Transcript A Mesoscale Tour of the Pacific Northwest

A History of Modern Synoptic
Meteorology
Ancient Synoptic Ideas
• The Book of Job (37:22) states, “Fair weather cometh out
of the north”, which is true for many midlatitude locations
when cold, high-pressure systems are found north of the
observer
• In his comprehensive volume, Meteorologica, Aristotle
suggested that a close examination of the sky could
provide useful forecasts.
• One of Aristotle’s pupils, Theophrastus of Ereos, noted in
his volume On Weather Signs that “If the breezes come
from the east or south, rain is indicated; if from the west or
north, breezes and cold weather.”
And, Of Course, Many Ancient Societies
Summarized Synoptic Wisdom in Sayings
•
"Red Sky at night, sailor's delight. Red sky in the morning, sailor take
warning.”
•
"Clear moon, frost soon.”
•
"Halo around the sun or moon, rain or snow soon."
•
"Rainbow in the morning gives you fair warning.”
But there was a major problem: prior to roughly
1750 meteorological instruments lacked the
accuracy, repeatability and common scales that
are mandatory for a viable meteorological
network.
A thermoscope constructed by Galileo Galilei around 1600 A.D. Picture
courtesy of Museo Galileo
By the late 1700s, reasonable (sufficiently
precise and repeatable)
weather instruments became available and
some “universal” scales were developed
In 1665 Christian Huygens suggested
using the freezing and boiling points of
water as standards, and in 1724 Daniel
Fahrenheit proposed a scale with 32F for
freezing and 212F for boiling and used
this scale in the first mercury
thermometers. In 1742 Anders Celsius
introduced the temperature scale that
bears his name (using 0 and 100°C for
the freezing and boiling points,
respectively).
More and more people took
observations….and some early
networks were started (observers
mailed their observations to a
central site or to each other)
Early Synoptic Networks
• 1792 the Mannheim (or Palatine) network included 39 stations
from France, Germany, Italy, Scandinavia, Poland and Russia.
• In the United States a formal observation program was
initiated in l816 under the auspices of the surgeon-general of
the army; army surgeons were required to take three
observations per day of pressure, temperature, state of sky and
winds. Bu 1853, nearly 100 army posts were providing daily
reports.
• Other American networks were organized under the auspices
of the U.S. Navy and the Smithsonian Institution.
The First Synoptic
Meteorologist?
Ben Franklin, the Synoptician
• In a letter dealing with the origin of northeast U.S. storms,
dated May 12, 1760, he concluded, based on the visibility
of a 1743 lunar eclipse at various locations, that the storm
was moving to the northeast even though the winds at the
surface were from the northeast.
• The implication of these observations, which Franklin
clearly appreciated, is that might be possible to predict
storm motion if information could move faster than the
storms, something that was not practical during the late
18th century.
Observing Networks Led to the
First Synoptic Maps
Perhaps the first surface
weather map was
created by H. W.
Brandes in 1820 for
March 6, 1783. The
arrows indicate wind
direction and the lines
show the deviation of
pressure from average
conditions
•
One of the weather maps
created by Elias Loomis in
his groundbreaking paper
on the storms of February
1842. Surface wind
direction is indicated by
arrows and the deviations
from average pressure are
shown by the dashed lines.
Temperatures are indicated
by dotted lines and the sky
or precipitation type by the
color shading. This map
indicates a strong lowpressure center over the
Ohio Valley, rain on the
coast, and snow-laden
northwesterly winds to the
west.
The New Synoptic Charts
Stimulated a Great Debate on the
Nature of Midlatitude Weather
Systems During the First Half of
the 19th Century
Why Do Cyclones Form?
• During the first half of the nineteenth
century several competing theories of the
origin and development of midlatitude
storms were proposed:
– the linear two-current theory of Heinrich Dove
– the centrifugal theory of William Redfield
– the thermal or convective hypothesis of James
Espy.
Heinrich Dove: two opposing
currents
• Dove, one of Europe's great authorities on meteorology
and later Director of the Prussian Meteorological Institute,
proposed that midlatitude storms result from the interaction
of two linear, opposing wind currents:
– a cold, dry, current originating in the polar latitudes
– a warm, humid flow from the equatorial zone.
• According to his theory, weather changes occur as one
current displaces the other. Furthermore, Dove insisted
that the resulting disturbances are not necessarily vortical
or rotating in nature.
