Research Interests
My main research focus is on investigating mid latitude storms, including how to forecast them better from a few days out to a season, how they may change under global warming, and their immense societal impacts. The distinctive characteristic of my research lies in the fact that I employ a wide range of tools in my research, ranging from analyses of gridded atmospheric analyses and state of the art climate model simulations to learn about the basic characteristics of the phenomena, examination of actual observations to validate what have been learnt from the gridded data, and dynamical studies using a suite of intermediate/mechanistic models to achieve better understanding of these observed phenomena. My major research interest areas are:
For more details, see my publications
Selected Highlights
The following highlights have been published as Highlights
in BAMS, EOS, or in news articles. These serve as examples of my research
interests over the years.
To Improve Seasonal
Storm Track Forecasts, Look to the Tropical Stratosphere
News article link: https://www.somas.stonybrook.edu/2018/03/28/somas-research-aims-to-improve-seasonal-storm-track-forecasts-look-to-the-tropical-stratosphere/
People have become
familiar with “bomb cyclones” this winter, as several powerful winter storms
brought strong winds and heavy precipitation to the U.S. east coast, knocking
out power and causing flooding. With strength that can rival that of
hurricanes, bomb cyclones get their name from a process called bombogenesis, which describes the rapid intensification
they undergo within 24 hours as they move along the coast. These winter storms
tend to form and travel within narrow “atmospheric conveyor belts”, called
storm tracks, which can change location over a period of years.
Scientists have
extensively studied potential causes behind these year-to-year changes in
attempt to better forecast storm tracks and their extreme impacts, but new
research from scientists at the Stony Brook University (SBU) School of Marine
and Atmospheric Sciences, funded by NOAA Research’s MAPP Program, identifies
another crucial controlling force. After analyzing 38 years of model data, the
research team found that an alternating pattern of winds high up in the
tropical stratosphere, called the Quasi-Biennial Oscillation (QBO), affects
significant year-to-year changes in both the North Pacific and North Atlantic
storm tracks. … Read
more
Journal Article:
Wang, J., Kim, H.-M. & Chang, E. K. M. (2018). Interannual
Modulation of Northern Hemisphere Winter Storm Tracks by the QBO. Geophysical
Research Letters, 45. https://doi.org/10.1002/2017GL076929
Projected Significant Increase in
Extreme S.H. Extratropical Cyclone Frequency
BAMS 98, May 2017, p.886-887,
Paper of Note
Extratropical cyclones
are responsible for much of the high impact weather in the mid-latitudes,
including heavy precipitation, high winds, and coastal storm surges. Thus how
these cyclones may change in the future is of much general interest. Previous
studies have suggested a poleward shift in the location of these cyclones, but
how their intensity may change remains uncertain, especially in terms of
maximum wind speed. This study shows that under global warming,
state-of-the-art climate models systematically project a significant increase
in the frequency of extreme cyclones in the Southern Hemisphere (S.H.).
In the study,
projected changes in extreme cyclones in the Southern Hemisphere, based on 26
models participating in the Coupled Model Intercomparison
Project Phase 5 (CMIP5), have been presented. Multiple definitions of extreme
cyclones have been examined, including intensity exceeding constant thresholds
of sea level pressure perturbations, 850 hPa
vorticity, and 850 hPa winds, as well as variable
thresholds corresponding to a top-5 or top-1 cyclone per winter month in these
three parameters and the near surface winds. Results show that CMIP5 models
project a significant (much more than 10% in most cases) increase in the
frequency of extreme cyclones in all four seasons regardless of the definition,
with over 88% of the models projecting an increase. Spatial patterns of
increase are also consistent, with the largest increase projected between 45°S
and 60°S, extending from the South Atlantic across the South Indian Ocean into
the South Pacific. Projected increase in cyclone intensity is largely
consistent with the projected increase in the temperature gradient in the
Southern Hemisphere upper troposphere and lower stratosphere.
