Who is the Mesoscale Group?
The Mesoscale Group in the Department of Atmospheric Sciences at the University
of Washington is the graduate research group of Professor Robert A. Houze,
Jr., 
who came to the department in 1972 immediately upon completing his Ph.
D. in Meteorology at the Massachusetts Institute of Technology. He studied
under Dr. Pauline Austin, one of the pioneers of radar meteorology. At
the University of Washington he teaches undergraduate general meteorology
courses and graduate courses in cloud physics and dynamics, and he leads
the Mesoscale Group, which consists of 4-5 graduate students, occasional
postdoctoral associates, a software engineer, and several undergraduate
research aides. In 1993 Professor Houze published the text Cloud
Dynamics. This book grew out of his graduate courses and the Mesoscale
Group's research on clouds and precipitation.
How the Mesoscale Group does its work
Professor Houze and his graduate students, staff, and postdoctoral associates
have participated in projects in Africa, India, Malaysia, Australia, the
tropical Pacific, the Gulf of Mexico, Florida, the U.S. Great Plains, Switzerland,
Italy, and the Pacific Northwest of North America. These projects have documented
the characteristics of precipitation-producing clouds of some of the most
important rainfall regimes in the world. The Mesoscale Group specializes
in the collection and analysis of meteorological radar data from these projects,
which have used meteorological research radars on land, ships, aircraft,
and satellite. The approach of the group's research is to examine the radar
data collected in these projects in the context of all other pertinent simultaneous
observations to synthesize empirical conceptual models of the observed precipitating
cloud systems.
Ultimately, the interpretation of the data through the analysis of radar
data, other observations, and modeling leads to theoretical insights. Over
the years, the Mesoscale Group's work has shown how the melting of ice influences
downdrafts, how gravity waves affect the development of cells in thunderstorms,
how the large-scale atmosphere responds to the presence of thunderstorms through
a spectrum of wave-like motions, how the latent heating in precipitating
cloud systems affects the larger-scale atmospheric circulation, and how flow
near and over mountains can enhance the precipitation in extratropical cyclones.
Early field projects and research directions
In the early 1970's, the Mesoscale Group began its work by participating in field studies of midlatitude frontal clouds in the northwestern U.S. (the CYCLES Project) and precipitating clouds over the tropical ocean (the Global Atmospheric Programme Atlantic Tropical Experiment, known as GATE). The CYCLES studies were among the first to use aircraft and Doppler radar to study cyclonic precipitation in a mountainous region, and the GATE studies were the first to use shipborne research radars in the tropics. These early studies set in motion two lines of research, which the group continues to the present day: tropical convection, midlatitude convection, and precipitation processes in mountainous regions.
Tropical convection
Africa
The GATE (1974) studies highlighted tropical squall lines
.
Using ship radar data obtained in GATE, the Mesoscale Group showed that
a large fraction of the tropical convective precipitation had a "stratiform
component", i.e., a large region of relatively uniform rain formed by
the melting of snow falling out of a region of relatively gentle but
widespread
upward air motions in the upper atmosphere. These stratiform precipitation
regions dominate the area of rain from cloud shields seen in satellite
pictures
of large convective cloud systems.
The Asian Monsoon
In 1978-79, the Mesoscale Group participated in two international monsoon experiments: winter and summer MONEX. These studies continued to document the convective and stratiform dichotomy of large convective cloud systems, most notably over the maritime continent of Indonesia/Malaysia
. The Mesoscale Group installed a radar on the coast of Borneo and used
the radar and microphysical instruments on board a U. S. NOAA P3 "hurricane
hunter" aircraft. These studies elucidated the diurnal variation of the
monsoonal cloud systems and the characteristics of the ice particles
precipitating
out of the upper levels of the cloud systems. These MONEX projects
were among the first to use the NOAA P3 "hurricane hunter" aircraft in
convection outside of hurricanes, and they showed that many of the results
of GATE
applied in this part of the world as well as in the tropical eastern Atlantic. 
After 20 years, in April-June 1999, the Mesoscale Group returned to the
Asian monsoon and participated in another ship-based radar experiment,
called JASMINE, in which ship radar data 
were collected in convection over the
Indian Ocean during the developing stages of the Asian summer monsoon.
As an active monsoon period develops, the rain occurs first over the ocean.
The shipborne radar dataset showed that the convection over the ocean
during the developing monsoon was dominated by large convective systems
that propagated seaward out over the Bay of Bengal as a result of the
heating of the land during the day. These systems resembled the large
convective systems seen over other tropical oceans.
Hurricanes
In the 1980's Professor Houze participated in hurricane research
flights. These studies were among the first to use airborne Doppler radar
to document simultaneously the airflow and precipitation structure in
hurricanes.