L
James Espy: The Thermal Theory
of Cyclones
• James Espy (1830's) proposed a convective or thermal
hypothesis of storm origin.
• Espy noted that storms are usually associated with clouds and
precipitation and thus must be areas of rising motion. He
suggested that storms are analogous to huge heat engines,
being driven by latent heating in the ascending air columns.
• Such heating forces the isobaric surfaces above the storm to
rise, resulting in mass outflow at upper levels and the
diminution of pressure at the surface.
• Espy did not indicate any circulation around midlatitude
cyclones, but noted only a radial component towards their
centers. Anticyclones were assumed to be the hydrostatic
reflection of cold air above the surface.
Thermal Theory
• Thermal Theory
conceptual model was
dominant in the 1830s and
for several subsequent
decades.
• Warm core with
hurricane-like circulation
• Accepted as main
mechanism for much of
the 19th century
Low
Espy 1831
William Redfield: Centrifugal
Mechanism
• William Redfield (1831) produced synoptic charts that
showed that storm winds possess rotational and
translational components.
• He suggested that the rotational motion were created when
the Northeast Trades pressed against "obstructions" such as
the islands of the Caribbean Sea. Once in rotation, air
would be pulled away from the storm's center by
centrifugal forces, thus producing a lowering of pressure in
the core of the storm.
• Although stressing the vortical motion around
disturbances, Redfield did note the existence of weak
convergence towards their centers.
Redfield Mechanism
Elias Loomis
• As more surface observations became available during the
first half of the nineteenth century some of the
contradictory elements of the above theories were
resolved.
• Completing a detailed series of case studies, Elias Loomis
(1841), using the best synoptic charts created to date,
showed conclusively that storms, at least in the United
States, possess both inward and vortical motion.
• Loomis accepted the basic tenets of the convectional
theory, but went considerably further by suggesting that
the initial upward motion that established the latent heating
originated in the uplifting of warm air by cold air currents.
• Loomis’s discussion of the meeting of such currents is one
of the earliest descriptions of what later was to become
known as a cold front. The figure below, taken from his
work, presents what was probably the first cross section of
a cold front.
• Loomis also suggested that the rotation of air around low
centers was caused by the deflection of the radial currents
by the rotation of the earth.
Competing Ideas of Cyclone
Airflows During the Mid-1800s
Espy
Redfield
Loomis
Dove
The Advent of Weather
Forecasting
Weather Forecasting
• Without a means for rapid dissemination of meteorological
observations, early synoptic charts could only be created
weeks or months after the actual collection of data.
• Therefore, weather forecasting at this time was limited to
the "local method," in which surface and cloud
observations at a location were compared to "models" of
weather evolution in order to predict future changes.
• The advent of telegraphy around 1840-45 made possible
the rapid distribution of weather data, so that synoptic
charts could be prepared quickly enough to be
operationally useful for diagnosing and forecasting the
weather.
The Telegraphic Communication
Revolution
• By 1849 a telegraphic network
was organized in the United States
for the transmission of daily
meteorological observations for a
collection of stations.
• In England during the l851
World's Fair, a telegraphic
company prepared daily weather
maps for display, and by 1859 the
British Meteorological
Department began to operationally
distribute weather information
using this new technology.
The internet of the 19th
century
The Revolution
•
With a rapid increase in the number of observing sites, the
demonstrated usefulness of synoptic charts, and the availability
of telegraphy, governments around the globe began to
establish operational meteorological services.
• In l854 Admiral Fitzroy, famed captain of Darwin's ship the
Beagle, was appointed head of the new British Meteorological
Service, which in l861 began issuing daily forecasts and storm
warnings.
• In 1870 the United States included the remnants of the
Smithsonian observation network within a new national weather
service located within the Signal Service of the U.S. Army.
Daily forecasts and warnings began the next year.
• Most of the larger European countries also set up government
meteorological services during this period.
First Real-Time Weather Maps
“Ol Probs”
•Cleveland
Abbe
(“Ol’
Probabilities”), who led the
establishment of a weather
forecasting division within the
U.S. Army Signal Corps.
•Produced the first known
communication of weather a
weather forecast (including the
term “probability”).
Professor Cleveland Abbe, who issued the first public
“Weather Synopsis and Probabilities” on February 19,
1871
On May 7, 1869, Abbe proposed to the Cincinnati
Chamber of Commerce "to inaugurate such a system, by
publishing in the daily papers, a weather bulletin, which
shall give the probable state of the weather and river for
Cincinnati and vicinity one or two days in advance”.