(a) Climatology and (b) projected
change in the frequency of cyclones with extreme near surface winds in Southern
Hemisphere winter. Contours show frequency of cyclone occurrences (in % of
time). Shades in panel (b) indicate model agreement, with orange (red)
indicating over 2/3 (4/5) of the CMIP5 models project an increase in the number
of extreme cyclones. There are no grid boxes at which over 2/3 of the models
project a decrease.
Results of this study,
together with those from previous studies that suggest cyclone extreme
precipitation may increase under global warning, indicate that extratropical
cyclones may give rise to more severe impacts in the future. Current research
efforts focus on understanding how Northern Hemisphere extreme cyclones may
change under the competing influences of the projected increase in the upper
level temperature gradient and substantial decrease in the near surface
temperature gradient under global warming.—Edmund Chang (Stony Brook
University), “Projected Significant Increase in the Number of Extreme
Extratropical Cyclones in the Southern Hemisphere,” Journal of Climate, 30,
4915-4935 (2017).
Stony Brook
Professors Create Hurricane Prediction Model for New York State
News article link: https://www.somas.stonybrook.edu/2015/04/22/stony-brook-professors-create-hurricane-prediction-model-for-new-york-state/
A new prediction
model tailored to forecast the number of tropical cyclones that could hit New
York State in a given season has been developed by three Stony Brook University
professors. For the coming Atlantic hurricane season, June 1 through Nov. 30,
their forecast is for below-normal activity for tropical storms and
depressions, hurricanes and post-tropical storms. Over the past 35 years, the
number of storms hitting the state has run from zero to two a year, said Hye-Mi Kim, assistant professor in the School of Marine and
Atmospheric Sciences. This year there’s a 19 percent probability, with 43
percent being the average, for one or more storms hitting the state, said Kim,
who developed the new forecast approach with professors Edmund K.M. Chang and Minghua Zhang of the School of Marine and Atmospheric
Sciences. Their article on the model was posted Thursday on “Weather and
Forecasting,” an online journal of the American Meteorological Society. … Read
more
Journal Article:
Kim, H.-M., E.K.M. Chang, and M. Zhang, 2015: Statistical-dynamical seasonal
forecast for tropical cyclones affecting New York State. Wea. Forecasting, 30, 295-307,
doi:10.1175/WAF-D-14-00089.1.
Why is the Atlantic Storm Track
Stronger Than its Pacific Counterpart?
BAMS 90, October 2009, p.
1438-1440, Conference Notebook
One puzzle confronting atmospheric dynamists is why during the Northern
Hemisphere (NH) mid-winter, the Atlantic storm track is stronger than its
Pacific counterpart. In this study, Chang and Lin used a suite of mechanistic
and GCM experiments to examine this issue, and found that diabatic
heating in the mid-latitudes, which is mainly forced by the continent/ocean
distribution, appears to be the most important factor leading to this
phenomenon.
During the cool season, weather in the mid-latitudes is dominated by
passages of cyclones and anticyclones. In the NH, these weather producing
systems are organized into two storm tracks, with peaks over the Pacific and
Atlantic oceans. Theory suggests that these storms develop by tapping the
available potential energy associated with the north-south temperature gradient
of the background flow. It is observed that the temperature gradient is much
stronger near the entrance of the Pacific storm track, thus one might expect
that the Pacific storm track should be stronger than its Atlantic counterpart.
However, under various quantitative measures, such as eddy kinetic energy and
poleward heat transport by these storms, the Atlantic storm track comes out to
be stronger.
In this study, a dry global circulation model is used to examine the
contributions made by orographic and diabatic forcings in shaping the zonal asymmetries of the NH winter
storm tracks. The model is forced by orography and fixed diabatic
heating mimicking observed heating. By design, the model mean flow is forced to
bear a close resemblance to the observed mean flow. The model also provides a
decent simulation of the storm tracks. In particular, the maxima over the
Pacific and Atlantic, and minima over Asia and North America, are fairly well
simulated. The model also successfully simulates the observation that the
Atlantic storm track is stronger than the Pacific storm track.
