Richard D. Farley

Bulk Parameterization of the Snow Field in a Cloud Model

Abstract A two-dimensional, time-dependent cloud model has been used to simulate a moderate intensity thunderstorm for the High Plains region. Six forms of water substance (water vapor, cloud water, cloud ice, rain, snow and hail, i.e., graupel) are simulated. The model utilizes the “bulk water” microphysical parameterization technique to represent the precipitation fields which are all assumed to follow exponential size distribution functions. Autoconversion concepts are used to parameterize the collision-coalescence and collision-aggregation processes. Accretion processes involving the various forms of liquid and solid hydrometeors are simulated in this model. The transformation of cloud ice to snow through autoconversion (aggregation) and Bergeron process and subsequent accretional growth or aggregation to form hail are simulated. Hail is also produced by various contact mechanisms and via probabilistic freezing of raindrops. Evaporation (sublimation) is considered for all precipitation particles outsi...

Numerical Simulation of Ice-Phase Convective Cloud Seeding

Abstract A two-dimensional time-dependent cloud model which covers a region 19.2 km × 19.2 km in the x and z directions with 200 m grid intervals, has been used to simulate silver iodide (AgI) seeding effects on strong convective clouds. The model is a set of conservation equations for momentum, energy and mass (air and water contents). One extra conservation equation is applied to trace the seeding agent which advects and diffuses along the flow field and interacts with the supercooled cloud fields. Contact and deposition nucleation are simulated. Only inertial impact and Brownian collection are considered as possible mechanisms for contact nucleation. Most of the AgI particles work as deposition or sorption nuclei in this study. Three different soundings are tested. Most of the effort is used in testing sounding H1, from Miles City, Montana, 29 July 1975. Seeding at a different place (see H1/P1), at a different time (case H1/T1), and with different amounts of AgI (cases H1/M1 and H1/M2) are simulated. T...

An examination of thunderstorm‐charging mechanisms using a two‐dimensional storm electrification model

The early, prelightning, electrification of a storm resulting from noninductive (NI) charging involving graupel, cloud ice/snow, and supercooled cloud water in a riming environment is studied using a comparative approach in a two-dimensional storm electrification model. The primary schemes examined are NI charge transfers based on the laboratory work of Takahashi [1978] and Saunders et al. [1991]. The NI mechanism, based on Takahashis work, develops a positive dipole (positive charge above negative) and electric fields approaching 185 kV m−1 as the cloud enters the dissipating stage. Charge transfers, based on the work of Saunders and colleagues, had to be reduced in magnitude to produce electrification that is consistent with the observations. In addition, the Saunders scheme produces an initially inverted dipole (negative charge above positive) which resolves to a positive dipole in the latter part of the simulation and produces electric fields approaching 250 kV m−1. Sensitivity tests show that the NI scheme, based on Takahashis work, is sensitive to the number concentration of ice crystals, whereas the Saunders-based scheme is much less sensitive to ice crystal numbers. The Saunders parameterization has strong positive charging of graupel at low effective liquid water content and low temperature. This positive charging can result in an unusual cloud-top charge structure when used at full value but is benign when the charging is reduced in magnitude. The charge structure resulting from the Saunders scheme is quite sensitive to the calculation of the effective water content, which determines the level of charge reversal. Both of the NI schemes are capable of producing electrification that approaches thunderstorm levels.

An intracloud lightning parameterization scheme for a storm electrification model

The parameterization of an intracloud lightning discharge has been implemented in our Storm Electrification Model. The initiation, propagation direction, and termination of the discharge are computed using the magnitude and direction of the electric field vector as the determining criteria. The charge redistribution due to the lightning is approximated assuming the channel to be an isolated conductor with zero net charge over its entire length. Various simulations involving differing amounts of charge transferred and distribution of charges have been done. Values of charge transfer, dipole moment change, and electrical energy dissipation computed in the model are consistent with observations. The effects of the lightning-produced ions on the hydrometeor charges and electric field components depend strongly on the amount of charge transferred. A comparison between the measured electric field change of an actual intracloud flash and the field change due to the simulated discharge shows favorable agreement. Limitations of the parameterization scheme are discussed.

A Numerical Case Study of Convection Initiation along Colliding Convergence Boundaries in Northeast Colorado

Abstract A numerical cloud model has been used to simulate convective storm development on 17 July 1987 in northeast Colorado. The study involves the simulation of convergence along atmospheric boundaries and the subsequent development of convection. The model was initialized using observed conditions for this case day from the Convection Initiation and Downburst Experiment (CINDE). A two-dimensional version of the Clark NCAR nested grid model is employed. Results indicate that convection in boundary line collision cases can be successfully simulated by using actual observed atmospheric data. Gradual deepening of the moisture layer in the convergence zone was shown to destabilize the local atmosphere leading to the initiation of deep convection on this day. The modeled storm approximated the depth and intensity of the observed storms and displayed many of the features of the actual event. Sensitivity studies revealed that the timing and intensity of convection along boundaries is greatly affected by alter...

