Geog 288 : Topics in Tropical Climate Objective : provide graduate students an overview of tropical climates, their

variations and changes over time and challenges in modelingSyllabus: 1) Fundamental concepts in tropical climates. 2) The trade winds, Hadley and Walker cells 3) Land-air interactions 4) Atmosphere and Ocean interactions: El Nino and Southern Oscillation and the Madden-Julian Oscillation ; 4) Monsoons: present, past and future; 5) Modeling tropical climates.

Grades: presence, seminars and labs.

Prerequisites: graduate standing.Mondays: 3:00-6:00 pm Ellison Hall 5824

Labs: TBAProf. Leila M. V. Carvalho

http://clivac.eri.ucsb.edu/

Objectives

Introduce the students the concept of tropical climates and the main

atmospheric mechanisms responsible for their changes

stimulate interactive classes to explore the tropical atmosphere

Syllabus• Introduction to tropical climates• Description of the characteristics of the tropical

atmosphere• The Hadley cell: trade winds, clouds and precipitation• The surface and atmosphere Interface: a) General features, b) Air-sea interactions c) Land-air

interactions • Monsoon Climates• El Nino/La Nina and the southern Oscillation• The Madden-Julian Oscillation • Modeling Tropical Climates

Bibliography• Observations of Surface to Atmosphere Interactions in the

Tropics: Michael Gargstang, David. R. Fitzjarrald. Oxford, (1999)

• Tropical Climatology: Glenn R. McGregor, Simon Nieuwold. Wiley, Second Ed (1998).

• The Asian Monsoon, Bin Wang. Springer (2006)• El Niño and the southern oscillation: multiscale variability

and global and regional impacts. Henry F. Diaz, Vera Markgraf. Cambridge University Press (2000)

• Additional journal papers will be suggested to cover other specific topics.

Grading

• Discussions during class: students should read the material and one or two will highlight the main points of the text.

• Practical activities or labs: include simple data analysis using available data.

Importance of tropical climates

• Tropical climates control the lives and economic activities of the population in their regions to a much greater extent than the midlatitude climates do.

• The inhabitants of these areas number ~ 45% of the world population (almost all living in the humid tropics, around 60% in southern and eastern Asia)

• Many tropical countries belong to the group of less developed, or developing nations, characterized by low standards of living and a strong economic concentration on agriculture and production of raw materials

Definition of ‘tropics’The word ‘tropics’ is derived from ‘Tropic of Cancer and Tropic of

Capricorn’ and ‘Tropics’ is essentially referred to ‘low latitudes’

However, is there a real climate boundary for the tropics? Let’s examine this issue further…

We show now Infrared (IR)Satellite images of the globe (merge of satellites). Remember:

IR: bright: ( Cold cloud tops – convective clouds and cirrus) dark : (low clouds or no clouds)

Tropic of Capricorn and Cancer

Climatic “Boundary” of the tropics:• What is the major common feature in the tropics?

a) Absence of a cold seasonb) Annual range of temperature

c) Atmospheric circulations dominated by easterlies in the tropics and westerlies in midlatitudes

d) Weak temperature gradientse) All the above

f) None of the above and something else

Let’s examine these issues…

a) Absence of cold season• Ok, consistent with the common sense that low latitudes “is

where winter never comes”…However, we need to define a limit (say 18oC) (Koeppen, 1936)This limit certainly separates cold from warm regionsDrawback: EXCLUDES TROPICAL HIGHLANDS WHERE

TEMPERATURE REMAIN FREQUENTLY BELOW THIS LIMITSolution: temperature can be reduced to sea level (that is,

transform the actual temperature into a new temperature as if the location was at sea level based on standard equations:

This is very fictitious in many continental areas and is subject to strong errors

Temperature in July

Temperature in January

Annual range of temperature

In general is only one or two degrees near the equator and increases with latitudeExhibits strong influence of continentalityMidlatitudes the annual range exceeds the mean daily range of temperature

Daily range of temperature > annual range of temperature

• This is an important climatic feature of the tropics – sometimes the line where the annual and daily temperature ranges are about equal has been taken as the outer limit of the tropics.

