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C. D. Stow
and
K. Jones

Abstract

A disdrometer for the measurement of the co-spectra of raindrop size and charge is described which can evaluate to a significant extent spurious data caused by unavoidable drop overlap within the charge detector volume. The sizes of individual drops in the range 0.1–2.5 mm radius and charges within the limits of magnitude 0.1–10 pC could be determined. The use of two size detectors enabled the measurement of drop velocity and the detection of drop overlap within the charge-sensitive volume to be made. Non-ideal data which arise from natural conditions, measuring sites, and from fundamental or unavoidable deficiencies in disdrometer design, could be tested and monitored automatically using a central processor under software control: TIME OUT, when a drop failed to pass through both size detectors; COINCIDENCE, caused by simultaneous occupation of both size detector volumes; SIZE MISMATCH, when the separate size measurements did not agree within limits predetermined by software; HIGH or LOW VELOCITY, when the actual drop velocity was not close to the terminal value expected from size measurement. The self-evaluating disdrometer cannot be designed to provide the minimum error content possible but offers the advantage of assessing the proportion of spurious data present; it is argued that this may be preferable to the situation in which the error content is lower but unknown. The performance of the instrument was assessed using individual drops generated in the laboratory and by exposure to natural rain falling through an aperture into a chamber shielded from wind and associated turbulence. The latter test was made at a non-ideal site in order to demonstrate the ability of the disdrometer to provide information on invalid data so that raw co-spectra may be corrected. In the preliminary tests described a substantial proportion of the drops possessed fall speeds significantly below their expected terminal velocity, in some cases as much as 30% less, and not more than 20% of the drops detected satisfied all criteria for acceptance. Further, an examination of drop arrival rates showed that not all data could be fitted to a Poisson-type distribution, either because of rapid changes in the mean arrival rate or on account of the clustering of drops. The potential seriousness of the drop overlap problem, which is fundamental to all methods of measurements, is emphasized in the trial analysis: uncertainties in the exact form of the size distribution, particularly for drops in the radius range 0.1–0.5 mm, render the design of any instruments of fixed entrance aperture size dubious; the co-spectra must be expected to show appreciable distortion unless data associated with drop overlap, particularly within the charge-sensitive volume, are excluded. Some improvements in the current disdrometer design are suggested.

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G. A. Jones
and
S. K. Avery

Abstract

The effects of the zonal mean circulation and planetary-wave winds on the distribution of nitric oxide in the 55–120 km height region is investigated. A time-dependent numerical model is used to investigate the interaction between planetary waves and the zonal mean circulation, and the effect of the circulation on the nitric oxide distribution is determined. The initial nitric oxide (NO) distribution is obtained by using a simple source/sink chemistry, vertical eddy diffusion, and advective transport by the zonal mean circulation. Changes in the initial NO distribution which result from the addition of planetary-wave winds are described. Planetary waves are found to induce a wave-like structure in the nitric oxide distribution which resembles that derived from observational data. Planetary waves can affect the nitric oxide concentration in two ways: first,through the wave-induced changes in the mean meridional circulation, and second, through the nitric oxide perturbation induced by wave winds themselves. The changes in total nitric oxide are due primarily to the zonal asymmetries in nitric oxide induced by the planetary waves. Implications of this result for explaining the winter anomaly are discussed.

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B. K. Jones
and
J. R. Saylor

Abstract

The shapes of falling raindrops are often significantly altered by drop oscillations, complicating dual-polarization radar methods that rely on a predictable, monotonic variation of drop axis ratio α with equivolume drop diameter d. This oscillation behavior varies with d so that time-averaged shapes, which are determined by oscillation mode, sometimes deviate from the d-dependent quiescent shape. The literature identifies a predominance of particular oscillation modes at discrete d, as well as the onset of oscillations at d ≈ 1 mm; however, the specific mechanisms of this phenomenon are unknown. In the present work, measurements of drop axis ratio α were obtained from observations of drops levitated in a vertical wind tunnel. Discordance of the present data with the literature suggests a correlation between oscillation mode and fall trajectory, as well as a steady-state mechanism for the excitation of specific modes for d = 1.3–3-mm drops.