Sensitivity experiments are performed by imposing and removing various
parts of the total forcings. Results of these
experiments suggest that NH extratropical heating is the most important
forcing. Zonal asymmetries in NH extratropical heating (basically heating over
the oceans, cooling over the continents) act to force the Pacific storm track
to shift equatorward, and the Atlantic storm track to shift poleward, attain a
southwest-northeast tilt, and intensify. It appears to be the main forcing
responsible for explaining why the Atlantic storm track is stronger than the
Pacific storm track. Tibet and the Rockies are also important, mainly in
suppressing the storm tracks over the continents, forcing a clearer separation
between the two storm tracks. In contrast, asymmetries in tropical heating
appear to play only a minor role in forcing the model storm track distribution.
Distribution of NH winter storm tracks, in terms of RMS 500 hPa geopotential height perturbations (contour interval 20
m) for synoptic scale waves, as simulated using the mechanistic model by Chang
and Lin: with both orographic and diabatic forcings (top panel), with diabatic
forcing alone (middle panel), and with orographic forcing alone (bottom). Note
that the Atlantic storm track is stronger than its Pacific counterpart in the
top two panels, with the orange shading indicative of perturbations of more
than 100 m.
To assess whether these dry model results are robust, several
experiments have also been conducted using the full physics CAM, comparing the simulated
circulations either in the presence or absence of mountains, forced either with
fixed SST distributions or coupled with a slab ocean model. The results of
these full GCM experiments confirm the mechanistic model results that even in
the absence of mountains, the Atlantic storm track would still be stronger than
the Pacific storm track, showing that it is diabatic
heating forced by continent/ocean distribution that acts to force a stronger
Atlantic storm track.
The question remains as to what aspects of the continent/ocean
distribution leads to this phenomenon. Preliminary results suggest that the
shapes of the continents may be an important factor, but more experiments need
to be conducted to clarify this – Edmund
K. M. Chang (Stony Brook University), and W. Lin, “Why Is the Atlantic Storm
Track Stronger Than its Pacific Counterpart?” presented at the 17th
Conference on Atmospheric and Oceanic Fluid Dynamics, 8012 June 2009, Stowe,
Vermont.
Related journal article: Chang, E.K.M.: Diabatic
and orographic forcing of northern winter stationary waves and storm tracks. J. Climate, 22, 670-688 (2009).
Estimating the number of
pre-satellite era North Atlantic tropical cyclones
EOS 88, #32, 7 August 2007, p.
319, AGU Journal Highlights – Mohi Kumar, Staff
Writer
Trends in hurricane occurrence
over the North Atlantic are difficult to determine because hurricanes likely
went undetected before satellite observations were available. To estimate the
occurrence of these undetected hurricanes, Chang and Guo
examined ship track records before and during the satellite era. They mapped
satellite-derived cyclone tracks from 1976 to 2005 against ship tracks from the
same time period to determine the probability that ships recorded wind speeds
high enough to detect a tropical cyclone. Then, they computed the probability
that ships that sailed between 1900 and 1965 made wind observations at similar
wind speeds, assuming that tropical cyclones from the satellite era were
present at the same rate during earlier years. From this, the authors found
which storm tracks were too distant from ships to be detected. They determined
that the number of tropical cyclones not making landfall likely was
underestimated by 1 or less per year after World War I. Thus, their results
suggest that the characteristics of North Atlantic tropical cyclone track
statistics might have changed during recent decades. (Geophysical Research Letters, 34, L14801, doi:10.1029/2007GL030169,
2007)
Based on HURDAT,
there is an apparent trend in the number of tropical storms in the Atlantic
during the 20th century. Several studies have blamed this trend on
undetected storms prior to the satellite era due to sparse observations over
the ocean. We have used ship observations to estimate whether this could be the
case, and our results suggested that the number of underestimated storms is
probably insufficient to explain away the trend. For details, see Chang and Guo (2007).