• However, this comparison is only possible over land• Over the oceans, where the air temperature are

almost entirely controlled by the surface water temperature, diurnal ranges are very small.

How about winds?

• Some meteorologists use another boundary of the tropics: the axis of the subtropical high pressure cells, that is: atmospheric circulations dominated by easterlies (in the tropics) and westerlies in mid-latitudes (see the satellite images again to understand what is meant).

Winds and Tropic of Capricorn and Cancer

Other factors: variation of winds

Winds also change with the time of the year

Precipitation and humidity

• Some geographers reserve the term “tropics” for regions where sufficient rainfall is received to carry out most forms of crop agriculture without irrigation (“humid tropics”)

• It is difficult to determine the amount of rainfall necessary to sustain crop agriculture without irrigation, as it depends of other factors such as temperature, wind speed, sunshine and seasonal distribution of rainfall, soil moisture, agricultural methods, etc.

Rainfall also exhibit a large seasonal variability in the tropics

Total Annual Rainfall

d) Weak temperature gradients

In conclusion…

• Precipitation, temperature, humidity and circulation are some of the important factors to identify tropical climates

• Tropical regions do show pronounced seasonal cycles in precipitation and circulation in some regions. Seasonal variations in temperature are less important than daily variations in temperature

• There is not a fixed physical boundary to define tropical regions. • Tropical meteorology is concerned about mechanisms that

explain the easterly winds, monsoons, seasonal variations in humidity and precipitation, hurricanes and typhoons, ENSO and oscillations that propagate in low latitudes.

• These phenomena are interconnected and affect mid and high latitudes as well.

Climate Diagnostic Center (CDC) http://www.cdc.noaa.gov/cgi-bin/data/getpage.plhttp://www.cdc.noaa.gov/cgi-bin/data/composites/printpage.pl

Websites of interest in this class

Moisture in tropical atmospheres

• Moisture plays a critical role for tropical atmospheres. Therefore, it is important to describe spatial variations of moisture and its variability with height

Absolute humidity is the density of water vapor,expressed as the number of grams of water vapor

contained in a cubic meter of air= (g/m3)

Ways of measuring moisture:

Specific humidity expresses the mass of water vapor existing in a given mass of air [g/kg]

The mixing ratio is a measure of the mass of water vapor relative to the mass of the other

gases of the atmosphere. (g/kg)

Relative humidity, RH, relates the ACTUAL amount of water vapor

in the air to the maximum possible at the current temperature.

RH = (specific humidity/saturation specific humidity) X 100%

Saturation for cold air

Saturation for warm air

If the air temperature increases, more water vapor

can exist, and the ratio of the amount of water vapor

in the air relative to saturation decreases.

More water vapor can exist in warm air than in cold air,

so relative humidity depends on both the actual moisture content and the air

temperature.

Evaluation of moisture profiles in tropical regions

There are two variables commonly used for this purpose: equivalent potential temperature θe and the total moist static energy Qs

Next slides will explore how these variables are defined

Understanding the formation of cloudsPressure

Volume expands and the parcel’s temperature decreases at a constant rate 10o/km: dry adiabatic process As it cools the air becomes saturated When that begins: Lifting Condensation Level (clouds are formed)Temperature decreases at a non constant rate: moist adiabatic lapse rate – (release of latent heat warms the atmosphere)

LCL: Cloud base :

Releases Latent heat:

HeatTemp

Γd

γs

The first Law of Thermodynamics

• Is the law that describes the relationships between heat, work and internal energy.

• It establishes the physical and mathematical framework to understand heating processes in our atmosphere, the formation of clouds, the thermodynamical modifications in parcels in movement, etc…

• Internal Energy u: measure of the total kinetic and potential energy of a gas

Kinetic energy: depend on molecular motions -> relationship with temperature

Potential energy: changes in the relative position of the molecules due to internal forces that act between molecules (small changes)

H

Suppose a closed system with one unity of mass

Suppose that this volume receives certain quantity of thermal energy q (joules) by ‘conduction’ and/or radiation. This system may do a certain amount of external work w (also measured in Joules) .

