Meteorology 3000- Mountain Weather and Climate

Assignment 4. Due September 27.

The goal of this assignment is to compute components of the surface energy balance. You will work in teams of 4-5 people to collect observations using several different sensors. Tables and text must be emailed or typed and turned in to me by the beginning of class next Friday.

Stefan-Boltzmann Law: IR = emissivity x constant x TxTxTxT. The constant is 5.67x10(-8) W/(m2 K4).

Data Collection

1. Because of the slow time response of the dial thermometers, immediately decide upon two adjacent locations in the vicinity of the INSCC building: one in direct sunlight and one in shade. One could be on a slope. Insert one dial thermometer into the soil at each location so that the dial is about an inch above the ground. Cover the thermometer so that it is not exposed to direct solar radiation.

2. Each person has a Kestrel hand held instrument to measure temperature, wind speed and dewpoint. Record measurements of these quantities from at least two sensors at regular intervals at your two locations from two different heights (1) near surface and (2) roughly 2 m. Record measurements of soil temperature at the same time.

3. Using the hand held IR gun and/or the IR sensor in the box, record the temperature of the surface at the same time that you are taking other measurements. Since you have to share the IR sensors, you will not have as many simultaneous observations of IR temperature.

4. As time permits, use the IR sensors to measure the surface temperature of other objects: trees, people, concrete, pavement, cars, buildings, etc. Be sure to record the location of the object relative to the solar zenith angle.

Analysis

5. Summarize the characteristics of your 2 sites: soil type, solar zentih angle, aspect, slope, etc.

6. It is very important to organize the data collected in a coherent table using metric units (all temperature observations should be first in C then in K in a separate column, wind in m/s). You may focus on data collected at 1 or 2 times, or average all of your data to estimate the mean conditions observed. Each of the following should be displayed in separate columns.

7. Using the Internet, text or other resources, estimate the incoming solar radiation (w/m2) at the top of the atmosphere at noon for the latitude of Salt Lake City.

8. Estimate from the figure in the text the albedo of your surface (take the midpoint value for the relevant soil type). Estimate the absorbed solar radiation (w/m2) based on the albedo and results from 7. This should be a positive number.

9. From direct measurements from the IR sensor in the box, estimate the temperature of the atmosphere radiating to the earth's surface. Use the temperature from this morning's SLC sounding to estimate the pressure from which the atmosphere is radiating back to the earth's surface. Using the Stefan Boltzmann Law and assuming the atmosphere is a blackbody (emissivity equal to 1), how much energy is being received at the earth's surface from the atmosphere?

10. Estimate the emissivity of the surface from the figure in the text (take the midpoint value for the relevant soil type). Estimate the infrared radiation (w/m2) emitted upwards from the surface from your measurements of the surface temperature.

11. Estimate the net infrared radiation lost by the surface (w/m2). This should be a negative number. What effects are we ignoring here? Hint: look around your site.

12. Estimate the net solar and terrestrial radiation (w/m2). Is the surface gaining energy or losing energy at this time?

13. Estimate the sensible heat flux using the following "bulk" formula: H (w/m2) = 1.73 x wind speed (m/s) x (surface temperature minus air temperature) (C). Is the surface losing or gaining energy to the atmosphere?

14. Estimate the latent heat flux using the following "bulk" formula: L (w/m2) = 3.61 x wind speed (m/s) x (surface vapor pressure minus air vapor pressure) (mb). Use the table of vapor pressure as a function of dew point to determine the air vapor pressure. Is dew observed at the surface? If not, assume that the surface vapor pressure is a few mb higher than the air vapor pressure. Is the surface losing or gaining energy to the atmosphere by evaporation/condensation?

15. Compute the difference, surface temperature minus soil temperature (C/K). Is energy being stored in the soil or is heat being conducted from the soil to the surface? How does this vary between the shady and exposed locations?

16. Compute the residual heat storage (w/m2) into the soil by summing the net radiation, sensible heat flux, and latent heat flux. Make sure each term has the right sign. Does your result agree with the results from 13?

Summary

17. To what extent do your results agree with the summary figures in the text on the surface eenergy balance? How would you explain any differences?

18. How would your results be different if the data were collected at night?

19. In a separate table, for each observation of the surface temperature of an object, estimate the emissivity of that object from the figure in the text. The IR gun assumes a constant emissivity of .95. So, if the actual emissivity is substantially different from .95, the temperature reading is in error. Using the Stefan Boltzmann Law, correct the surface temperature observations using the appropriate emissivity and estimate the "true" temperature of the object.