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L. Kuznetsov
,
K. Ide
, and
C. K. R. T. Jones

Abstract

Difficulties in the assimilation of Lagrangian data arise because the state of the prognostic model is generally described in terms of Eulerian variables computed on a fixed grid in space, as a result there is no direct connection between the model variables and Lagrangian observations that carry time-integrated information. A method is presented for assimilating Lagrangian tracer positions, observed at discrete times, directly into the model. The idea is to augment the model with tracer advection equations and to track the correlations between the flow and the tracers via the extended Kalman filter. The augmented model state vector includes tracer coordinates and is updated through the correlations to the observed tracers.

The technique is tested for point vortex flows: an N F point vortex system with a Gaussian noise term is modeled by its deterministic counterpart. Positions of N D tracer particles are observed at regular time intervals and assimilated into the model. Numerical experiments demonstrate successful system tracking for (N F , N D ) = (2, 1), (4, 2), provided the observations are reasonably frequent and accurate and the system noise level is not too high. The performance of the filter strongly depends on initial tracer positions (drifter launch locations). Analysis of this dependence shows that the good launch locations are separated from the bad ones by Lagrangian flow structures (separatrices or invariant manifolds of the velocity field). The method is compared to an alternative indirect approach, where the flow velocity, estimated from two (or more) consecutive drifter observations, is assimilated directly into the model.

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Kelvin K. Droegemeier
,
Steven M. Lazarus
, and
Robert Davies-Jones

Abstract

A three-dimensional numerical cloud model is used to investigate the influence of storm-relative environmental helicity (SREH) on convective storm structure and evolution, with a particular emphasis on the identification of ambient shear profiles that are conducive to the development of long-lived, strongly rotating storms. Eleven numerical simulations are made in which the depth and turning angle of the ambient vertical shear vector are varied systematically while maintaining a constant magnitude of the shear in the shear layer. In this manner, an attempt is made to isolate the effects of different environmental Felicities on storm morphology and show that the SREH and bulk Richardson number, rather than the mean shear in the low levels, determine the rotational characteristics and morphology of deep convection.

The results demonstrate that storms forming in environments characterized by large SREH are longer-lived than those in less helical surroundings. Further, it appears that the storm-relative winds in the layer 0–3 km must, on average, exceed 10 m s−1 over most of the lifetime of a convective event to obtain supercell storms. The correlation coefficient between vertical vorticityζ and vertical velocity w, which (according to linear theory of dry convection) should be proportional to the product of the normalized helicity density, NHD (i.e., relative helicity), and a function involving the storm-relative wind speed, has the largest peak values (in time) in those simulated storms exhibiting large SREH and strong storm-relative winds in the low levels. Even when the vorticity is predominantly streamwise in the storm-relative framework, giving a normalized helicity density near unity (as is the case in many of these simulations), significant updraft rotation and large w–ζcorrelation coefficients do not develop and persist unless the storm-relative winds are sufficiently strong.

The correlation coefficient between w and ζ based on linear theory is found to be a significantly better predictor of net updraft rotation than the bulk Richardson number (BRN) or the BRN shear, and slightly better than the 0-3-km SREH. Both the theoretical correlation coefficient and the SREH are based on the motion of the initial storm after its initially rapid growth. Linear theory also predicts correctly the relative locations of the buoyancy, vertical velocity, and vertical vorticity extrema within the storms after allowance is made for the effects of vertical advection. In predicting the maximum vertical vorticity both above and below 1.14 km, rather than the actual w and ζcorrelation, the 0–3-km SREH performs slightly worse than the BRN. The correlation coefficient, SREH, and BRN all do a credible job of predicting storm type. Thus, it is recommended that operational forecasters use the BRN to predict storm type because it is independent of storm motion, and the SREH to characterize the rotational properties of storms once their motions can be established.

