Research
Seven tenths of the Earth's surface is covered with oceans.
A majority of the remainder is uninhabitable landmass such as desert,
rain forest, and tundra.
We inhabit such a limited area on our planet that we tend to be unaware of
what occurs in the rest of the world.
Regional climate around where we live, however, is inseparable from the
complicated system of global climate.
In the last century, great progress was made in our understanding of
individual processes critical for climate formation.
At the same time, we have learned that it is awfully
difficult to disentangle those processes from the whole climate system.
A typical example is the El Niño/Southern Oscillation (ENSO) and
its impact on midlatitude climate.
Another notable case may be found in the controversies on climate change
associated with global warming.
Monitoring how the nature behaves on a global scale is essential
to understand not only the Earth's climate system as a whole but also
a tiny piece of it upon which human life depends.
Our research group focuses on two of the most important elements of the
Earth's climate: cloud and precipitation.
Over the past decades a number of satellites have been launched with various
sensors aboard that observe the distribution of clouds and rainfall across
the entire globe.
Progress in computer capability has provided us with "virtual eyes" to
look into the world such as a cloud-resolving model (CRM) and
a general circulation model (GCM).
We study cloud and precipitation climatology by means of
satellite observation and model simulation.
Climatology of Tropical Precipitation Systems
The nature of large-scale atmospheric circulation is fundamentally different
between the Tropics and higher latitudes.
The midlatitude atmosphere, where horizontal temperature gradient is sharp
in balance with the Coriolis force, has an internal mechanism to mix warm
and cold airs over thousands of kilometers.
We often see this happen on a weather chart as a frontal system
associated with a developing low.
The tropical atmosphere lacks this mechanism due to the weak Coriolis force.
All that is found in the tropical atmosphere, you might imagine, is beautiful
clouds towering in the clear, blue sky.
Towering clouds are indeed all that is needed for the tropical
atmosphere to drive its circulation.
An afternoon shower on a hot summer day leaves cool refreshing air
as it passes away.
It might surprise you to learn that rainshower actually "warms" the surrounding
air inside a developing convective tower.
This is due to latent heat released when water vapor condenses into
cloud droplets.
Two thirds of global rainfall falls in the Tropics,
where latent heat released from precipitating clouds is so great that it is
a major source of energy to fuel atmospheric circulation.
Our goal here is to understand tropical climate dynamics in light of the
role of precipitating cloud systems.
Follow the link below for more details on this topic.
Tropical Convection and Environmental Variability
Tropical clouds develop sometimes into a vigorous, intense cloud system such as squal lines
when certain environmental conditions are met.
The condtions favorable or unfavorable for the development of intense convection include
thermodynamic factors such as moisture and static stability as well as dynamic ones like
vertical wind shear and cold air intrusion.
Convective clouds, however, are not only a passive response to the external forcing but
affect the surrounding humidity, temperature, and wind fields and can ultimately modify
the large-scale environment.
Such an interplay between tropical convection and its environment is a key ingredient of
the tropical weather/climate system but is so difficult to disentangle we don't yet
fully understand how it works.
The lack of precise knowledge of how moist convection and large-scale dynamics are related
remains a formidable obstacle to further improving our skill in
numerical weather forecast and future climate prediction.
Satellite observations are not necessarily an optimal tool to look into a quickly varying
(over a few hours) atmosphere during the course of a cloud life cycle,
because low-Earth orbiting (LEO) satellties fly over a given point on the Earth only twice a day
at best.
An innovative aspect of the study underway is that the hourly or subdaily scale
variability is successfully, although only in statistical space, illustrated exclusively with
LEO satellite measurements alone.
How to do the trick and what it tells us about the convection-environment interaction
are outlined below (follow the link).
Equatorial Waves Coupled with Convection
A phenomenon specific to tropical and subtropical climate is a pair of
rainy and dry periods that take place by turns.
Seasonal change is an obvious factor responsible for repeating
wet and dry phases.
A faster cycle with a period of a month or two is also known to dominate
tropical weather patterns,
but the origin of its periodicity is less obvious.
This intraseasonal variability has been a popular and challenging
subject in tropical meteorology for years.
A pebble thrown into a pond makes ripples propagating away.
The atmosphere, as if it were a gigantic pond, is also filled with
invisible waves traveling around.
A wave of the atmosphere becomes visible when a disturbance of air creates
a lampshade-like cloud as occasionally observed in the downstream of mountains.
If wave-induced clouds grow deep and get organized to form a heavy storm,
the resultant thermodynamical impact on the surrounding atmosphere would be so
large that it could dynamically feed back the wave propagation.
This type of an atmospheric wave is called a convectively-coupled wave
or moist wave.
Of particular importance are convectively-coupled equatorial waves
that extend from the Indian ocean all the way to the Pacific ocean,
oscillating at periods as slow as a month or two.
Such synoptic- to planetary-scale modes are believed to be a key
for explaining tropical intraseasonal variability.
Follow the link below for more details on this topic.
Satellite Data Simulator Unit
Modern technologies gave birth to flying "artists" who paint
cloud and rainfall on a gigantic canvas known as satellite imagery.
The artists dedicate themselves to faithfully reproduce what they saw,
but we cannot figure out what they painted without proper interpretation.
A fair amount of work is required to transform a sequence of downlinked
digital signals into a map of rainfall patterns as appears in weather forecast.
Such a software to process satellite data is called a
retrieval algorithm.
A crucial component of remote-sensing retrieval algorithms is radiative
transfer simulation to synthesize satellite measurements.
Radiative transfer simulation deals with the interaction of radiation with
the Earth surface and atmospheric constituents, which involves a variety of
physical processes such as the scattering and absorption of radiation.
Computational schemes to solve radiative transfer problems are individually
optimized to different kinds of sensors for practical reasons.
For example, a radiative transfer code desinged for microwave frequencies
is generally not interchangeable with a visible/infrared simulation program.
Recent Earth-observing satellites carrying multiple sensors, however, have
led to increasing demands for a radiative transfer code applicable uniformly
to different types of sensors.
The Satellite Data Simulator Unit (SDSU) was developed for simulating
data from space-borne microwave radiometers, radars, and visible/infrared
imagers.
Potential application of the SDSU package is not limited to retrieval
algorithm development but also includes cloud-resolving models (CRMs).
It would help test and improve CRM performance to assess simulated
measurements of various satellite sensors
in comparison with actual observations.
Follow the link below for more information and download.