first tropical lecture jon m. schrage summer 2009
TRANSCRIPT
First Tropical Lecture
Jon M. SchrageSummer 2009
Why Is Tropical Meteorology a Separate Course?
• Too big to ignore
• Lots of differences between the tropics and the extratropics
• Interesting and unusual phenomena
Differences between Tropical and Extratropical Meteorology
• Physical Differences
• Observation Differences
• Paradigm Differences
Differences between Tropical and Extratropical Meteorology
• Physical Differences– Warmer at the surface– Colder aloft– Less stable– More thunderstorms– Higher moisture
content– Warm cloud processes– Weak Coriolis Force
– Weak temperature and pressure gradients (no fronts)
– Small annual cycle of temperature
– Relatively large diurnal cycle of temperature
Differences between Tropical and Extratropical Meteorology
• Observation Differences
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Differences between Tropical and Extratropical Meteorology
• Observation Differences– Mostly ocean– Mostly developing countries– Western interests are focused on wartime
needs– Emphasis on remote sensing
Differences between Tropical and Extratropical Meteorology
• Paradigm Differences
Paradigm: a philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated; broadly : a philosophical or theoretical framework of any kind (m-w.com)
Differences between Tropical and Extratropical Meteorology
• Paradigm Differences– Paradigms in Meteorology:
• Polar Front Theory• Air Parcel Theory• Quasigeostrophic Theory
Differences between Tropical and Extratropical Meteorology
• Paradigm Differences– Paradigms in Meteorology:
• Polar Front Theory• Air Parcel Theory• Quasigeostrophic Theory
Not all of these work as well in the tropics as they do in the extratropics!
Differences between Tropical and Extratropical Meteorology
• Paradigm Differences– Paradigms in Meteorology:
• Polar Front Theory• Air Parcel Theory• Quasigeostrophic Theory
Not all of these work as well in the tropics as they do in the extratropics!
Tropical meteorologists have their own paradigms, too:
• e.g., Oscillations
Definitions of the Tropics
• Latitudinal Definitions (used in McGregor and Nieuwolt)– e.g. 23.5°N to 23.5°S– e.g. 5°N to 5°S are the “deep tropics”
Definitions of the Tropics
• Climate Definitions (a la Koeppen)
Definitions of the Tropics
• Kinematic Definitions– e.g., where there are trade winds at the
surface
Definitions of the Tropics
• From Buckle:
Definitions of the Tropics
• In this class, the tropics are where tropical weather happens.
Observing the Tropics
Jon M. Schrage
1. Observing SST
2. Observing the Atmosphere
• Divergent and Rotational Flow
3. Observing Precipitation
Observing SST
• The ocean absorbs tremendous amounts of sunlight.
• All of this energy must eventually be transferred back into the atmosphere.
Observing SST
• Overall, the ocean has a very LOW albedo.
Observing SST
• SST is the single most important variable over the tropical oceans.– SST is the major factor controlling the flux of
sensible heat and latent heat into the atmosphere.
• Keep in mind that winds are important, too!
3 Ways to Observe SST
1. Buoys• Moored or drifting• A maintenance nightmare
Drifting Buoys
TAO Array (fixed)
Example from the TAO Array
3 Ways to Observe SST
2. Ships
• Merchant ships• Disadvantages:
• Taken by nonprofessionals• Calibration issues• SST depends on the weight of the cargo• Limited to shipping lanes
3 Ways to Observe SST
3. Remote Sensing from Space– SST is one of the easiest things to estimate
from space– Sea water radiates at known emissivities
3 Ways to Observe SST
3. Remote Sensing from Space– Problems:
• Calibration issues• Bulk temperature v. skin temperature
Key Features
Some salient features of the global sea surface temperature distribution include:
1. Meridional SST gradients are weak in the tropics and great at midlatitudes.
2. Zonal temperature gradient across the tropical Pacific is large.
3. Almost no annual cycle of SST in the tropics (except in the Western Atlantic).
4. Warmest waters are in small, enclosed subtropical basins…
Key Features
5. The Western Pacific Warm Pool– the warmest open-ocean water in the world.
Key Features
6. The Eastern Pacific Cold Tongue is due to:
1. The cold “Humbolt Current” of the South Pacific Gyre.
2. Upwelling off the coast of Peru and Chile.
3. Coriolis Force deflects the equatorial current, causing divergence and upwelling.
Observing the Tropical Atmosphere
At the surface:
• Over land, observations are taken as at midlatitudes, except there are far fewer obs.
At the surface:
• Over water, observations are taken by mariners and buoys…
At the surface:
…or surface wind speeds can be estimated by remote sensing, which infers the roughness of the sea from scattering of microwave radiation.
Aloft:There are very few radiosonde stations in the tropics over land, and almost none over tropical oceans.
How do we compensate for this lack of data?
1. Remote sensing (i.e., “retrieval”) of temperature, water vapor, precipitable water, etc.
How do we compensate for this lack of data?
2. Cloud tracked winds
How do we compensate for this lack of data?
3. Global AnalysesModels assimilate all sources of data and develop a global depiction of the atmosphere.
Describing the Wind
Wind is a vector quantity, so it always takes two numbers to describe the wind at a given point:
1.Direction and speed2.U and V3.Streamfunction and Velocity Potential
Helmholtz’s Theorem
• Circulation at low latitudes tends to be driven by the divergent outflow from tropical convection.
