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D. J. Raymond
and
A. M. Blyth

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.

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E. M. Blyth
,
A. J. Dolman
, and
J. Noilhan

Abstract

A meso-β-scale model is used to model a frontal intrusion in southwest France during HAPEX-MOBILHY. The skill of the model to reproduce the observed variation in temperature, humidity, and wind speed over the domain is reasonable within the limitations of the model parameterizations and initialization procedure, although there were errors in the timing and positioning of the front. A stable boundary layer was both observed and modeled over the forested area. The associated negative sensible heat flux provided the energy to sustain evaporation from the wet forest canopy under conditions of low radiation. A large wind shear over the stably stratified boundary layer provided the required turbulent kinetic energy to maintain the downward transport of sensible heat. Sensitivity experiments showed that local rainfall with a full forest cover changed from 2.9 to 3.8 mm, which represents a 30% increase when compared with a bare-soil domain. Half of this increase is from positive feedback of the intercepted water that reevaporates. The high roughness length of the forest, with its associated physical and dynamical effects, accounts for the rest of the increase in rainfall and for the accompanying increase in soil moisture.

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William A. Cooper
,
Sonia G. Lasher-Trapp
, and
Alan M. Blyth

Abstract

The objective of this study is to address the problem of the production of rain in warm cumulus clouds that has been observed to occur within about 20 min. A hybrid model approach is used where a microphysical parcel model is run along trajectories produced by a 3D cloud model, with sufficiently high resolution to allow explicit representation of the effects of entrainment and mixing. The model calculations take the next step from the previous study, which showed that entrainment and mixing can accelerate the diffusional growth of cloud droplets to the production of raindrops by collision and coalescence. The mechanism depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. The resulting broadening favors collisions among droplets in the main peak of the droplet size distribution, which leads to the production of raindrop embryos. However, this production and the subsequent growth of the embryos to become raindrops only occur in regions of relatively high cloud water content. The modeling framework allows an objective test of this sequence of events that explain the seemingly contradictory notions of the enhancement of cloud droplet growth as a result of entrainment and mixing and the need for substantial cloud water content for collision and coalescence growth. The results show that raindrops can be produced within 20 min in warm cumulus clouds. The rain produced is sensitive to giant aerosols, but modification of the modeling framework is required to conduct a more robust test of their relative importance.

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Alan M. Blyth
,
William A. Cooper
, and
Jørgen B. Jensen

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 measurements of air motions in small cumulus clouds.

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P. H. Austin
,
M. B. Baker
,
A. M. Blyth
, and
J. B. Jensen

Abstract

We have analyzed small-scale fluctuations in microphysical, dynamical and thermodynamical parameters measured in two warm cumulus clouds during the Cooperative Convective Precipitation Experiment (CCOPE) project (1981) in light of predictions of several recent models. The measurements show the existence at all levels throughout the sampling period of two statistically distinct kinds of cloudy regions, termed “variable” and “steady,” often separated by transition zones of less than ten meters. There is some evidence for microphysical variability induced by local fluctuations in thermodynamic and dynamic parameters; however, the predominant variations are of a nature consistent with laboratory evidence suggesting that mixing is dominated by large structures. Entrainment appears to occur largely near cloud top but the data presented here do not permit identification of a mechanism for transport of the entrained air throughout the cloud.

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J. B. Jensen
,
P. H. Austin
,
M. B. Baker
, and
A. M. Blyth

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 spectral comparison and the results of the N-vs-F analysis support the hypothesis that cloudy and environmental air interact on fairly large scales with subsequent homogenization of the large-scale regions. This description is consistent with recent models of mixing in turbulent flows.

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Alan M. Blyth
,
Sonia G. Lasher-Trapp
,
William A. Cooper
,
Charles A. Knight
, and
John Latham

Abstract

Observations of the formation of the first radar echoes in small cumulus clouds are compared with results of a stochastic coalescence model run in the framework of a closed parcel. The observations were made with an instrumented aircraft and a high-powered dual-wavelength radar during the Small Cumulus Microphysics Study (SCMS) in Florida. The principal conclusion is that coalescence growth on giant and ultragiant nuclei may be sufficient to explain observations.

The concentration of cloud droplets varied from under 300 cm−3 when surface winds were from the ocean, to over 1000 cm−3 when the wind direction was from the mainland. Although there is a slight tendency for the altitude of the first 0-dBZ echo to be lower on average in maritime than in continental clouds there were several cases where it was higher. The model results suggest that the lack of correlation is consistent with drops forming on giant and ultragiant nuclei. The first 0-dBZ echo was observed to form at higher altitudes in clouds with stronger updrafts.

