Students will learn the following:

- Understand how the Earth finds an energy balance.

- Explore how each of the four ways energy is exchanged at the Earth's surface.

- Examine the net downward surface heat flux.

The format suggested for this lesson is a data lab. Since it requires no laboratory equipment, the class size can range from a small student seminar to a medium sized lecture hall. The only mitigating factor related to class size is the necessity for each student (or perhaps pairs if the instructor elects to make the lab report a paired activity) to have a computer terminal or laptop. The class does not need to have a SmartBoard or LCD projector, since the lab work will be conducted at individual computers, but access to multimedia equipment is preferred. The hands-on portion of the lab requires adequate space. See "Description and Teaching Materials" below for link to the source lab.

The structure and primary components of this lab lesson is sourced from Columbia University's Earth Environmental Systems Climate (EESC) course, Lab #3: Surface Energy and Water Balance.

I. Introduction

Until now, we have only considered the energy balance at the top of the atmosphere. Now it is time to examine the energy balance at the surface of the Earth. There are four ways in which energy is exchanged at the Earth's surface: shortwave radiative flux, longwave radiative flux, sensible heat flux, and latent heat flux. In this lab, we will examine all of these components of the surface energy budget. Since the latent heat flux, caused by evaporation, plays such an important role in the surface energy budget, we will also consider the surface water budget.

The total rate of energy exchange between the atmosphere and the Earth surface is the net of all surface fluxes:

Fnet = SW - LW - SHF - LHF

Where:

Fnet is the net downward heat flux into the surface (positive downward)

SW is the portion of incoming solar radiation absorbed by the surface (positive downward)

LW is the net outgoing longwave radiation (positive upward)

SHF is the sensible heat flux, or heat transferred from the surface to the atmosphere by turbulent motion and dry convection (positive upward)

LHF is the latent heat flux, or heat extracted from the surface by evaporation (positive upward)

The relative importance of these various parts of the surface energy budget will vary according to season and location. To understand the contribution of each component to the total surface energy balance, we will first look at how each individual component varies in time and space. Then we will put it all together and examine the net downward surface heat flux. Note that because LW, SHF, and LHF are generally positive, they cool the surface, which is warmed primarily by solar radiation.

II. Metadata

In this portion of the lab, you will use data from the National Centers for Environmental Prediction (NCEP) Reanalysis project. NCEP generates these datasets using a type of atmospheric model known as a data assimilation model. In general, models predict the future behavior of the atmosphere using information about its current state (i.e., its initial conditions) and solving systems of equations that describe how the atmosphere will evolve in time. Data assimilation models work by giving the model updated measurements as it runs, so that the output is a combination of measured values where they are available, and modeled values where they are not. These datasets are often used by climate researchers because they provide the sort of continuous, global coverage produced by models, but also contain all of the information available from observations.

The data you will be using are monthly means of the NCEP reanalyses for the period 1948-1998. Coverage is global, with a horizontal resolution of 2.5° longitude by 2.5° latitude. Go to the NCEP/NCAR Reanalysis Project web site to learn more about the reanalysis project.

II. Lab Instructions

A. Radiative heat flux (SW & LW)

Consider the solar energy absorbed by the surface (land or ocean) of the Earth as given by the climatology of surface absorbed solar radiation. Passing through the atmosphere, some solar radiation is reflected or absorbed by clouds, aerosol through the atmosphere, or gas molecules. Of the remainder, some is reflected and the rest is absorbed by the surface. View the data for January and July by clicking the averaged over each calendar month. View the data for January and July by clicking the 5^th box in the "Views" section. This view should have coasts that help you differentiate land and ocean. Consider the following questions:

· What is the range of values of surface absorbed solar flux?

· At which latitudes is the most sunlight absorbed in January and where is the least sunlight absorbed? Is more absorbed in the Northern or Southern Hemisphere? How is the situation different in July?

· Why does surface absorbed shortwave radiation vary by latitude over the oceans? Over land?

· Do you expect the magnitude of surface absorbed shortwave to be larger than, smaller than, or equal to the shortwave incident at the top-of-the-atmosphere? Why?

Task 1: Warmed by the sun, the planet's surface emits longwave radiation upward. The atmosphere absorbs much of it and re-emits its own longwave radiation back down. The amount that escapes is generally larger than the downward one. Open up the dataset for net outgoing longwave radiation, which is the upward radiation minus the downward radiation. View the fields for January and July (and others if you like), and answer the following questions: (answers should apply to all months)

· How does the range of values of net surface longwave flux compare to the range of surface absorbed shortwave flux (one sentence)?

· Where do the maxima (>80 W/m²) in the net longwave flux occur? Where are the minima (<30 W/m²) (two sentences)?

· What difference between max and min regions accounts for the difference (two sentences)?

**To answer this question, think about the factors that control the up and down components separately. Consider also the temperature and moisture conditions (warm/cold, dry/humid) that would cause the biggest and smallest difference between upward flux from the surface and downward flux from the atmosphere.

