Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: J. Joss x
  • Refine by Access: All Content x
Clear All Modify Search
J. Joss and A. Waldvogel

Abstract

Full access
J. Joss and A. Waldvogel

Abstract

The standard deviation of the rain intensity R and the reflectivity factor Z are derived theoretically for R and Z values which are calculated from measured distributions of raindrop size. The derivation is based on the assumption that the distribution of raindrop size follows a negative exponential law. The Poisson distribution is assumed for the number of the counted drops with diameters between D and D+dD. We found that a large sample is necessary to get a good estimate of R and Z; for example, in a widespread rain with a rainfall rate of 1 mm hr−1, a filter paper with an area of ˜1 m2 must be exposed during 1 sec to obtain, with a probability of 68%, a Z value which deviates less than 20% from the mean.

Full access
H. P. Roesli, J. Joss, and M. Schüepp

Abstract

The possibility is discussed that evaporation in the subcloud layer might be a major factor controlling the efficiency of rain enhancement by seeding, under the simplistic assumption that seeded clouds produce larger proportions of small raindrops than unseeded clouds. The concept of an evaporation parameter, E = ∫0HΔTdh, is introduced, and it is shown that this quantity is a measure of the amount of water evaporated from a raindrop falling through an air column of height H and with a wet-bulb depression profile ΔT(h). Based on the evaporation parameter a simple numerical model is constructed, which models the evaporation process as a function of air mass conditions and raindrop-size distributions. This makes it possible to estimate what changes in rainfall at the ground might occur for given changes in rainfall at cloud base. Numerical results are obtained for ambient conditions of the Swiss hail suppression project Grossversuch III. These results establish the potential importance of evaporation in attempts at rain enhancement and suggest the types of observations required for further study. The need of careful measurements of the drop-size distribution beneath cloud base is especially stressed.

Full access
J. G. Esler and N. Joss Matthewman

Abstract

Vortex displacement stratospheric sudden warmings (SSWs) are studied in an idealized model of a quasigeostrophic columnar vortex in an anelastic atmosphere. Motivated by the fact that observed events occur at a fixed orientation to the earth’s surface and have a strongly baroclinic vertical structure, vortex Rossby waves are forced by a stationary topographic forcing designed to minimize excursions of the vortex from its initial position. Variations in the background stratospheric “climate” are represented by means of an additional flow in solid body rotation. The vortex response is determined numerically as a function of the forcing strength M and the background flow strength Ω.

At moderate M it is found that a large response, with many features resembling observed displacement SSWs, occurs only for a narrow range of Ω. Linear analysis reveals that for this range of Ω the first baroclinic azimuthal wave-1 Rossby wave mode is close to being resonantly excited. A forced nonlinear oscillator equation is proposed to describe the nonlinear behavior, and a method for determining the relevant coefficients numerically, using unforced calculations of steadily propagating vortex “V states,” is adopted. The nonlinear equation predicts some qualitative details of the variation in the response at finite M. However, it is concluded that strongly nonlinear processes, such as wave breaking and filament formation, are necessarily quantitatively important in determining the amplitude of the near-resonant response at finite M.

Full access
L. Li, W. Schmid, and J. Joss

Abstract

Motion vectors of radar echo patterns can be obtained by applying a cross-correlation method (e.g., the TREC method) to radar data collected several minutes apart Here an extension of TREC, called COTREC, is presented. Based on constraints and a variational technique, this extension is an efficient objective analysis method for smoothing the motion vectors and forcing them to fulfill the continuity equation. COTREC corrects the apparently wrong vectors that are often caused by failures of TREC. This allows us to identify regions of growth and decay of radar echoes.

For different types of precipitation (convective and widespread), radar data were collected for evaluation of COTREC in complex orography. A comparison between the radial velocity components of retrieved fields of echo motion and the measured Doppler velocity has been made. A marked reduction of the differences with respect to the measured Doppler field was obtained for COTREC, as compared to TREC vectors.

