the wmc when used to bound Gaussian homogeneous turbulence, but that no reflection scheme can satisfy the wmc when applied at a location where the probability distribution for the normal velocity is asymmetric, or locally inhomogeneous. Thus, there is no well-mixed reflection scheme for inhomogeneous or skew turbulence.

Longwave Radiation from Clear Skies in Niger: A Comparison of Observations with Simple Formulas

Abstract: Observations of downward longwave radiation from clear skies over Niger, West Africa, are compared with values calculated from atmospheric profiles and with estimates from several equations based on screen-level variables. It is shown that the Eppley pyrgeometer measurements are in error during daylight hours but that this error can be satisfactorily corrected. Measured atmospheric profiles of temperature and vapor density are used to tailor Brutsaert's equation to the dry-season atmospheric conditions in Niger. The adjusted form of Brutsaert's equation describes the data excellently, illustrating the fact that its analytical derivation makes it more easily adaptable than other similar, but empirically derived, equations. There is considerable variation in the output of thew empirical equations, especially during the dry season, but those equations that include a vapor-pressure term perform best.

Rainfall Estimation Using Polarimetric Techniques at C-Band Frequencies

Abstract: The accuracy of radar measurements and their derived parameters, such as rainfall rate, are compromised by errors caused by propagation effects at C-band frequencies. The radar measurements of reflectivity factor Z and differential reflectivity ZDR are affected by the absolute and differential attenuation through the rain medium. Another useful radar-derived parameter, differential propagation phase shift ΦDP, is contaminated by the differential phase on backscatter δ, which attains significant values in rainfall at C-band frequencies. In this paper we present a technique to correct these propagation and backscatter effects by application of an algorithm that corrects first Z and ZDR, using relationships between the specific and differential attenuations versus phase shift, which is followed by estimation of the differential backscatter phase shift parameter δ from the corrected ZDR. Simulation results are presented to demonstrate the effectiveness of this correction procedure for two cases: (a) ...

Parameterization of surface heat fluxes above forest with satellite thermal sensing and boundary-layer soundings

Abstract: Satellite-derived surface temperature measurements can be used in conjunction with temperature and wind soundings in the boundary layer to determine the surface sensible heat flux from forests at the regional scale. The underlying formulation is derived on the basis of similarity principles applied to the unstable turbulent boundary layer in which the scalar roughness for sensible heat is determined by calibration. This is illustrated herein with data acquired during the HAPEX–MOBILHY (Hydrologic Atmospheric Pilot Experiment-Modelisation du Bilan Hydrique), which was conducted in the spring and summer of 1986 over a 100-km × 100-km square in southwestern France. The northwest third of the area is covered by the Landes forest. The surface temperatures for a 25-km × 25-km area in this forested region were determined from the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) channels 4 and 5. Atmospheric collections were applied with LOWTRAN-7 and measured atmospheric profiles, as well as with...

Use of Weather Radar for Flood Forecasting in the Sieve River Basin: A Sensitivity Analysis

Abstract: Weather radar, in combination with a distributed rainfall-runoff model, promises to significantly improve real-time flood forecasting. This paper investigates the value of radar-derived precipitation in forecasting streamflow in the Sieve River basin, near Florence, Italy. The basin is modeled with a distributed rainfall-runoff model that exploits topographic information available from digital elevation maps. The sensitivity of the flood forecast to various properties of the radar-derived rainfall is studied. It is found that use of the proper radar reflectivity-rainfall intensity (Z-R) relationship is the most crucial factor in obtaining correct food hydrographs. Errors resulting from spatially averaging radar rainfall are acceptable, but the use of discrete point information (i.e., raingage) can lead to serious problems. Reducing the resolution of the 5-min radar signal by temporally averaging over 15 and 30 min does not lead to major errors. Using 3-bit radar data (rather than the usual 8-bit ...

Estimating effective data density in a satellite retrieval or an objective analysis

Abstract: An attempt is made to formulate consistent objective definitions of the concept of 'effective data density' applicable both in the context of satellite soundings and more generally in objective data analysis. The definitions based upon various forms of Backus-Gilbert 'spread' functions are found to be seriously misleading in satellite soundings where the model resolution function (expressing the sensitivity of retrieval or analysis to changes in the background error) features sidelobes. Instead, estimates derived by smoothing the trace components of the model resolution function are proposed. The new estimates are found to be more reliable and informative in simulated satellite retrieval problems and, for the special case of uniformly spaced perfect observations, agree exactly with their actual density. The new estimates integrate to the 'degrees of freedom for signal', a diagnostic that is invariant to changes of units or coordinates used.