Cleveland Abbe released the first public weather
forecast on September 1, 1869.
Following the signing by President Ulysses S. Grant of
an authorization to establish a system of weather
observations and warnings of approaching storms, on
February 19, 1871, Abbe issued the first “official”
public Weather Synopsis and Probabilities based on
observations taken at 7:35 a.m.
An early example of a report:
"Synopsis for past twenty-four hours; the barometric
pressure had diminished in the southern and Gulf
states this morning; it has remained nearly stationary
on the Lakes. A decided diminution has appeared
unannounced in Missouri accompanied with a rapid
rise in the thermometer which is felt as far east as
Cincinnati; the barometer in Missouri is about fourtenths of an inch lower than on Erie and on the Gulf.
Fresh north and west winds are prevailing in the
north; southerly winds in the south. Probabilities
[emphasis added]; it is probable that the low pressure
in Missouri will make itself felt decidedly tomorrow
with northerly winds and clouds on the Lakes, and
brisk southerly winds on the Gulf."
Push Back
• The new technology of telegraph-based weather prediction was
not welcomed by many in the tradition-bound scientific
establishment of the day, who saw the new applied science as
inconsistent with the proper theoretical approach to scientific
inquiry. Furthermore, some fishing fleet owners objected to his
storm warnings, which kept their ships in port.
• A special committee of the British Royal Society recommended
to the Board of Trade that storm warnings and daily forecasts be
discontinued, and in December 1866 they were terminated.
• The physicist François Arago, the director of the Paris
Observatory before Le Verrier, made his establishment position
quite plain:
– “Whatever may be the progress of sciences, never will observers that are
trustworthy, and careful of their reputation, venture to foretell the state of
the weather.”
Fortunately, the value of the new
telegraph-enabled weather maps
and the infant science of weather
prediction were so obvious to so
many, that national weather
services and their new products
were destined to spread rapidly
during the second half of the 19th
century.
Weather Prediction Technology
of the Later 1800s
• The essential approach…simple temporal
extrapolation.
• No fronts, but they understood that
discontinuities existed.
• Little understanding of the evolution of
weather systems.
• After a period of rapid increase in forecast
skill, predictive skill leveled off.
Atmospheric “Model”
The Theoretical Basis for
Modern Synoptic Meteorology is
Established During the Second
Half of the 1800s
• Between l850 and 1870 there were several major conceptual
and theoretical advances in atmospheric kinematics and
dynamics.
• Although the basic equations of motion of a solid body on a
rotating sphere were known since the beginning of the
nineteenth century, William Ferrel (1855) was the first to
formulate the dynamical equations of fluid motion for a
rotating planet.
• Ferrel also suggested that large-scale atmospheric motions are
basically hydrostatic and geostrophic.
• A geostrophic relationship between pressure and winds was
also implied by C. H. D. Buys Ballot (1857) in the rule: "
Winds always blow, in the northern hemisphere, with high
barometer to the right and low barometer to the left of the
direction in which they blow." Buys-Ballot also noted the
proportionality between wind speed and pressure gradients
Atmospheric Thermodynamics
• The period l850-1870 also brought major advances in
atmospheric thermodynamics including:
– the demonstration of the mechanical equivalent of heat
– the derivation of the first law of thermodynamics
– the accurate description of dry and moist adiabatic processes.
• Furthermore, researchers such as Theodore Reye and H. Peslin
established criteria for the vertical stability of air parcels.
• Thus, by the end of this period the basic dynamic and
thermodynamic principles governing atmospheric flows
were known.
Structural Details of Cyclones
• As more synoptic data became available during the second
half of the nineteenth century, several individuals
attempted to describe the horizontal and vertical structure
of synoptic systems.
• For example, Fitzroy (l863) presented a storm model that
explicitly showed polar and tropical currents spiraling into
cyclonic disturbances . He retained the mechanistic
approach of Dove in which storms formed in the horizontal
shear between two, contrasting air streams.
By 1860s the idea of two main airflows
(warm and cold) and rotation was accepted
cold
Fitz-Roy
1863
warm
• However, in contrast with Espy, Buchan and Mohn
(1870s) and others stressed the asymmetric geometry of
cyclone structure, with upward motion in the forward
sector of the storm and descending motion to its rear.
Warm air in front, cold air behind.
• An asymmetric structure was also noted by Abercromby
(l883) who created a schematic model of the distribution of
clouds, precipitation, winds and pressure relative to the
center of cyclonic storms.
