Alan M. Blyth

A stochastic mixing model for nonprecipitating cumulus clouds

Abstract Recent work suggests that nonprecipitating cumulus clouds must be considered as aggregates of many parcels always moving toward buoyancy equilibrium, but otherwise subject to a multiplicity of fates. For example, some parcels originating at low levels seem to ascend to their level of undilute neutral buoyancy before mixing with the environment, while others mix at intermediate levels. We present a model for a cumulus cloud incorporating these ideas. In spite of the apparent simplicity of the model, it compares favorably with observations.

The Convective and Orographically Induced Precipitation Study:A Research and Development Project of the World Weather Research Program for Improving Quantitative Precipitation Forecasting in Low-mountain Regions

Abstract The international field campaign called the Convective and Orographically-induced Precipitation Study (COPS) took place from June to August 2007 in southwestern Germany/eastern France. The overarching goal of COPS is to advance the quality of forecasts of orographically-induced convective precipitation by four-dimensional observations and modeling of its life cycle. COPS was endorsed as one of the Research and Development Projects of the World Weather Research Program (WWRP), and combines the efforts of institutions and scientists from eight countries. A strong collaboration between instrument principal investigators and experts on mesoscale modeling has been established within COPS. In order to study the relative importance of large-scale and small-scale forcing leading to convection initiation in low mountains, COPS is coordinated with a one-year General Observations Period in central Europe, the WWRP Forecast Demonstration Project MAP D-PHASE, and the first summertime European THORPEX Regional...

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.

A Study of the Source of Entrained Air in Montana Cumuli

Abstract Data gathered by the University of Wyoming King Air, the Atmospheric Environmental Services Twin otter and an NCAR Queen Air were used in thermodynamic analyses to determine the sources of environmental air entrained into cumulus clouds. The measurements were made in clouds ranging from small cumuli a few kilometers deep to a large supercell system. Previous results have indicated that the source of entrained air in continental cumuli is generally above the flight level, often near cloud top. The results reported here, however, suggest that the source of entrained air is close to, or slightly above, the observation level of the aircraft, even when the aircraft descends through different levels in the cloud. The results are consistent with the idea that cumulus clouds consist of thermal-like elements from which the least buoyant mixed parcels are shed off and the most buoyant mixed parcels may continue with the general ascent. A schematic model of cumulus convection is presented and supported by m...

Turbulent Mixing, Spectral Evolution and Dynamics in a Warm Cumulus Cloud

Abstract The analysis of Paluch suggests that some cumuli contain cloudy air from only two sources: cloud base and cloud top. A framework is presented for the investigation of droplet spectral evolution in clouds composed of air from only these two sources. The key is the investigation of the dependence of droplet concentration N on the fraction of cloud base air F in a sample of cloudy air. This N-vs-F analysis is coupled with an investigation of droplet spectral parameters to infer the types and scales of entrainment and mixing events. The technique is used in a case study of a small, nonprecipitating continental cumulus cloud which was sampled during the 1981 CCOPE project in eastern Montana. The mixing between cloudy and entrained air in this cloud often appears to occur without total removal of droplets, although there is evidence that total evaporation occurs in some regions with low liquid water content. The observed droplet spectra are compared with those calculated from an adiabatic parcel model....

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

Extension of the Stochastic Mixing Model to Cumulonimbus Clouds

Abstract The stochastic mixing model of cumulus clouds is extended to the case in which ice and precipitation form. A simple cloud microphysical model is adopted in which ice crystals and aggregates are carried along with the updraft, whereas raindrops, graupel, and hail are assumed to immediately fall out. The model is then applied to the 2 August 1984 case study of convection over the Magdalena Mountains of central New Mexico, with excellent results. The formation of ice and precipitation can explain the transition of this system from a cumulus congestus cloud to a thunderstorm.

Entrainment and Detrainment in Numerically Simulated Cumulus Congestus Clouds. Part I: General Results

Abstract This paper is the first in a three-part series in which a three-dimensional numerical model is run at high resolution to simulate cumulus congestus clouds in three dimensions with the principal goal of understanding the mechanisms associated with entrainment and detrainment. The clouds are contained within a nested grid having a 50-m uniform grid spacing; the model does not allow precipitation or ice formation and achieves saturation through bulk condensation. The prescribed environment is that associated with nonprecipitating New Mexican cumulus clouds observed on 9 and 10 August 1987. The convection is initiated using continuous surface heating, including a central Gaussian component to represent the effects of an isolated mountain range. Several regions of concentrated surface heating are used during the first hour to condition the environment. The turbulent motion thereby introduced into the boundary layer is crucial for the accurate simulation of the clouds. The simulated clouds extend verti...