Wave Packets and Pacific Cyclone
Development
BAMS 86, 2005, p. 912-913, Paper
of Note
In the upper troposphere, medium
scale baroclinic waves tend to be organized into wave
packets. During the Northern Hemisphere winter, these wave packets propagate
across
It is well known that in the
Northern Hemisphere winter, one of the major peaks in cyclogenesis lies over
the western Pacific. While it is clear that the location of the peak is related
to the presence of strong ocean-continent temperature contrasts, the precise
conditions under which explosively deepening cyclones (or ‘bombs’) develop over
the region are not well understood. Previous studies have documented some
environmental preconditioning that can significantly impact the rate of cyclone
deepening. In this study, by stratifying cyclone development statistics based
on the configuration of wave packets over
Lagged composites, based on cyclone event over West
Pacific on day 0 (panel i), showing development of cyclone
over the west Pacific (panels f-j), and evolution of upper tropospheric wave
train (panels a-e), propagating from Asia across the Pacific, approaching
western US on day +1. Note the two branches of wave activity over Asia on days
-3 and -2 (panels a-b). Adapted from Chang (2005).
More detailed examination of the
structure of the cyclones that developed under the influence of the upper level
wave packets indicates that the dynamics involved in the cyclone development
may well depend on whether the wave packet has originated from the northern or
the southern branch. Moreover, while cyclone development is found to be clearly
enhanced when wave packets propagate from
Jet Variations and Storm Track
Variability
BAMS 85, February 2004, p. 164,
Paper of Note
A striking observation of the
Pacific storm track is the negative correlation between the storm track and jet
strengths during midwinter (where storm track strength is measured by transient
eddy variance). This relation is not obvious, given that geographically, the
storm tracks peak in regions where the jet stream—and correspondingly, the
vertical wind shear—are strongest.
Observations show that the
midwinter Pacific jet is narrower during years when it is strong. With linear
models we are able to reproduce the observed decrease of storm strength with
shear, if we take into account the narrowing of the jet. This is the case
because a narrower jet can decrease the meridional scale of the perturbations,
which, according to baroclinic instability theory,
hampers growth. A common suggestion is that perturbations are weaker when the
jet is strong because they move faster out of the unstable storm-track region
and thus have a shorter period of growth. We find that we need to take into
account that the jet narrows when it strengthens; otherwise, the increase in
growth rate is strong enough to counteract the effect of increased advection speed.
One question that arises is
whether our results can also explain the observation that the midwinter Pacific
storm track is weaker than the fall and spring tracks (the midwinter
suppression). While a similar jet width-strength relation is observed on a seasonal
time scale, linear model calculations suggest that seasonal variations in jet
width are not large enough to counter the effects of seasonal changes in baroclinicity to fully explain the midwinter suppression.
Seasonal variations in jet width can, however, explain the observation that the
midwinter suppression is much more prominent during strong jet years.
In the
Possible Trends in Northern
Hemisphere Storm Tracks
BAMS 83, March 2002, p. 348,
Paper of Note
Results of a new analysis suggest
that both the Pacific and Atlantic storm tracks intensified by about 30% from
the 1960s to the 1990s, and that the month-to-month and interannual
variations of the two storm tracks are significantly correlated. Our analysis
also shows that much of the storm track variability is not related to
previously documented modes of low frequency variability.
Mid-latitude weather and climate
during the cool season are closely related to changes in the location and
intensity of the storm tracks. For example, precipitation anomalies over the
western
An important issue is that spurious climate perturbations could be introduced
into atmospheric analyses due to changes in the observing system. Our
preliminary analysis of radiosonde observations along the storm tracks suggests
that the upward trend based on radiosonde observations alone appears to be less
than that based on the reanalysis data. Unfortunately, since the peaks of the
storm tracks lie over the oceans (where there are no radiosonde stations with
continuous records), the variations over the storm track peaks cannot be
verified. Thus it is crucial that other direct evidence be found to
substantiate the magnitude of the storm track variability, especially over the
oceans.--Edmund K. M. Chang and Yunfei Fu. “Interdecadal Variations in Northern Hemisphere Winter Storm
Track Intensity,” appearing in the 15 March Journal
of Climate, 15, 642-658 (2002).