But what about vertical structure?
• With only sporadic soundings taken by balloons and kites, and
only a scattering of mountain top observing sites, nineteenth
century meteorologists lacked the direct measurements required
to clearly define the upper-level structure of synoptic systems.
• Using ten years of cloud observations, Ley found diffluence
and divergence above and in front of surface cyclones, as well
as a general westward tilt with height from the surface up to
cirrus level.
• A decade later Wladimir Koppen explained the backwards tilt
as the hydrostatic result of the asymmetric temperature
distribution of cyclones, with the trough axis tilting towards the
cold air.
Ley: Structure from Cloud
Tracking
Koppen: Temperature
Asymmetry and Vertical Tilt
Thermal Theory on Defense
• New observations and theoretical work were undercutting its
basic foundation.
• With ever growing experience in synoptic analysis, it became
clear to many investigators (such as Loomis, l877) that
barometric minima can form with little or no precipitation.
• At the same time, observations at mountain stations (e.g., Hann,
l877) strongly suggested that, at least in Europe, anticyclones
were relatively warm above a shallow surface layer.
• Mountain reports also indicated that cyclones were generally
associated with cold air, in contradiction to the thermal theory
which demanded a column of warm air to explain the low
pressure at the surface.
Thermal Theory on Defense
• It was becoming clear that the temperature structure of cyclones
was highly asymmetric and that horizontal advection of warm
air, not latent heating, was the major source of warming.
• Several researchers proposed that horizontal temperature
gradients played a crucial role in the development and
evolution of cyclones. For example, Mohn (1870) suggested
that cyclone intensity might be proportional to the temperature
differences across a storm, and Reye (1873) attributed the
stronger storms of winter to the larger temperature contrasts of
that season.
• Koppen and Miller (1882) suggested that the movement of
synoptic disturbances are controlled by upper-level steering
currents, and that dynamical forcing of divergence aloft was
crucial for maintaining the upward motion of storm systems.
But what was the energy source
of cyclones?
• In 1897 Vilhelm Bjerknes developed the circulation theorem,
which showed how baroclinicity could produce acceleration
of air parcels.
• Making use of a series of vertical soundings at the Blue Hill
Observatory near Boston, J.W. Sanstrom, a student of
Bjerknes, found that the pressure and density structure of a
cyclone produced sufficient acceleration to explain the
observed winds.
• Detailed descriptions of the energetics of synoptic
disturbances were produced by Bigelow (1903) and
Margules (1903); the latter demonstrated that the kinetic
energy of storms originated in the available potential energy
resident in horizontal temperature gradients.
Did Meteorologists Know About Fronts
Before the Norwegian Cyclone Model?
• The existence and importance of lines of discontinuity in
pressure and temperature became increasingly evident in
the latter 19th and early 20th centuries..
• For example, Shaw and Lempert (1906) performed
mesoscale analyses of the structure and movement of
frontal zones in Northern Europe; to them the abrupt
changes in pressure suggested "fault lines" in their isobaric
analyses.
• Lempfert and Corless (1910) created vertical cross sections
through the "line squalls" or fronts, as we now call them.
In work that clearly foreshadowed the polar front model of
the Bergen School, Shaw (1911) presented a schematic
cyclone model that clearly showed wind and temperature
discontinuities associated with cold and warm fronts (but
without the names!)
But how can cold and warm air
sit side by side in a stable
equilibrium?
• Margules set up a mesoscale network of mountain and
low-level stations to examine the structure of temperature
discontinuities; an important question at that time was
how contiguous air masses of different temperature and
density could remain in a stable configuration.
• Margules (1906) showed that the Coriolis force allows
cold and warm air to exist side by side, and that the
interface between the two air mass slopes at an angle
determined by the magnitude of the Coriolis force, the
temperature gradient, and the horizontal wind shear.
The Next Major Advance
• The Norwegian Cyclone Model, around
1920
The Bergen School
• The roots of modern synoptic meteorology are often traced
to the seminal papers of the Bergen School that were
produced during and shortly after World War I by J.
Bjerknes, H. Solberg, T. Bergeron, and others.
• The patriarch of the Bergen school, Vilhelm Bjerknes,
possessed a background in electro and hydrodyanmics.
First at Leipzig and then at Bergen, Norway, Bjerknes and
a talented group of colleagues and assistants (including his
son Jack Bjerknes) considered the problems of
atmospheric structure and dynamics.