12 uuwq

Differences will cause changes in the internal energy

Where 1 is before and 2 after the change

This is the First Law of

Thermodynamics

In the differential form

dq is the differential increment of heat added to the system, dw is the differential element of work done by the systemdu is the differential increase in internal energy of the system

Changes in du depend only on

the final and initial state: functions of

state

dudwdq (34)

Adiabatic Processes

• If a material undergoes a change in its physical state (e.g., pressure, volume, or temperature)

without any heat being added to it or withdrawn from it, the change is said to be

• ADIABATICdq=0

Definition of Specific Heat at constant pressure Cp

Suppose an expansion in which pressure is kept constant

The material is allowed to expand as heat is added to it and its temperature rises, as pressure remains constant. In this case, a certain amount of heat added to the material will have to be expended to DO WORK as the system expands against constant pressure of its environment We can also define a specific

heat at constant pressure cp

constantpp dT

dqc

Potential Temperature θ

• Is defined as the temperature that the parcel of air would have if it were expanded or compressed adiabatically from its existing pressure and temperature to a standard pressure po (generally taken as 1000hPa)

• This concept is useful for many reasons. One of them is to compare masses of air from different altitudes and from different regions

Definition of Potential temperature

• R≈Rd= 287 J K-1 kg-1 and cp ≈ 1004 J K-1 kg-1

• R/cp ≈ 0.286• Po= 1000 mb (or hPa)• Potential temperature is conserved in dry

adiabatic processes

pcR

o

p

pT

/

Definition of equivalent potential temperature

• Equivalent potential temperature is the temperature of a parcel of air after it is subjected to dry adiabatic expansion until it is saturated, then to moist (or pseudoadiabatic expansion) until all moisture is precipitated out of the volume of air and lastly to adiabatic compression to the initial pressure

Equivalent Potential Temperature

pcR

oaee p

pT

/

• R≈287 J K-1 kg-1 and cp ≈ 1004 J K-1 kg-1

• R/cp ≈ 0.286• Po= 1000 mb (or hPa)• is conserved in moist

adiabatic processes

Tc

LwTT

pae exp

• L = latent heat of condensation

• W=mixing ratio• Cp= specific heat at constant

pressure• T=temperature

Total moist static energy• The moist static energy is a thermodynamic variable that describes

the state of an air parcel, and is similar to the equivalent potential temperature.

• The moist static energy is a combination of a parcel’s kinetic energy due to temperature, potential energy due to its height above the surface, and the latent energy due to water vapor present in the air parcel.

• It is a useful variable because it is conserved during adiabatic ascent and descent

gzqLTcQ vps g = acceleration of gravityz= heightq= specific humidity and L latent heat of evaporation

The importance of moist static energy

• The atmosphere can hold a certain amount of moisture and heat. If the moist static energy (MSE) becomes too high, there should be a mechanism to release the excess of energy. Convection is a good way to release the excess of energy

• For example, we see that in monsoon regions the moist static energy at low levels in the atmosphere increases during the dry season (evaporation and sensible heat increases), reaches the maximum during the pre-monsoon season and decreases during the monsoon season (moisture is converted into precipitation during the monsoon).

Example for the South America Monsoon Region

Monsoon Cycle

Solar and terrestrial radiation and the energy balance in tropical regions

Length of the day during the year

At the equator: 12:07 min (3.5 min for the sun to disappear at sunset and sunrise)In the low latitudes the difference between the shortest and longest day of the year increases by about 7 minutes per degree of latitudes; it is about 71 minutes at 10o and 146 minutes at 20o

(60N)

(45S)(30N)

(20S)

(10N)

Elevation of the sun at noon time

Importance of the sun elevationThe same beam is spread in a larger area: the greater the spreading, the less intense radiation is

Incoming radiation is received at 90o angle (low latitudes)

More obliquely : same radiation distributed to a larger area: less energy/area

Third way in which the tilt of the axis influences heating is in determining the amount of atmosphere that sunlight must

penetrate before reaching surfaceThe greater the thickness of the atmosphere the more the beam is weakened by reflecting back the light, sometimes absorption by particles in the air

• Tropical latitudes, while never receiving the high daily maxima reached near the poles, receive relatively large amounts of insolation throughout the year. When insolation losses in the earth’s atmosphere are considered, latitudinal differences become smaller.