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R. R. Burton
,
A. M. Blyth
,
Z. Cui
,
J. Groves
,
B. L. Lamptey
,
J. K. Fletcher
,
J. H. Marsham
,
D. J. Parker
, and
A. Roberts

Abstract

The ability to predict heavy rain and floods in Africa is urgently needed to reduce the socioeconomic costs of these events and increase resilience as climate changes. Numerical weather prediction in this region is challenging, and attention is being drawn to observationally based methods of providing short-term nowcasts (up to ∼6-h lead time). In this paper a freely available nowcasting package, pySTEPS, is used to assess the potential to provide nowcasts of satellite-derived convective rain rate for West Africa. By analyzing a large number of nowcasts, we demonstrate that a simple approach of “optical flow” can have useful skill at 2-h lead time on a 10-km scale and 4-h lead time at larger scales (200 km). A diurnal variation in nowcast skill is observed, with the worst-performing nowcasts being those that are initialized at 1500 UTC. Comparison with existing nowcasts is presented. Such nowcasts, if implemented operationally, would be expected to have significant benefits.

Significance Statement

A freely available, easy-to-use nowcasting package has been applied to satellite-retrieved rainfall rates for West Africa, and extrapolations have useful skill at up to 4 h of lead time.

Open access
Sonia Lasher-Trapp
,
Shailendra Kumar
,
Daniel H. Moser
,
Alan M. Blyth
,
Jeffrey R. French
,
Robert C. Jackson
,
David C. Leon
, and
David M. Plummer

ABSTRACT

The Convective Precipitation Experiment (COPE) documented the dynamical and microphysical evolution of convection in southwestern England for testing and improving quantitative precipitation forecasting. A strong warm rain process was hypothesized to produce graupel quickly, initiating ice production by rime splintering earlier to increase graupel production and, ultimately, produce heavy rainfall. Here, convection observed on two subsequent days (2 and 3 August 2013) is used to test this hypothesis and illustrate how environmental factors may alter the microphysical progression. The vertical wind shear and cloud droplet number concentrations on 2 August were 2 times those observed on 3 August. Convection on both days produced comparable maximum radar-estimated rain rates, but in situ microphysical measurements indicated much less ice in the clouds on 2 August, despite having maximum cloud tops that were nearly 2 km higher than on 3 August. Idealized 3D numerical simulations of the convection in their respective environments suggest that the relative importance of particular microphysical processes differed. Higher (lower) cloud droplet number concentrations slow (accelerate) the warm rain process as expected, which in turn slows (accelerates) graupel formation. Rime splintering can explain the abundance of ice observed on 3 August, but it was hampered by strong vertical wind shear on 2 August. In the model, the additional ice produced by rime splintering was ineffective in enhancing surface rainfall; strong updrafts on both days lofted supercooled raindrops well above the 0°C level where they froze to become graupel. The results illustrate the complexity of dynamical–microphysical interactions in producing convective rainfall and highlight unresolved issues in understanding and modeling the competing microphysical processes.

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Energy and Water Cycles in a High-Latitude, North-Flowing River System

Summary of Results from the Mackenzie GEWEX Study—Phase I

W. R. Rouse
,
E. M. Blyth
,
R. W. Crawford
,
J. R. Gyakum
,
J. R. Janowicz
,
B. Kochtubajda
,
H. G. Leighton
,
P. Marsh
,
L. Martz
,
A. Pietroniro
,
H. Ritchie
,
W. M. Schertzer
,
E. D. Soulis
,
R. E. Stewart
,
G. S. Strong
, and
M. K. Woo

The MacKenzie Global Energy and Water Cycle Experiment (GEWEX) Study, Phase 1, seeks to improve understanding of energy and water cycling in the Mackenzie River basin (MRB) and to initiate and test atmospheric, hydrologic, and coupled models that will project the sensitivity of these cycles to climate change and to human activities. Major findings from the study are outlined in this paper. Absorbed solar radiation is a primary driving force of energy and water, and shows dramatic temporal and spatial variability. Cloud amounts feature large diurnal, seasonal, and interannual fluctuations. Seasonality in moisture inputs and outputs is pronounced. Winter in the northern MRB features deep thermal inversions. Snow hydrological processes are very significant in this high-latitude environment and are being successfully modeled for various landscapes. Runoff processes are distinctive in the major terrain units, which is important to overall water cycling. Lakes and wetlands compose much of MRB and are prominent as hydrologic storage systems that must be incorporated into models. Additionally, they are very efficient and variable evaporating systems that are highly sensitive to climate variability. Mountainous high-latitude sub-basins comprise a mosaic of land surfaces with distinct hydrological attributes that act as variable source areas for runoff generation. They also promote leeward cyclonic storm generation. The hard rock terrain of the Canadian Shield exhibits a distinctive energy flux regimen and hydrologic regime. The MRB has been warming dramatically recently, and ice breakup and spring outflow into the Polar Sea has been occurring progressively earlier. This paper presents initial results from coupled atmospheric-hydrologic modeling and delineates distinctive cold region inputs needed for developments in regional and global climate modeling.

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