First Tropical Lecture

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


• 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


• Observation Differences

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• Observation Differences– Mostly ocean– Mostly developing countries– Western interests are focused on wartime

needs– Emphasis on remote sensing


• 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)


• 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!




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”


• Climate Definitions (a la Koeppen)


• Kinematic Definitions– e.g., where there are trade winds at the

surface


• From Buckle:


• 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


2. Ships

• Merchant ships• Disadvantages:

• Taken by nonprofessionals• Calibration issues• SST depends on the weight of the cargo• Limited to shipping lanes


3. Remote Sensing from Space– SST is one of the easiest things to estimate

from space– Sea water radiates at known emissivities


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.


2. Cloud tracked winds


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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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.


• So, where do we get the “divergent” and “rotational” winds?

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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.


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.


• Why do we care how much it rains at sea?– NOT for agricultural reasons– NOT to predict runoff


• Why do we care how much it rains at sea?– Rain water is generally cooler than the

underlying ocean.– Therefore, rain generally reduces SST.


• Why do we care how much it rains at sea?– Rain water is fresh. Some ocean circulations

are driven by salinity gradients.


• Why do we care how much it rains at sea?– Rain water is fresh. Establishes “fresh water

lenses” that are very stable.


• 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.


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.


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


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


3. Budget Techniques– Our observing systems are better at tracking

properties of the atmosphere that persist:• Temperature• Relative humidity• Winds

– Even globally!


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.


3. Budget Techniques

What can change the amount of moisture (Q) in this box? (DQDT)



Moisture could be horizontally advected in or out of the box. (HADQ)



Moisture could be vertically advected in or out of the box. (VADQ)



Or moisture could condense or evaporate in the box…

…but we don’t measure this! (Q2)



Q2 = -[ DQDT + HADQ + VADQ ]




Using things we DO measure…

…to estimate something we do

NOT!



What can change the amount of dry static energy (S) in this box? (DSDT)



Energy could be horizontally advected in or out of the box. (HADS)



Energy could be vertically advected in or out of the box. (VADS)



Energy could be radiated in or out of the box. (QR)



Or heat could be released by condensation in the box…

…but we don’t measure this! (Q1)



Q1-QR = -[ DSDT + HADS + VADS ]




Using things we DO measure…

…to estimate something we do

NOT!


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.

First Tropical Lecture

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