• Isolated in Norway, they made use of a dense network of
surface observations.
The 1918 Paper
• In l918 the younger Bjerknes (Jack) completed his famous
paper, "On the Structure of Moving Cyclones," which for
the first time identified warm and cold fronts (although
they were called by different names), described the three
dimensional motions and related cloud and precipitation
structure in the vicinity of fronts, and inferred the source of
kinetic energy of cyclonic systems, i.e., the release of
potential energy stored in horizontal temperature gradients.
1922 Paper
• A few years later a sequel paper by Bjerknes and Solberg
entitled "Life Cycle of Cyclones and the Polar Front
Theory of Atmospheric Circulation" introduced the idea
that cyclones form as wave disturbances on the boundary
separating tropical and polar air masses (“the polar front”),
and that this development occurs in a predictable life cycle.
• This cycle begins with an open wave, and then as the faster
moving cold front catches up to the warm front, the
cyclone begins to occlude. By the end of the occlusion
process warm air exists only aloft; thus all that remains is a
dying vortex, completely embedded in the cold air
Concept
of
Evolution
of
Cyclones
Bjerknes and
Solberg
1922
Stationary Polar Front
Wave Forming on Polar Front
Wave Amplifies
Occlusion as Cold Front
Catches Up to Warm Front
Occlusion Lengthens and System Weakens
Introduced the Concept of Air
Flows in Cyclones
Warm and Cold Occlusions
Norwegian Cyclone Model
(NCM)
• It was an important and revolutionary advance at the time.
• Provided a coherent consistent picture of airflows, clouds,
and precipitation of cyclones and fronts. First to connect
three dimensional trajectories with clouds and precipitation.
• Provided a model for frontal and cyclone evolution, aiding
future prediction
• Still found in many textbooks today almost verbatim!
• Over flat land away from water and terrain, reality often
approximates gross characteristics of the NCM.
• However, there are some major problems with the
Norwegian Cyclone model that have been revealed by
modern observations and modeling.
Some Problems With The
Norwegian Cyclone Model
• Different structures and evolutions of fronts
and cyclones often observed over water and
over/downstream of mountain barriers.
• Does not properly consider the role of the
middle to upper troposphere.
• No upper levels fronts.
• Major deficiencies regarding the occlusion
process.
• Does not properly consider that cyclogenesis
and frontogenesis occur simultaneously.
Lack of Upper Air Observations
• In 1920 very sparse upper air observations:
– Mountain stations
– Kites and pilot balloons
– Limited aircraft observations.
Navy bi-plane with meteorgraph on starboard wing strut, taking
meteorological measurements for pressure, temperature, and humidity
Manned flights were inefficient for routine observations as costs were
high and often grounded during poor weather.
Pilot Balloons Provided Winds
Beginning of Reliable Upper Air
Observations
• The first true radiosonde that sent precise
encoded telemetry from weather sensors
was invented in France by Robert Bureau.
Bureau coined the name "radiosonde" and
flew the first instrument on January 7, 1929.
Rapid Expansion of the Upper
Air Network During the 1930s
and 1940s.
Upper Air Data Impact
•
•
•
•
Seminal work of Rossby and the “Chicago
School” in the late 1940s, early 1950s.
Gave a 3D picture of what was happening
Long and short waves
Jet streams
Upper flow steered storms, and thus
provided a tool for forecasting cyclone
movement.
Upper Air Data Leads to
Important Advances
• The structure of these waves, their relationship to the mean
tropospheric winds, and the existence of upper-tropospheric jet
streams were carefully documented by Rossby, Fultz,
Cressman and others at the University of Chicago.
• In a famous 1939 paper, Rossby used the principle of
conservation of absolute vorticity to derive the well-known
Rossby wave formula, which describes the wavelength and
phase speed of upper level waves, given the characteristics of
the mean flow.
• This relationship was used to forecast the movement and
evolution of upper level wave features, and is probably the
first example of the quantitative use of a dynamical principle
in weather prediction.
During the l930's and 40's important new theoretical insights
into the structure and development of synoptic systems
• Bjerknes (l937), Sutcliffe (l947), Bjernkes and Holmboe (l944)
and others showed how upper and lower level flow fields
simultaneously evolve during cyclone development.
• Furthermore, they demonstrated that cyclone intensification was
controlled by the net divergence in a vertical column, which in
turn was a function of phase shift with height and the advections
of vorticity and temperature.