• Circulation at high latitudes tends to be driven by the rotational flow governed by troughs and ridges.
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Helmholtz’s Theorem
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The divergence of the real wind equals the divergence of the divergent wind.
The vorticity of the real wind equals the vorticity of the rotational wind.
The rotational wind has no divergence.
The divergent wind has no vorticity.
Helmholtz’s Theorem
• So, where do we get the “divergent” and “rotational” winds?
R
D
V
V
Divergent wind is the GRADIENT OF VELOCITY POTENTIAL, χ.
Rotational wind is parallel to contours of STREAMFUNCTION, ψ, with low values of ψ to the flow’s LEFT.
• Rotational winds are parallel to contours of streamfunction, much like geostrophic winds are parallel to contours of height.
• The stronger the gradient of streamfunction, the stronger the rotational wind—much like geostrophic winds are stronger when the height gradient is stronger.
• Divergent winds flow from regions of low velocity potential to regions of high velocity potential, OPPOSITE the way the pressure gradient force goes from areas of high pressure to areas of low pressure..
• The stronger the gradient of velocity potential, the stronger the divergent wind—much like pressure gradient force is stronger when the height gradient is stronger.
Helmholtz’s Theorem
Why do we partition the flow like this?
1. Some models use streamfunction and velocity potential (instead of pressure and heights, for example).
2. Some tropical weather features are more easily seen/identified in the velocity potential than in other atmospheric variables.
3. Velocity potential and streamfunction are defined and useful on the equator.
Tropical Charts: Streamfunction
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/gfswk2.gif
Observing Precipitation
Observations of precipitation in the tropics are greatly complicated by the fact that most of the surface area of the tropics is ocean.
Observing Precipitation
• Why do we care how much it rains at sea?– NOT for agricultural reasons– NOT to predict runoff
Observing Precipitation
• Why do we care how much it rains at sea?– Rain water is generally cooler than the
underlying ocean.– Therefore, rain generally reduces SST.
Observing Precipitation
• Why do we care how much it rains at sea?– Rain water is fresh. Some ocean circulations
are driven by salinity gradients.
Observing Precipitation
• Why do we care how much it rains at sea?– Rain water is fresh. Establishes “fresh water
lenses” that are very stable.
Observing Precipitation
• Why do we care how much it rains at sea?– Latent heat release in the tropics is an
important component of the global energy balance.
– Improves model results.
Observing Precipitation
1. In situ instruments• Work well over land, but not at sea:
• Can’t distinguish between rain and sea spray.• Gauges need to point straight up at all times.• Who empties the gauge on a buoy?• Tipping bucket rain gauges have problems at
sea.
Observing Precipitation
2. Remote Sensing:• ACTIVE remote sensing:
• Coastal radar stations• Ship-borne radar• Radar from space-TRMM
Ship radar on the
Ron Brown.
Coming Soon: GPM
• One “core” satellite with a radar and microwave imager
• A “constellation” of satellites with identical microwave imagers
Observing Precipitation
2. Remote Sensing:• PASSIVE remote sensing:
• GPI: GOES Precipitation Index• Ordinary IR satellite images
• Microwave Remote Sensing• Emission and scattering of microwave radiation by
hydrometeors
Observing Precipitation
3. Budget Techniques– Our observing systems are better at tracking
properties of the atmosphere that persist:• Temperature• Relative humidity• Winds
– Even globally!
Observing Precipitation
3. Budget Techniques– The goal of budget techniques is to use our
“good” understanding of the atmosphere to produce estimates of precipitation.
– Cannot estimate rain from a single storm, but over a large region, over a period of time, they can work very well.
Observing Precipitation
3. Budget Techniques
What can change the amount of moisture (Q) in this box? (DQDT)
Observing Precipitation
3. Budget Techniques
Moisture could be horizontally advected in or out of the box. (HADQ)
Observing Precipitation
3. Budget Techniques
Moisture could be vertically advected in or out of the box. (VADQ)
Observing Precipitation
3. Budget Techniques
Or moisture could condense or evaporate in the box…
…but we don’t measure this! (Q2)
Observing Precipitation
3. Budget Techniques
Q2 = -[ DQDT + HADQ + VADQ ]
Observing Precipitation
3. Budget Techniques
Q2 = -[ DQDT + HADQ + VADQ ]
Using things we DO measure…
…to estimate something we do
NOT!
Observing Precipitation
3. Budget Techniques
What can change the amount of dry static energy (S) in this box? (DSDT)
Observing Precipitation
3. Budget Techniques
Energy could be horizontally advected in or out of the box. (HADS)
Observing Precipitation
3. Budget Techniques
Energy could be vertically advected in or out of the box. (VADS)
Observing Precipitation
3. Budget Techniques
Energy could be radiated in or out of the box. (QR)
Observing Precipitation
3. Budget Techniques
Or heat could be released by condensation in the box…
…but we don’t measure this! (Q1)
Observing Precipitation
3. Budget Techniques
Q1-QR = -[ DSDT + HADS + VADS ]
Observing Precipitation
3. Budget Techniques
Q2 = -[ DQDT + HADQ + VADQ ]
Using things we DO measure…
…to estimate something we do
NOT!
Observing Precipitation
3. Budget Techniques• Whether by the “apparent moisture sink”
(Q2) or the “apparent heat source” (Q1) method, good estimates of condensation and/or latent heating.