Lab: 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 between the surface and the atmosphere: 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 exchange of energy between the atmosphere and the surface is the net of all of the different fluxes:

Fnet = SW - LW - SHF - LHF


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 components of the surface energy budget will vary according to season and location. To try to understand the contribution of each component to the total surface heat flux, 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.

Similarly, we can consider the water balance of the atmosphere. We will examine maps of sources and sinks of water vapor in the atmosphere and calculate their sum to find the net water balance for different locations.

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 it will evolve in time given the initial conditions. 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 based on the measurements. 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.

These data are monthly means of the re-analyses for the period 1948-1998. Coverage is global, with a horizontal resolution of 2.5 degrees longitude by 2.5 degrees latitude. Go to the NCEP/NCAR Reanalysis Project Overview web site to learn more about the reanalysis project.

III. Lab Instructions

A. Radiative heat flux (SW & LW)

Task 1: Consider the solar energy absorbed by the surface (land or ocean) of planet Earth as given by the climatology of surface absorbed solar radiation. As it passes through the atmosphere, some solar radiation is reflected or absorbed by clouds, aerosol particles, or gas molecules. Of the remainder that is incident at the surface, some is reflected and the rest absorbed by the surface. The climatology tells us how much solar energy is being absorbed by each square meter of surface area on the planet during each second of time, averaged over each calendar month. View the data for January and July. Draw coasts to help you differentiate land and ocean. Answer the following questions:

Task 2: 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 data set for net outgoing longwave radiation (up minus down). This is radiation going up minus radiation coming down. View the field for January and July, and answer the following questions.

B. Non-radiative heat flux (SHF & LHF

Task 3: 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 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:

Task 4: 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. However, if the overlying air is already humid, evaporation will be decreased. Examine the pattern of LHF in January and July. Describe and explain the following:

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

Task 5: 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. Link to the net downward surface heat flux. Make an animation of the seasonal cycle by typing "Jan to Dec" in the time window and clicking "Redraw." Answer the following questions:

Task 6: Go back to the data page to which you originally linked, activate the Expert Mode, calculate the annual mean net surface heat flux (by adding "[T] average" to the dialog), and view the result. You may want to redefine the limiting values on the color bar to a narrower range such as -100 to 100 to see the features better. From what locations and to what locations does the ocean need to transport heat to balance the surface inequalities of heat input?

D. Water balance in the atmosphere

Task 7: The source of water vapor for the atmosphere is surface evaporation and evapotranspiration (E). The sink for atmospheric water is precipitation (P). For the humidity to remain constant, averaged over the globe it must be true that P = E. But this need not be true at every individual location. If P > E somewhere, then that excess water was transported from somewhere else. Use the viewer to display the monthly climatological precipitation rate maps for the world. The units of these maps are kilograms of condensed water per second per square meter (i.e., the flux of water) reaching the Earth's surface as rain, hail, or snow, averaged separately for each calendar month. Because these are rates per second, and at most times it is not raining, the values are very small. You can obtain the daily average rainfall amounts by returning to the page from which you originally accessed the data and clicking on "Filters." At the lower end of the window find the small box where you can convert from "kg/m2/s" to "kg/m2/day." Describe the following aspects of the January rainfall distribution:

Task 8: We can calculate the evaporation rate from the latent heat flux data by dividing the latter by the latent heat of vaporization L = 2.5 x 106 Joules/Kg. Link again to the latent heat flux climatology. In the Expert Mode window type "2.5E06 div 86400 mul", which divides the LHF by L and converts the units from Kg/m2/s to Kg/m2/day (1 day = 86400 s). Then redefine the units by typing on a new line in the Expert Mode window "/units (kg/m2/day) def". View the result for January and July and answer the following questions:

Task 9: Just as we did for net radiation at the top-of-the-atmosphere last week, we can define the atmosphere's net water balance by taking the difference between evaporation and precipitation. Now view the annual average water balance. Answer the following questions:

IV. Data

V. Lab Report Instructions

Write a lab report (as per the Lab Report Format ) summarizing the findings of your investigation. Use the questions posed in the lab instructions as a guide for a scientific text that describes the data fields you are observing and the connection between them and the material you studied in class. Incorporate your answers to the following questions into your lab report:

Updated June 20, 2007
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