He collaborated with Dr. Frank Marks of the NOAA Hurricane Research Division
in Miami. Their first work in Hurricane Debby (1982) was featured on the
cover of the Bulletin of the American Meteorological Society in
1984. Their 1987 study of Hurricane Alicia (1983) showed that the hurricane
eyewall precipitation had a stratiform component of precipitation with
air motion and precipitation growth processes similar to those in other
types of tropical cloud systems. This work received the Distinguished
Authorship Award of the National Oceanic and Atmospheric Administration
Environmental Research Laboratories. They continued their work on Hurricane
Norbert (1984) in which they performed the first dual aircraft coordinated
radar and cloud microphysical documentation of a hurricane.
Northern Australia
In
1987 Professor Houze and the Mesoscale Group planned and participated
in
an aircraft radar investigation of the precipitating clouds of the Australian
monsoon, off the northern coast of Australia (the Equatorial Mesoscale
Experiment,
EMEX). This project led to new insight into how the large-scale monsoon
circulation adjusts to the presence of the precipitating clouds.
West Pacific
The Mesoscale Group spent much of the period November 1992-February 1993 in the
Solomon
Islands to participate in tHe Tropical Ocean Global Atmosphere Research
Program Coupled Ocean Atmosphere Experiment (TOGA COARE). Four aircraft
with Doppler radars and two ship radars obtained observations of convective
cloud systems over the tropical western Pacific "warm pool," the region
of warmest ocean water in the world. The Mesoscale Group's work in TOGA
COARE was featured on the cover of the Bulletin of the American Meteorological
Society in 1995. The group published numerous papers on the TOGA COARE
dataset. 
By
using the aircraft and shipborne radar data to identify details of the convective
and stratiform components of the cloud systems, these studies showed how
the environment responds to the cloud systems and how the convective systems
that develop to very large size develop very deep layers of inflow feeding
large sloping updrafts. The dataset from TOGA COARE was so extensive that
these studies were able to characterize the natural variability of the convective
structures. Other studies used airborne measuremnts to identify the characteristics
of the raindrop size distributions in the convective and stratiform regions
of these cloud systems.East Pacific
In 1997, the Group led a ship-based field expedition to the eastern tropical Pacific for the Tropical Eastern Pacific Process Study (TEPPS) to investigate deep convection in the intertropical convergence zone. This project obtained detailed radar data in the convective systems occurring in connection with easterly waves in the intertropical convergence zone. The cruise also obtained radar data in drizzle formation in oceanic stratus clouds of Baja California.The Tropical Rainfall Measuring Mission (TRMM)
In the mid 1980's Professor Houze became a
member
of a team designing the science program for a satellite which would carry
a radar into space and document the structure and intensity of tropical
convective clouds. The satellite, launched in 1997, has been highly successful
at providing a long and detailed record of tropical cloud systems. Professor
Houze and his students have contributed several papers on the implications
of the satellite radar measurements regarding the structure of tropical
precipitating clouds. His group pioneered th
e
subdivision of the radar data into convective and stratiform components
and has produced important results on how the large-scale atmosphere responds
to to the tropics-wide pattern of convection seen by the TRMM satellite.
Professor Houze was also responsible for ground validation of the TRMM satellite
at a ground radar station at Kwajalein in the Republic of the Marshall Islands.
As part of this ground validation effort the group participated in an intensive
field study at the Kwajalein ground validation site (KWAJEX, July-September
1999) in which 3 aircraft and a ship were used in coordination with the
Kwajalein ground radar.
Midlatitude convection
In
the mid-1980s to early 1990's, the Mesoscale Group examined midlatitude
convection. The group extended its studies of large convective systems
to include midlatitude thunderstorms over Kansas and Oklahoma as part
of the PRESTORM project. The group found that convective and stratiform
structures in these storms were similar to the tropical cloud systems
studied in GATE and MONEX. They documented one of the most intensively
studied storms in the history of meteorology- the 10-11 June 1985 squall
line over Kansas and Oklahoma. Using ground based Doppler radars the Mesoscale
Group showed for the first time the feature that has come to be known
as the "rear inflow jet" in the stratiform precipitation region of the
storm. The Bulletin of the American Meteorological Society featured
these radar data in a cover story in 1989 which pointed out that weather
forecasters would be able to identify the rear inflow jet in standard
Doppler radar data available in weather stations.
In the early 1990's, Professor Houze collaborated on studies of hailstorms
in central Switzerland. Those studies showed how the mountainous terrain
of that region modified structures of convective systems that were otherwise
similar to those seen in the U. S. over flatter terrain.
