Search Results
You are looking at 1 - 3 of 3 items for
- Author or Editor: Shalva Tzivion (Tzitzvashvili) x
- Refine by Access: All Content x
Abstract
The evolution of raindrop spectra with altitude through collisional collection/breakup sedimentation and evaporation is presented. Two-moment treatment of sedimentation and evaporation is developed to complement Part I (Feingold et al.) of this series. We have obtained an accurate, stable numerical scheme for evaporation that enables the investigation of the effect of evaporation on spectra subject to entrainment of strongly subsaturated air (including ventilation). The method includes provision for treatment of the variation of the sub/supersaturation within a time step in a dynamical framework. Results confirm that steady-state raindrop spectra are characterized by a bimodal or trimodal structure that becomes evident shortly after evolution commences. After sufficient evolution, peaks become clearly defined at 0.25 mm and 0.8 mm and further evolution with altitude affects only the relative magnitude of these peaks. It is shown that the evaporation process is not only dependent on the subsaturation of ambient air but is also strongly dependent on the shape of the drop spectrum. Evaporation tends to increase the number of the smallest raindrops (≤ 0.1 mm) at the expense of the larger drops but does not modify the position of the peaks. The effect of drop spectral evolution on radar reflectivity (Z) and scavenging (Λ) profiles is studied.
Abstract
The evolution of raindrop spectra with altitude through collisional collection/breakup sedimentation and evaporation is presented. Two-moment treatment of sedimentation and evaporation is developed to complement Part I (Feingold et al.) of this series. We have obtained an accurate, stable numerical scheme for evaporation that enables the investigation of the effect of evaporation on spectra subject to entrainment of strongly subsaturated air (including ventilation). The method includes provision for treatment of the variation of the sub/supersaturation within a time step in a dynamical framework. Results confirm that steady-state raindrop spectra are characterized by a bimodal or trimodal structure that becomes evident shortly after evolution commences. After sufficient evolution, peaks become clearly defined at 0.25 mm and 0.8 mm and further evolution with altitude affects only the relative magnitude of these peaks. It is shown that the evaporation process is not only dependent on the subsaturation of ambient air but is also strongly dependent on the shape of the drop spectrum. Evaporation tends to increase the number of the smallest raindrops (≤ 0.1 mm) at the expense of the larger drops but does not modify the position of the peaks. The effect of drop spectral evolution on radar reflectivity (Z) and scavenging (Λ) profiles is studied.
Abstract
The evolution of raindrop spectra below cloud base in subsaturated atmospheres is traced with the aid of an axisymmetrical rainshaft model which includes the detailed warm microphysical treatment presented in parts I and II of this series. As input to the model, a stationary cloud provides rainfall with a predetermined drop spectrum. Mass loading and evaporative cooling generate downdrafts below cloud base. For near-adiabatic lapse rates and moderate mass loading, microbursts develop. For a given liquid water content, the magnitude of these downdrafts depends primarily on the lapse rate of temperature, but also on the drop spectrum injected at cloud base. For a given liquid water content, spectra comprising a relatively large number of small drops tend to generate significantly stronger downdrafts than spectra with a greater component of large drops. It is shown that drop collection and breakup may also affect the magnitude of the generated downdrafts significantly. When spectra comprising mainly small drops evolve to create larger drops, or when spectra comprising mainly large drops evolve to create smaller drops, neglect of collection and breakup can modify the downdrafts by up to about 50%. It is shown that in a steady state situation the drop spectra evolve toward bi- or trimodal spectra as predicted by simple rainshaft models with fixed dynamics.
Abstract
The evolution of raindrop spectra below cloud base in subsaturated atmospheres is traced with the aid of an axisymmetrical rainshaft model which includes the detailed warm microphysical treatment presented in parts I and II of this series. As input to the model, a stationary cloud provides rainfall with a predetermined drop spectrum. Mass loading and evaporative cooling generate downdrafts below cloud base. For near-adiabatic lapse rates and moderate mass loading, microbursts develop. For a given liquid water content, the magnitude of these downdrafts depends primarily on the lapse rate of temperature, but also on the drop spectrum injected at cloud base. For a given liquid water content, spectra comprising a relatively large number of small drops tend to generate significantly stronger downdrafts than spectra with a greater component of large drops. It is shown that drop collection and breakup may also affect the magnitude of the generated downdrafts significantly. When spectra comprising mainly small drops evolve to create larger drops, or when spectra comprising mainly large drops evolve to create smaller drops, neglect of collection and breakup can modify the downdrafts by up to about 50%. It is shown that in a steady state situation the drop spectra evolve toward bi- or trimodal spectra as predicted by simple rainshaft models with fixed dynamics.
Abstract
A new, accurate, efficient method for solving the stochastic collection equation (SCE) is proposed. The SCE is converted to a set of moment equations in categories using a new analytical form of Bleck&'s approach. The equations are written in a form amenable to solution and to a category-by-category analysis of drop formation and removal. This method is unique in that closure of the equations is achieved using an expression relating high-order moments to any two lower order moments, thereby restricting the need for approximation of the category distribution function only to integrals over incomplete categories. Moments in categories are then expressed in terms of complete moments with the aid of linear or cubic polynomials. The method is checked for the case of the constant kernel and a linear polynomial kernel. Results show that excellent approximation to the analytical solutions for these kernels are obtained. This is achieved without the use of weighting functions and with modest computing time requirements. The method conserves two or more moments of the spectrum (as required) and successfully alleviates the artificial enhancement of the collection process which is a feature of many schemes.
Abstract
A new, accurate, efficient method for solving the stochastic collection equation (SCE) is proposed. The SCE is converted to a set of moment equations in categories using a new analytical form of Bleck&'s approach. The equations are written in a form amenable to solution and to a category-by-category analysis of drop formation and removal. This method is unique in that closure of the equations is achieved using an expression relating high-order moments to any two lower order moments, thereby restricting the need for approximation of the category distribution function only to integrals over incomplete categories. Moments in categories are then expressed in terms of complete moments with the aid of linear or cubic polynomials. The method is checked for the case of the constant kernel and a linear polynomial kernel. Results show that excellent approximation to the analytical solutions for these kernels are obtained. This is achieved without the use of weighting functions and with modest computing time requirements. The method conserves two or more moments of the spectrum (as required) and successfully alleviates the artificial enhancement of the collection process which is a feature of many schemes.