B. Non-radiative heat flux (SHF & LHF)

Task 2: Link to the sensible heat flux climatology. SHF is heat extracted from the surface by turbulent air motion and dry convection. The amount of SHF depends mainly on the temperature difference between surface and the overlying air. The actual temperatures of the surface (land or ocean) and the overlying air don't matter; it's the gradient between them that controls the magnitude and direction of the heat flux. Describe and explain the following (look at least at Jan and Jul):

· The differences between the SHF from land and from ocean (i.e., explain why surface- atmosphere interactions should behave differently over land and ocean and how that gives rise to what you observe).

· The outstanding features in the pattern of sensible heat flux over the oceans (one sentence). Choose "draw land" and reset the range of values to -50 to 200 W/m2. Pay particular attention to the ocean areas just east of the eastern coasts of North America and Asia and the changes that occur there from January to July.

· The pattern of fluxes in the equatorial Pacific Ocean. To see this more clearly, limit the map boundaries to 30°S - 30°N and 120°E - 60°W, use the map option that masks out land, and set the range of colors and contours to 0 - 60 W/m2.

Task 3: Link to the latent heat flux climatology. LHF is heat transferred by evaporation of water from the surface. Water requires a great deal of energy to change phase from liquid to gas. When a molecule of water evaporates, it gets the necessary energy from its surrounding surface, which then lowers the temperature of the surface. The temperature of the newly evaporated vapor molecule doesn't change, since all of the energy goes toward breaking free from the liquid phase. Wet surfaces (ocean, vegetated land, moist soil) can potentially evaporate large quantities of moisture. If the overlying air is already humid, however, evaporation will be decreased. Examine the pattern of LHF in January and July.

· Note the Jan/July differences over land and ocean. Explain these differences in terms of the Clausius-Clapeyron relation (one sentence).

The relative magnitude of LHF and SHF over the tropical oceans (which is bigger and why?). You may want to mask out land for this comparison (one sentence).

C. Earth's surface energy budget (SW, LW, SHF, & LHF)

Task 4: The difference between absorbed solar and the sum of the three other components you have looked at is the total rate of exchange of energy between the atmosphere and the surface. Look at the map for annual mean net surface heat flux given to you and answer the following questions in short answer:

· Where does the ocean gain heat from the atmosphere? What difference in this region is responsible (which heat flux component is different from others at the same latitude?)?

· Where does the ocean lose heat to the atmosphere, and which component is most responsible?

· What do the relative magnitudes of net surface heat flux into the ocean and land tell you about their relative ability to store heat?

· From what locations and to what locations does the ocean need to transport heat to balance the surface inequalities of heat input?

Final Questions:

· Based on your observations what are the two largest components of surface heat flux over the ocean? Over land? What is the reason for the difference?

Suppose that the Earth's climate continues to warm and the relative humidity of the atmosphere remains approximately the same as it is now (i.e., the effects of added water vapor and warmer temperatures offset each other). What changes do you expect at the surface in SW, SH, LH, incoming, outgoing and net LW? Your answer should take less than five sentences.

Below are the links for source material and resources:

· EESC course page (accessible to Columbia University faculty and students only: <a>https://courseworks.columbia.edu/cms/</a>; publicly accessible version will have 2011 EESC lab materials (updates forthcoming) http://eesc.columbia.edu/courses/ees/climate/syllabus.html

Handouts and Directions:

· Lab instructions (data)

Background Information for instructors/TAs:

Instructors/TAs may find it useful to refer to lecture notes from EESC 2010 course (<a>https://courseworks.columbia.edu/cms/</a>):

· Atmospheric Forces, Balances, and Winds

· Moisture & Clouds

Equipment/Supplies:

· No equipment necessary

Data Lab

· Computer lab or moveable laptops with Internet access and Excel.

· LCD projector

· Handouts - lab instructions

· "Writing a Lab Report" (may have already been disseminated)

Lab

Lab instructions

Surface Energy and Water Balance (LAB)
The Word version of the modular unit
7_CM_EarthsRadiationBudget.doc

Instructors/TAs may find it useful to refer to lecture notes from EESC 2010 course (https://courseworks.columbia.edu/cms/); updates to the publicly accessible site forthcoming (http://eesc.columbia.edu/courses/ees/climate/syllabus.html): · Atmospheric Forces, Balances, and Winds · Moisture & Clouds

Students summarize their findings in a lab report.

Instructors/TAs may find it useful to refer to lecture notes from EESC 2010 course (https://courseworks.columbia.edu/cms/); updates to the publicly accessible site forthcoming (http://eesc.columbia.edu/courses/ees/climate/syllabus.html): · Atmospheric Forces, Balances, and Winds · Moisture & Clouds · Surface absorbed solar radiation. · net outgoing surface longwave radiation. · sensible heat flux climatology · latent heat flux climatology. Please see Columbia University's IRI/LDEO Climate Data Library for data sets associated with this lab (http://iridl.ldeo.columbia.edu/)