Numerical Modeling of Hailstorms and Hailstone Growth. Part III: Simulation of an Alberta Hailstorm—Natural and Seeded Cases

Abstract This paper reports on simulations of a multicellular hailstorm case observed during the 1983 Alberta Hail Project. The field operations on that day concentrated on two successive feeder cells which were subjected to controlled seeding experiments. The fist of these cells received the placebo treatment and the second was seeded with dry ice. The principal tool of this study is a modified version of the two-dimensional, time dependent hail category model described in Part I of this series of papers. It is with this model that hail growth processes are investigated, including the simulated effects of cloud seeding techniques as practiced in Alberta. The model simulation of the natural case produces a very good replication of the observed storm, particularly the placebo feeder cell. This is evidenced, in particular, by the high degree of fidelity of the observed and modeled radar reflectivity in terms of magnitudes, structure, and evolution. The character of the hailfall at the surface and the scale ...

Some Surprising Results from Simulated Seeding of Stratiform-Type Clouds

Abstract Studies have been conducted to determine the cloud seeding potential of stratiform type clouds using a two-dimensional, time-dependent cloud model. An atmospheric sounding from Villanubla, Spain, in February 1980, was used to initialize the model. The model is designed to allow mesoscale convergence in the lower levels and divergence in the upper levels, which results in a stratiform-type cloud in this Spanish situation. The seeding of clouds using either dry ice or silver iodide has been tested and rather surprising results are indicated. The silver iodide seeding simulations produce strong dynamic responses in the model clouds, even with small amounts of supercooled liquid available and a few natural ice crystals per liter in the cloud. These effects occur in a nearly moist adiabatic layer as well as in a convectively unstable layer. The effects appear to be due to the heat released as the liquid freezes and the cloudy environment switches from liquid saturation to ice saturation. Cloud vertica...

Numerical modeling of lightning‐produced NOx using an explicit lightning scheme: 1. Two‐dimensional simulation as a “proof of concept”

[1] We use the two-dimensional (2D) version of our Storm Electrification Model to test its potential for studying lightning-produced NO x . We assume that NO production is a function of energy dissipation and calculate this value from the electric field before and after each lightning flash. We use a production rate of 9.2 x 10 16 molecules joule -1 to generate the NO. Using a limited set of chemical reactions involving NO, NO 2 , and O 3 , we simulated a small storm with 10 intracloud lightning flashes produced over a 2-min span. Their energy dissipation ranged between 0.024 and 0.28 GJ. The simulation was run an additional 18 min after the cessation of lightning. Our results show that the parameterization produced NO mixing ratios internal to the cloud of the order of 10 ppbv after the most energetic flashes and 1-2 ppbv in the upwind portion of the anvil toward the end of the simulation. These mixing ratios are shown to be comparable to observations in a generic sense. Comparison with the C-shaped profiles developed by Pickering et al. [1998], also using a 2D model, show similarities, but our results are more weighted toward larger values at higher altitudes than those of Pickering et al. This may be due to differences in the length of the simulation, a lack of cloud-to-ground lightning in our work, a lack of reactive chemistry in Pickering et al., or the use by Pickering et al. of the assumption of Price et al. [1997] that intracloud flashes dissipate one tenth the energy of cloud-to-ground flashes. We show, using recent observational data and an analysis of the assumptions of Price et al., that this one tenth energy dissipation assumption is not appropriate. We conclude that our use of an explicit lightning scheme to study NO production at the process level is a viable methodology.

Numerical Modeling of Hailstorms and Hailstone Growth. Part II: The Role of Low-Density Riming Growth in Hag Production

Abstract The past several years have seen a renewed interest in the importance of low-density riming growth to the development of hailstones. This paper reports on the results of a study that incorporates the physical factors controlling the density of the rime deposit in a two-dimensional, time-dependent numerical cloud model with discretized treatment of the graupel/hail size distribution. Comparisons are made between cases in which the mass-diameter relationship is fixed based on a priori assumed particle densities and cases in which the mass-diameter relationship is allowed to change in accordance with the variable particle density diagnosed from the riming density relationship and past growth history. Compared to the fixed particle density treatment common to earlier work, ice particles of lower density have enhanced surface and cross-sectional area for particles of equal mass, which in turn, increases the effective ventilation experienced by the particles and their capture volumes, thus allowing enh...

The 1 August 1981 MOPE Storm: Observations and Modeling Results

Abstract Observations made by the T-28 armored research aircraft, radar, and other data sources were used to study an eastern Moutana hailstorm that developed on 1 August 1981 during the Cooperative Convective Precipitation Experiment season. The storm featured a wide, persistent, vertically oriented updraft with speeds exceeding 35 m s−1. Hailstones of over 5 cm diameter were collected at the ground, while the T-28 encountered hail up to about 2.5 cm diameter. Them was no evidence of feeder cells or a weak echo region. The IAS two-dimensional, time-dependent “bulk water” model was run on this case, using a sounding from this day. Some areas of agreement between the simulation and observations include the maximum updraft speed, cloud top height, presence of a rounded cloud dome, and maximum radar reflectivity. The simulation failed to properly model the width and orientation of the updraft, as well as its long lifetime. The IAS hail category model was also run on this case. This model features 20 categori...