• Charney (l947) and Eady (l949) showed that cyclone development
was the result of baroclinic instability, and that cyclone scale and
growth rates could be explained by this theory.
• Charney also showed that the hydrodynamical equations could be
significantly simplified (the quasi-geostrophic approximation) and
still retain the essential physics describing synoptic and planetary
scale circulations.
The Development of NWP
• Vilhelm Bjerknes in his
landmark paper of 1904
suggested that NWP was
possible.
– A closed set of equations
existed that could predict
the future atmosphere
(primitive equations)
– But NWP wasn’t practical
then because there was no
reasonable way to do the
computations and sufficient
data for initialization did
not exist.
L. F. Richardson: An Insightful
But Unsuccessful Attempt
• In 1922 Richardson
published a book
Weather Prediction by
Numerical Process that
described an approach to
solving the primitive
equations: solving the
equations on a grid using
finite differences.
L. F. Richardson
• He attempted to make a numerical forecast using a
mechanical calculator
• Unfortunately, the results were not good, probably
because of problems with his initial conditions.
L. F. Richardson
• He imagined a giant theater filled with
human calculators…
• So NWP had to wait until a way of doing
the computations quickly was developed
and more data…especially aloft… became
available.
NWP Becomes Possible
• By the mid to late 1940’s there was an
extensive upper air network, plus many
more surface observations. Thus, a
reasonable 3-D description of the
atmosphere was possible.
• Also during this period digital
programmable computers were becoming
available…the first..the ENIAC
The Eniac
The Last Piece of the Puzzle
• Meteorologists realized that useful
numerical weather predictions were
possible using a simplified equation set that
was easier to solve.
• The Barotropic Vorticity Equation
(conservation of absolute vorticity) was
suggested as a first step
First NWP
• The first successful numerical prediction of
weather was made in April 1950, using the
ENIAC computer at Maryland's Aberdeen
Proving Ground
• The prediction was for 500 mb height,
covered North America, using a twodimensional grid with 270 points about 700
km apart.
• The results showed that even primitive
NWP was superior to human subjective
prediction. The NWP era had begun.
Evolving NWS
• Early 50s: one-level barotropic model
• Late 50s: Two-level baroclinic QG model (just
like Holton!)
• 1960s: Primitive equation models of increasing
resolution and number of levels.
• Resolution increases (distance between grid points
decrease): 1958: 380 km, 1985: 80 km, 1995: 40
km, 2000: 22 km, 2002: 12 km
NWP Improvements in the Later
20th Century
• Better resolution
• Rapidly increasing data for initialization
from weather satellites, radars, more surface
observations, and other sources.
• Better models: better numerics and physics
P
Forecast Skill Improvement
NCEP operational S1 scores at 36 and 72 hr
over North America (500 hPa)
National Weather Service
75
S1 score
65
"useless forecast"
55
36 hr forecast
72 hr forecast
45
Forecast
Error 35
10-20 years
Better
"perfect forecast"
25
15
1950
1960
1970
Year
1980
Year
1990
2000
Post 1950: Synoptic Meteorology Transitions
into a Quantitative Science
• Rapid Advancement in the understanding of
frontal structure and development
– Realization that frontogenesis often accompanies
cyclogenesis (e.g., Philips 1956 GC experiment)
– Understanding that large scale (QG) confluence is
not sufficient for rapid frontal collapse (need
convergence of ageostrophic motions-SawyerEliassen equation)
– Observation that fronts can collapse into very
sharp transition not unlike gravity currents
(Sanders 55, Shapiro et al 1987)
Understanding of Frontal
Structures
• Upper level fronts exist near the tropopause
and are primarily the result of differential
vertical motion (Reed and Sanders 53, Reed
55, and many more)
• Frontal precipitation is organized into
mesoscale rainbands, with further
substructures (cores) (Houze and Hobbs
1982)
Mesoscale Rainbands
A Few Other Major Advances
• “PV Thinking” becomes a major tool for
cyclogenesis studies (Hoskins, MacIntyre and
Robertson, 1985). The invertibility of PV becomes
a major cyclogenesis research tool.
• The realization that cyclogenesis can occur away
from fronts in cold air streams, producing polar
lows, comma clouds, and synoptic cyclones.
• Numerical simulation becomes the dominant
approach to studying synoptic structure and
dynamics.
Advances
• Understanding of the
impact of terrain
from global to
mesoscale
• Understanding the
role of extratropical
transition and
downstream Rossby
wave development.