A retardation of COTREC-derived motion compared to Doppler-derived motion was found in orographic precipitation. This retardation may have two causes: 1) a tendency of radar patterns to become stationary (triggered) on upsloping orography; and 2) the influence of ground clutter and shielding, also highly correlated with orography. While the first reflects the fact that propagation of echoes (by growth/decay) and translation of echoes (with the wind) are two different phenomena, the second cause is an artifact produced by the method of observation (radar) but mitigated with Doppler techniques (by suppressing the stationary ground clutter).

COTREC may be useful for nowcasting, especially in orographically complex areas: for orographic precipitation as well as for severe convective storms, the technique predicts the echo development approximately 20 min ahead, and there is good hope to extend the forecasting period.

Full access
N. Joss Matthewman and J. G. Esler

Abstract

The fundamental dynamics of “vortex splitting” stratospheric sudden warmings (SSWs), which are known to be predominantly barotropic in nature, are reexamined using an idealized single-layer f-plane model of the polar vortex. The aim is to elucidate the conditions under which a stationary topographic forcing causes the model vortex to split, and to express the splitting condition as a function of the model parameters determining the topography and circulation.

For a specified topographic forcing profile the model behavior is governed by two nondimensional parameters: the topographic forcing height M and a surf-zone potential vorticity parameter Ω. For relatively low M, vortex splits similar to observed SSWs occur only for a narrow range of Ω values. Further, a bifurcation in parameter space is observed: a small change in Ω (or M) beyond a critical value can lead to an abrupt transition between a state with low-amplitude vortex Rossby waves and a sudden vortex split. The model behavior can be fully understood using two nonlinear analytical reductions: the Kida model of elliptical vortex motion in a uniform strain flow and a forced nonlinear oscillator equation. The abrupt transition in behavior is a feature of both reductions and corresponds to the onset of a nonlinear (self-tuning) resonance. The results add an important new aspect to the “resonant excitation” theory of SSWs. Under this paradigm, it is not necessary to invoke an anomalous tropospheric planetary wave source, or unusually favorable conditions for upward wave propagation, in order to explain the occurrence of SSWs.

Full access
P. Meischner, C. Collier, A. Illingworth, J. Joss, and W. Randeu

The European Union COST (Cooperation in Science and Technology) action on advanced weather radar systems is described. The associated five-year research project, which began in early 1993, has the objective to develop guideline specifications for a future generation of European operational radar systems. The authors describe the status of the project, the results reached so far in assessing and reviewing the potential improvements to conventional radars, the products and application of Doppler radar data, the contribution of polarimetric radars to the improvement of quantitative precipitation measurements and for nowcasting, and the possible development of electronically scanned systems. Problems to be tackled in the remaining years of the project are assessments of future technological feasibility, market forecasts, and cost/benefit investigations for the varied requirement profiles across Europe. It is intended to generate a high-level specification for the next generation of weather radars in Europe.

Full access
J. G. DeVore, A. T. Stair, A. LePage, D. Rall, J. Atkinson, D. Villanucci, S. A. Rappaport, P. C. Joss, and R. A. McClatchey

Abstract

This paper describes a newly designed Sun and Aureole Measurement (SAM) aureolegraph and the first results obtained with this instrument. SAM measurements of solar aureoles produced by cirrus and cumulus clouds were taken at the Atmospheric Radiation Measurement Program (ARM) Central Facility in Oklahoma during field experiments conducted in June 2007 and compared with simultaneous measurements from a variety of other ground-based instruments. A theoretical relationship between the slope of the aureole profile and the size distribution of spherical cloud particles is based on approximating scattering as due solely to diffraction, which in turn is approximated using a rectangle function. When the particle size distribution is expressed as a power-law function of radius, the aureole radiance as a function of angle from the center of the solar disk also follows a power law, with the sum of the two powers being −5. This result also holds if diffraction is modeled with an Airy function. The diffraction approximation is applied to SAM measurements with optical depths ≲2 to derive the effective radii of cloud particles and particle size distributions between ∼2.5 and ∼25 μm. The SAM results yielded information on cloud properties complementary to that obtained with ARM Central Facility instrumentation. A network of automated SAM units [similar to the Aerosol Robotic Network (AERONET) system] would provide a practical means to gain fundamental new information on the global statistical properties of thin (optical depth ≲ 10) clouds, thereby providing unique information on the effects of such clouds upon the earth’s energy budget.

Full access