In 1991, 
the
Mesoscale Group participated in a project in the vicinity of Cape Canaveral,
Florida. This project obtained a variety of very high resolution data
from three ground-based radars as well as an airborne radar. Using a new
approach to analyzing radar data, the Mesoscale Group analyzed high-resolution
dual-Doppler radar data and dual-polarimetric radar data from the Florida
project to determine in more detail the process by which the stratiform
precipitation evolves in a convective cloud system.
Precipitation in mountainous regions
The Mesoscale Group's studies of precipitation in mountainous regions begin with the CYCLES Project in the late 1970's and early 1980's. The main result of this effort was to identify a systematic substructure within frontal precipitation systems that was manifested as "rainbands". The CYCLES Project studies focussed on the frontal systems in the Pacific Northwest, as they moved over the lowlands just before moving over the mountains. Doppler radars were used along with airborne microphysical measurements. This work done in colloboration with Professor Peter Hobbs's Cloud Physics Group at the University of Washington led to a classification of the rainbands embedded in frontal precipitation and to observations of the internal air motions and precipitation particle growth modes within the various types of rainbands.In the early to mid 1990's, the Mesoscale Group participated in the COAST
Project, which made airborne Doppler radar measurements of frontal cloud
systems approaching the U.S.-Canadian Pacific Northwest coast and moving
over the near-coastal mountain ranges. These flights showed that the frontal
systems were stronger over the ocean before landfall, and that as they approached
land the fronts deformed their shapes in response to the rugged coastline,
and barrier jets formed ahead of the coastal terrain.
In the late 1990's, the Mesoscale Group participated in the Mesoscale
Alpine Project (MAP). This project was a large international cooperative
effort aimed to understand the processes leading to heavy precipitation
on the southern side of the Alps, a region noted for severe flooding
in
association with midlatitude frontal systems passing over the Alpine
range. The NCAR S-Pol Doppler polarimetric radar was operated in the
region of
the Lago Maggiore at the base of the Alps along with several other ground
based radars. The radar data and other observations provided new insight
into the mechanism by which the mountains turn the frontal precipitation
into potentially flood producing storms. The data indicate that when
the
low-level flow from the Mediterranean is neutral to weakly unstable it
rises quickly and unimpeded over the terrain. The air rising over the
first foothills leads to the formation of large raindrops forming by
coalescence
and graupel particles growing by riming. 
These
enhanced microphysical processes are thus key to the precipitation enhancement
as the front moves toward and over the mountains. Situations in which
the low level flow was cold, stable, and apparently blocked were characterized
by relatively stagnant flow at low levels. Precipitation enhancement
occurred over the lower slopes of the Alps but was not as strong as
in the unblocked
cases. A Doppler on Wheels (DOW) truck-mounted radar was used in
the valleys of the Alps in MAP and together with aircraft Doppler radar
data it revealed that during blocked-flow precipitation events the cool
air at low levels drained down the valley in opposition to upslope flow.
The Mesoscale Group published several papers on these orographic precipitation
processes in a special issue of the Quarterly Journal of the Royal
Meteorological Society dedicated to MAP.
In 2001, the "Improvement of Microphysical Parameterization through
Observational Verification Experiment II (IMPROVE II)" was carried out
to study the precipitation processes in frontal systems moving over the
Cascade
Mountains
of Oregon. 
This project was similar to MAP in that the NCAR S-Pol radar was again
used, this time at the base of the Cascade Mountains. It was combined
with a NOAA/ETL vertically pointing S-band radar located near the crest
of the range. Airborne Doppler radar data, aircraft ice particle sampling
and other data were also obtained. This study showed that the Pacific
systems had a lot in common with the stable blocked cases observed in
MAP. As the main band of frontal rainfall passed over the lower slopes
of the Cascades precipitation enhancement the lower layer of air was
blocked.
But the blocked air was bounded above by strong shear as the air at higher
levels strongly flowed over the barrier. The shear layer spawned turbulence,
which in turn produced pockets of enhance upward motion and precipitation
particle growth by riming and aggregation. Thus the Mesoscale Group
has
produced a study that shows that even in a stable frontal situation,
the response of the stable flow to the mountain barrier leads
to a rearrangement of the flow that allows cells of enhanced precipitation
growth and fallout.
This result combined with the MAP studies shows that whether the air
approaching the mountain barrier is stable or unstable, enhancement
of precipitation
fallout occurs on the lower slopes of the mountain range.
The future
In the coming years, the Mesoscale Group plans to return to research on hurricanes. We will be trying to understand how hurricane rainbands interact with the eyewall region and produce storm intensity changes. The group will also continue its work on tropical convection, probably over the African continent, where some of the most intense and electrified convection occurs, and where the convection may be affected by Saharan air and dust particles. And the group will continue work on orographic precipitation over the west coast of North America, and will begin to study precipitation over the Himalayas, and the mountains of Taiwan.
