Geog 288 : Topics in Tropical Climate

1. Geog 288 : Topics in Tropical Climate • Objective : provide graduate students an overview of tropical climates, their variations and changes over time and challenges in modeling • Syllabus: 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: TBA • Prof. Leila M. V. Carvalho • http://clivac.eri.ucsb.edu/

2. 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

3. 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

4. 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.

5. 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.

6. 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

7. 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)

8. Tropic of Capricorn and Cancer

9. Climatic “Boundary” of the tropics: • What is the major common feature in the tropics? • Absence of a cold season • Annual range of temperature • Atmospheric circulations dominated by easterlies in the tropics and westerlies in midlatitudes • Weak temperature gradients e) All the above f) None of the above and something else Let’s examine these issues…

10. 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 regions Drawback: EXCLUDES TROPICAL HIGHLANDS WHERE TEMPERATURE REMAIN FREQUENTLY BELOW THIS LIMIT Solution: 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

11. Temperature in July

12. Temperature in January

13. Annual range of temperature • In general is only one or two degrees near the equator and increases with latitude • Exhibits strong influence of continentality • Midlatitudes the annual range exceeds the mean daily range of temperature

14. 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.

15. 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

16. Other factors: variation of winds Winds also change with the time of the year

17. 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.

18. Rainfall also exhibit a large seasonal variability in the tropics

19. Total Annual Rainfall

20. d) Weak temperature gradients

21. 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.

22. Websites of interest in this class Climate Diagnostic Center (CDC) http://www.cdc.noaa.gov/cgi-bin/data/getpage.pl http://www.cdc.noaa.gov/cgi-bin/data/composites/printpage.pl

23. 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

24. Ways of measuring moisture: 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)

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

26. 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)

27. 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 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. 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.

28. Evaluation of moisture profiles in tropical regions

29. There are two variables commonly used for this purpose: equivalent potential temperature θe and the total moist static energy QsNext slides will explore how these variables are defined

30. Understanding the formation of clouds Pressure Releases Latent heat: γs • 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) Γd LCL: Cloud base : Temp Heat

31. 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…

32. Internal Energy u: measure of the total kinetic and potential energy of a gas H 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)

33. Suppose a closed system with one unity of mass • Suppose that this volume receives certain quantity of thermal energyq (joules) by ‘conduction’ and/or radiation. • This system may do a certain amount of external work w(also measured in Joules) . Differences will cause changes in the internal energy Where 1 is before and 2 after the change

34. In the differential form (34) • dq is the differential increment of heat added to the system, • dwis the differential element of work done by the system • du is the differential increase in internal energy of the system This is the First Law of Thermodynamics Changes in du depend only on the final and initial state: functions of state

35. 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 • ADIABATIC dq=0

36. Definition of Specific Heat at constant pressure Cp

37. 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

38. 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

39. 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

40. 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

41. Equivalent Potential Temperature • L = latent heat of condensation • W=mixing ratio • Cp= specific heat at constant pressure • T=temperature • 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

42. 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 g = acceleration of gravity z= height q= specific humidity and L latent heat of evaporation

43. 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).

44. Example for the South America Monsoon Region Monsoon Cycle

45. Solar and terrestrial radiation and the energy balance in tropical regions

46. Length of the day during the year (60N) (45S) (30N) (20S) (10N) • 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

47. Elevation of the sun at noon time

48. Importance of the sun elevation The 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

49. Third way in which the tilt of the axis influences heating is in determining the amount of atmosphere that sunlight must penetrate before reaching surface The greater the thickness of the atmosphere the more the beam is weakened by reflecting back the light, sometimes absorption by particles in the air

50. 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