Cloud Dynamics and Development of
Precipitation
Alan Blyth
NCAS and University of Leeds
25 October 2012
New Mexico thunderstorm project, 1984
Alan Blyth
Dynamics and Precip 2/15
Four Doppler Radars
Raymond and Blyth, (1989)
Alan Blyth
Dynamics and Precip 3/15
First echoes
Alan Blyth
Dynamics and Precip 4/15
Development of Precipitation
Alan Blyth
Dynamics and Precip 5/15
Origin and growth of graupel
Alan Blyth
Dynamics and Precip 6/15
Model of particle trajectories
Alan Blyth
Dynamics and Precip 7/15
Persistence of cloud systems
• The greatest amount of convective precipitation will be produced by
stationary large storm systems, or back-building storms.
• Browning (1977) described the different types of storms based on the
strength and movement of cells and the relative position of the updrafts and
downdrafts
• Many storms persist because the new updraft forms to one side of the
downdraft, or the storm is made up of multiple sequential cells initiated by
topography.
Alan Blyth
Dynamics and Precip 8/15
Line of clouds over Boscastle, Golding et al (2005)
• Sequence of convective storms developed along the north coast of
Cornwall
• Each storm element started as a non-precipitating cumulus
• Rapid cloud development started as each cell encountered convergence
• New cells formed upstream near the original location, so each initial shower
spread out into a line of storm cells
Alan Blyth
Dynamics and Precip 9/15
Formation of Downdrafts
• Despite the recognition for several decades that convective downdrafts are
key elements in cumulonimbus evolution and impacts, prediction remains
very poor
• Downdrafts occur at the end of the chain of cloud microphysical changes,
as a result of hydrometeors falling through the air, and cooling it by
evaporation.
• Although significant advances have been made through observations and
models (Wakimoto, 2001), we still do not accurately know the sensitivity of
the downdrafts to hydrometeor distributions, to ambient thermodynamics,
nor to the fluid dynamics of the system.
Alan Blyth
Dynamics and Precip 10/15
Transition from shallow to deep
convection
Khairoutdinov and Randall (2006) found that
• Cumulus convection starts shallow,
gradually develops into congestus, and
becomes deep only toward the end of
simulation
• Shallow clouds too small to penetrate
deep into the troposphere, since
diluted by entrainment
• Precipitation and associated cold pools
needed to generate thermals big
enough to support the growth of deep
clouds
Alan Blyth
Dynamics and Precip 11/15
Entrainment
Alan Blyth
Dynamics and Precip 12/15
Not just cold pools?
Raymond and Blyth, (1992)
Alan Blyth
Dynamics and Precip 13/15
Major Questions to address in field campaign
• How does entrainment occur in cumulus clouds?
• What is dynamic structure of clouds (thermals, multiple thermals,
displaced cells, etc) and how does it change with time?
• How does precipitation form and develop?
• How does warm rain form and what is role of supercooled raindrops
in production of ice and precipitation?
• How do downdrafts develop and what is their role in the
development and persistence of the clouds?
• What is the 4D structure of convergence lines in the BL?
• What is the temperature and water vapour structure in the BL and
how does it change with time?
Alan Blyth
Dynamics and Precip 14/15
Data required to address questions
• Vertical velocities from aircraft
• Vertical velocities from Wyoming Cloud Radar
• Size and structure of eddies from Wyoming Cloud Lidar
• 3D Doppler radar data
• Good observations of 1st precip echoes
• Rapid scans of developing clouds
• Good in-situ observations of the development of warm rain
• Good in-situ observations of the first ice particles
• Good in-situ observations of the growth of ice particles, secondary
processes and development of precipitation.
• Temperature, water vapour and air motions in BL
Alan Blyth
Dynamics and Precip 15/15