SOLAR RADIATION RECEIVED AT SURFACE

The fate of the solar radiation

• Let’s consider that there is a constant supply of radiation at the top of the atmosphere. Let’s assume that it corresponds to 100 unities

20% + 6% +4%= 30% = Earth Albedo

Radiation available to heat the surface of the planet (direct and diffused)

UV by Ozone (7%) and Near IR by water vapor CO2 (12%) 19%

20%

51%

4%

The fate of the solar radiation

• Aerosols (air pollution, smoke, industrial areas, volcanic eruptions, dust storms) can alter the fractions of backscattered and absorbed radiation locally and globally).

• Remember also that seasonal and regional differences can be very large

(e.g., water surfaces generally absorb solar radiation much more readily than land areas)

Earth-Atmosphere Interactions

surface

Emission IR Radiation Latent Heat (evaporation)

Sensible Heat (ConductionConvection)

AtmosphereEmission IR, Latent H, Sens H

Absorbed directly from the Sun Lost to space – from what was

absorbed

Returns to Earth

Radiation from Earth that is lost to space without interaction with the atmosphere

Sun-Earth-Atmosphere interactions

Surface heats with solar radiance and transfer energy to the atmosphere

Emission IR Radiation Latent Heat (evaporation)

Sensible Heat (ConductionConvection)

Greenhouse gases absorb, heat the atmosphere and emit up and down

Emission IR, Latent H, Sens H

Absorbed directly from the Sun

Lost to space – from what was absorbed in the atmosphere = -64

Returns to Earth-96

Radiation from Earth that is lost to space without interaction with the atmosphere:IR ~11μm

-117

-6

117-6=+111

-23 -7

+19

(19+111+23+7=160)

=+96-117-23- 7 =-51 = net income solar radiation

+96

• The total radiation inputs and outputs from the earth-atmosphere system are referred to as the radiation balance.

• For conditions that radiation inputs to the earth’s surface or atmosphere are greater (less) than the net radiation balance is positive (negative)

• Net radiation = (net shortwave) + (net longwave)= (input SW – output SW) + (input LW – output LW)

For the Earth Surface: (Solar ↓ – albedo↑) + (Atmos ↓– Emitted ↑)(55 – 4) +(96-117) = 30For the Atmosphere(Solar absorbed↓) + (Surface ↓– Emitted ↑)(19) +(111- 96 -64) = -30Same value but opposite sign!!!HOWEVER, THIS SITUATION DESCRIBED IS FOR THE GLOBE AND FIGURES

ARE GLOBAL AVERAGES.

The Radiation Balance

Energy Balance• A mechanism is certainly

required to transport the surplus of energy at the earth’s surface into the atmosphere where a deficit exist.

• Convection transports sensible and latent heat away from the earth surface (7% and 23% ) respectively

• (=30% ENERGY BALANCE)

Surface Heating

Radiation Balance: Regional and seasonal differences occur:

DJF

JJA

Components of the Energy Balance• The balance of net radiation surplus at the earth’s surface and

the upward fluxes of sensible and latent heat are then referred to as the energy balance

• The main components of the energy balance are the fluxes of net radiation (Q), sensible heat (H), latent heat (E)and subsurface heat (G).

• Q= (H + E + G) = 7 + 23 + 0 = 30• G is the subsurface heat flux that is zero on an annual basis as

flow into and out of the ground are approximately zero• Energy balances are important concept for interpreting climates

at any temporal or spatial scale : Any net radiation surplus must be ‘consumed’ by either sensible heat of the atmosphere, by evaporation (latent heat) or heating the subsurface.

surface

Net Radiation

Latent Heat (evaporation)

Sensible Heat (ConductionConvection)

QE

H- +

-+

Bowen Ratio = sensible heat (H) / Latent heat (E)

>1 Sensible heat dominates (dry environment)

<1 Domination of Evaporation (wet environment)

Tropical Oceans = 0.1; wet tropical forest = 0.1-0.3 ; semi-arid desert 2-6

Desert > 10Tropical urban surfaces, 1.5-5

Quiz for fun: Find what figure represents: dry grass land;Wet dry monsoon climate; tropical deserts; humid equatorial