Finally, the ability of the NHD to characterize storm type and rotational properties is examined. Computed using the storm-relative winds, the NHD shows little ability to predict storm rotation (i.e., maximum w-ζcorrelation and maximum vertical vorticity), because it neglects the magnitudes of the vorticity and storm-relative wind vectors. Histograms of the disturbance NHD show a distinct bias toward positive values near unity for supercell storms, indicating an extraction of helicity from the mean flow by the disturbance, and only a slight bias for multicell storms.

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Ronald M. Thorkildson
,
Kathleen F. Jones
, and
Maggie K. Emery

Abstract

On 24 November 2005, 11 lattice steel towers of a high-voltage electrical transmission line running along the edge of an escarpment were damaged by an accumulation of rime on overhead ground wires. Cold air pooling in the Columbia basin of eastern Washington several days before the failure led to the formation of low-level fog and low clouds with temperatures below freezing at the elevation of the transmission line. The liquid water content profile of the cloud formed by air rising over Badger Mountain north of Wenatchee, Washington, is estimated using the air temperature, dewpoint temperature, and air pressure as measured at Wenatchee in the Columbia River valley below the line. Cloud median volume droplet diameters are estimated using typical droplet concentrations. The validity of the computed liquid water content is determined by comparing the measured cloud-base heights at Wenatchee with the calculated cloud-base heights. The mass and density of ice accreted on the ground wires and conductors of the transmission line are modeled using assumed wind speeds at the top of the escarpment with the estimated cloud properties. Results are compared with the density and mass of an ice sample retrieved from the field. This event is compared with other modeled in-cloud icing events from 1973 to 2007 using the period of record of Wenatchee weather data. This paper illustrates an approach for estimating the severity of in-cloud icing on the wires of transmission lines subject to cloud liquid water contents that have been enhanced by the local terrain.

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Yvette P. Richardson
,
Kelvin K. Droegemeier
, and
Robert P. Davies-Jones

Abstract

Severe convective storms are typically simulated using either an idealized, horizontally homogeneous environment (i.e., single sounding) or an inhomogeneous environment constructed using numerous types of observations. Representing opposite ends of the spectrum, the former allows for the study of storm dynamics without the complicating effects of either land surface or atmospheric variability, though arguably at the expense of physical realism, while the latter is especially useful for prediction and data sensitivity studies, though because of its physical completeness, determination of cause can be extremely difficult. In this study, the gap between these two extremes is bridged by specifying horizontal variations in environmental vertical shear in an idealized, controlled manner so that their influence on storm morphology can be readily diagnosed. Simulations are performed using the Advanced Regional Prediction System (ARPS), though with significant modification to accommodate the analytically specified environmental fields. Several steady-state environments are constructed herein that retain a good degree of physical realism while permitting clear interpretation of cause and effect. These experiments are compared to counterpart control simulations in homogeneous environments constructed using single wind profiles from selected locations within the inhomogeneous environment domain. Simulations in which steady-state vertical shear varies spatially are presented for different shear regimes (storm types). A gradient of weak shear across the storm system leads to preferred cell development on the flank with greater shear. In a stronger shear regime (i.e., in the borderline multicell/supercell regime), however, cell development is enhanced on the weaker shear flank while cell organization is enhanced on the strong shear side. When an entire storm system moves from weak to strong shear, changes in cell structure are influenced by local mesoscale forcing associated with the cold pool. In this particular experiment, cells near the leading edge of the cold pool, where gust front convergence occurs along a continuous line, evolve into a bow-echo structure as the shear increases. In contrast, simulated cells that remain relatively isolated on the flank of the cold pool tend to develop supercellular characteristics.

