| METR 104: Our Dynamic Weather (w/Lab) |
Lab Exercise #2: Solar Radiation & Temperature Part VI: An Even More Complex Computer Model |
Dr. Dave Dempsey Dept. of Geosciences SFSU, Spring 2012 |
(5 points)
(Lab Section 1: Wed., April 4; Lab Section 2: Fri., April 6)
Learning Objectives. After completing this activity, you should be able to:
Prior Knowledge Required:
I. Introduction. This lab activity continues our development and testing of a computer model of the daily temperature cycle at the earth's surface introduced in Lab #2, Part IV and Lab #2, Part V. We're doing this to try to develop a sense of how we might use (a) experience with observations and (b) basic physical principles, to understand and forecast surface temperature over the course of one to several days.
In this lab you'll configure a more sophisticated version of the computer model, run experiments with it, and evaluate it based on your experience reading surface weather observations displayed on meteograms.
II. An Even More Sophisticated Computer Model of the Daily Temperature Cycle
In Lab #2, Part V, you experimented with a computer (STELLA) model of the daily temperature cycle that included one mechanism by which the earth's surface could gain heat (absorption of solar radiation) and one by which the surface can lose heat (emission of longwave infrared (LWIR) radiation). You could modify the model's behavior by specifying the degree of cloudiness and by changing the nature of the surface from land to water.
That model produced a daily temperature pattern that was realistic in some ways but not in others, compared to actual observations of the daily temperature cycle under similar conditions. In particular, it produced a temperature maximum in the afternoon and cooling thereafter until near (just after) sunrise, as we commonly see in observations of the real atmosphere. The daily maximum temperature, although a little too high, wasn't too bad, but the minimum temperatures were much colder than we'd expect to see, and so the daily temperature range (the difference between the minimum and maximum temperature) was too large. We conclude that, although that model did much better than its predecessor did in Lab #2, Part IV, it must not yet be complete—there is likely one or more physical mechanisms affecting surface temperature that the model doesn't account for yet. What might it (or they) be?
We know that greenhouse gases and clouds absorb longwave infrared radiation emitted by the earth's surface. We also know that greenhouse gases and clouds emit longwave infrared radiation of their own, that they emit part of that radiation downward, and that the surface absorbs it. Hence, we'll include in the model this additional source of heat for the surface.
We also know that when two objects at different temperatures are in direct contact, heat will "flow" from the warmer one to the cooler one (so the warmer one cools off and the cooler one warms up). This is the process of conduction of heat. In particular, when air in contact with the earth's surface is warmer or colder than the surface, heat will conduct from one to the other, and the surface will gain or lose heat. We'll try to represent this process in the model.
Finally, we know that when water evaporates, heat in the water transforms into latent heat in the water vapor, reducing the amount of heat in the remaining water. (We experience this directly when we overheat, produce sweat, and feel cooler when the sweat evaporates from our skin.) We'll try to represent evaporative cooling in the model (especially from the oceans, less so from land).
With these three new physical process added, the Law of Conservation of Energy applied to the earth's surface and written in a form that describes how the surface gains and loses heat and how its temperature responds as a result, can be written like this:
|
The rate at which the earth's surface temperature changes |
∝ (is proportional to) |
The rate at which the heat content of a layer of the earth's surface changes |
||||
| = | The rate at which the surface absorbs solar radiation |
+ | The rate at which the surface absorbs longwave infrared radiation emitted downward by greenhouse gases and clouds |
− | The rate at which the surface emits longwave infrared radiation |
|
| ± | The rate at which heat conducts between the surface and the atmosphere |
− | The rate at which the surface loses heat due to evaporation of water, converting heat into latent heat |
Using this relationship, we can calculate how fast the surface temperature changes and estimate temperature in the near future (at least, if the model is complete and accurate).
We can run this more sophisticated model, compare the results to observed daily temperature cycles, and see how well the model performs. That is, we can evaluate, or validate, the model. If the model does well, it could be useful for helping us to understand better how the atmosphere works and for making temperature forecasts. (We'd need further experience and validation of the model to be sure.) If the model doesn't do well, we have to question any assumptions that underlie the physical relationship as we've applied it above, or perhaps take into account physical processes that are important but that we've neglected.
III. Instructions. Respond in writing to questions posed in sections D and E below. Turn in your responsesat the end of lab, along with the plot that you print in Section D.