Relationships between lightning activity and various thundercloud parameters: satellite and modelling studies

Abstract The lightning frequency model developed by Baker et al. [Baker, M.B., Christian, H.J., Latham, J., 1995. A computational study of the relationships linking lightning frequency and other thundercloud parameters, Q. J. R. Meteorol. Soc., 121, 1525–1548] has been refined and extended, in an effort to provide a more realistic framework from which to examine computationally the relationships that might exist between lightning frequency f (which is now being routinely measured from a satellite, using the NASA/MSFC Optical Transient Detector (OTD)) and a variety of cloud physical parameters. Specifically, superior or more comprehensive representations were utilised of: (1) glaciation via the Hallett–Mossop (H–M) process; (2) the updraught structure of the model cloud; (3) the liquid-water-content structure of the model cloud; (4) the role of the reversal temperature T rev in influencing lightning characteristics; (5) the critical breakdown field for lightning initiation; and (6) the electrical characteristics of the ice crystal anvil of the model cloud. Although our extended studies yielded some new insights into the problem, the basic pattern of relationships between f and the other parameters was very close to that reported by Baker et al. (1995). The more elaborate treatment of T rev restricted somewhat the range of conditions under which reverse-polarity lightning could be produced if the cloud glaciated via H–M, but confirmed the earlier conclusion that such lightning would not occur if the glaciation was of the Fletcher type. The computations yielded preliminary support for the hypothesis that satellite measurements of f might be used to determine values of the ice-content of cumulonimbus anvils: a parameter of climatological importance. The successful launch and continuing satisfactory functioning of the OTD [Christian, H.J., Goodman, S., 1992. Global observations of lightning from space, Proc. 9th Int. Conf. on Atmospheric Electricity, St. Petersburg, pp. 316–321; Christian, H.J., Blakesee, R.J., Goodman, S.J., 1992. Lightning imaging sensor (LIS) for the earth observing system. NASA Tech. Memorandum, 4350] make it possible—with a high degree of precision—to measure lightning location, occurrence time and frequency f over extensive areas of the Earths surface. Measured global distributions of lightning and associated lightning stroke radiance demonstrate that: lightning activity is particularly pronounced over the tropics, much greater over land than over the oceans, and exhibits great seasonal variability; lightning radiance tends to be greater over the oceans, less when lightning activity is high, and greater in the Northern Hemisphere winter than summer.

The influence of aerosol concentrations on the glaciation and precipitation of a cumulus cloud

The response of the glaciation and precipitation of U multi-thermal cumulus cloud to changes in the aerosol concentration has been assessed in a series of sensitivity tests with the UMIST Explicit Microphysics Model (EMM). A simulation of this cloud from the Met Office cloud-resolving model (CRM) has been utilized in these tests. This cumulus cloud was observed h aircraft during the initial stage of its growth over New Mexico on 10 August 1987. The growth of the simulated cloud is divided into two parts: a shallow phase followed by a deep phase. Maximum values of the cloud depth in these two phases were 5 and 9 km. respectively. In the EMM simulations including only the shallow phase, the precipitation efficiency was found to decrease substantially with increasing atmospheric concentrations of cloud condensation nuclei (CCN). Also, the graupel mixing ratio and total ice concentration ere found to decrease as normalized CCN concentration are increased above typical continental values. These changes are explicable in terms of: (I) the Hallett-Mossop (H-M) process at -3 to -8 degreesC and the freezing of supercooled raindrops in collisions with ice splinters dominating the glaciation and (2) the warm-rain process being more significant for the overall precipitation production than the ice process in these particular Simulations. The almost complete suppression of precipitation by extreme CCN concentrations corresponding to a forest-fire plume in the EMM simulation is consistent with the analysis by Rosenfeld of satellite images of Indonesian cumuli engulfed by smoke from biomass burning. A clear tendency for ice crystals to be smaller and more numerous in the anvil as found with increasing CCN concentrations beyond typical continental values in long-term simulations that included the deep phase. The sensitivity of the precipitation rate to the normalized CCN concentration was found to be relatively low in these deep cases. (Less)