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T. M. L. Wigley
,
K. R. Briffa
, and
P. D. Jones

Abstract

In a number of areas of applied climatology, time series are either averaged to enhance a common underlying signal or combined to produce area averages. How well, then, does the average of a finite number (N) of time series represent the population average, and how well will a subset of series represent the N-series average? We have answered these questions by deriving formulas for 1) the correlation coefficient between the average of N time series and the average of n such series (where n is an arbitrary subset of N) and 2) the correlation between the N-series average and the population. We refer to these mean correlations as the subsample signal strength (SSS) and the expressed population signal (EPS). They may be expressed in terms of the mean inter-series correlation coefficient as
n,N 2nNNN
N 2Nr̄N
Similar formulas are given relating these mean correlations to the fractional common variance which arises as a parameter in analysis of variance. These results are applied to determine the increased uncertainty in a tree-ring chronology which results when the number of cores used to produce the chronology is reduced. Such uncertainty will accrue to any climate reconstruction equation that is calibrated using the most recent part of the chronology. The method presented can be used to define the useful length of tree-ring chronologies for climate reconstruction work. A second application considers the accuracy of area-average precipitation estimates derived from a limited network of raingage sites. The uncertainty is given in absolute terms as the standard error of estimate of the area-average expressed as a function of the number of gage sites and the mean inter-site correlation.
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R. Meneghini
,
J. A. Jones
,
T. Iguchi
,
K. Okamoto
, and
J. Kwiatkowski

Abstract

Data from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar represent the first global rain-rate dataset acquired by a spaceborne weather radar. Because the radar operates at an attenuating wavelength, one of the principal issues concerns the accuracy of the attenuation correction algorithms. One way to test these algorithms is by means of a statistical method in which the probability distribution of rain rates at the high end is inferred by measurements at the low to intermediate range and by the assumption that the rain rates are lognormally distributed. Investigation of this method and the area–time integral methods using a global dataset provides an indication of how well methods of this kind can be expected to perform over different space–timescales and climatological regions using the sparsely sampled TRMM radar data. Identification of statistical relationships among the rain parameters and an understanding of the rain-rate distribution as a function of time and space may help to test the validity of the high-resolution rain-rate estimates.

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Edwin J. Adlerman
,
Kelvin K. Droegemeier
, and
Robert Davies-Jones

Abstract

A three-dimensional nonhydrostatic numerical model, the Advanced Regional Prediction System, is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm. During the 4-h simulation, the storm’s mesocyclone undergoes two distinct occlusions, with the beginning of a third indicated at the end of the simulation. The occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case.

Initial midlevel (3–7 km) mesocyclogenesis proceeds according to the “classic” picture, that is, via tilting of streamwise environmental vorticity. The development of an evaporatively driven rear-flank downdraft (RFD) signals the beginning of the occlusion process. The developing RFD wraps cyclonically around the mesocyclone, causing the gust front to surge outward. Simultaneously, the occluding mesocyclone rapidly intensifies near the surface. Trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near-ground (<500 m) streamwise vorticity produced by baroclinic generation, crosswise exchange, and streamwise stretching along descending parcel trajectories in the RFD. The surging gust front also initiates updraft development on the downshear flank at midlevels, resulting in a two-celled updraft structure. As the near-ground mesocyclone becomes detached from the gust front due to the developing occlusion downdraft, the upshear updraft flank weakens as its conditionally unstable inflow is cut off at low levels; at the same time, the downshear updraft flank continues to develop eastward. The end of the occlusion process is signaled as the old near-ground mesocyclone becomes completely embedded near the surface in divergent outflow beneath the decaying updraft and is advected away by the mean flow.

Near-ground mesocyclogenesis is initiated in the new updraft in a process nearly identical to that of the initial mesocyclone. However, after the first occlusion, near-ground equivalent potential temperature and buoyancy contours are fortuitously oriented such that streamwise baroclinic generation can proceed without delay. Thus, although the initial occlusion requires two hours to become fully organized, the second occurs only one hour later. In effect, the occlusion appears to set the stage for more rapid development of subsequent mesocyclones.

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