Lectures - Monday and Wednesday, 11:00 AM - 12:15 PM
Lab - Tuesday, 4:10 PM -7 PM
The average of many individual weather events over time periods of a month or more defines the statistical characteristics of the atmosphere; this is referred to as climate.
For example, some summer days are warm and dry, others are oppressively hot and humid; this is a weather statement. But summer in New York is always hotter than winter; this is a statement about New York's climate .
In this lab we are going to examine the monthly mean vertical and latitudinal structure of some of the basic variables that describe the atmosphere's climate and general circulation: temperature, humidity, zonal wind speed, and meridional wind speed. The term "zonal" refers to the west-east direction, and the term "meridional" refers to the south-north direction. Recall that any two dimensional vector can be resolved into two components that are perpendicular to one another. Zonal and meridional winds are the zonal and meridional components of the observed wind. All of these are presented as average values over all longitudes (called a "zonal mean" since it is an average in the west-east, i.e. "zonal" direction) and over fifteen-year periods for each of the months of the year. The results can be thought of as representing typical cross sections of the atmosphere at one latitude.
A note about pressure coordinates: During our lab this week we will continually refer to the variation of meteorological field with pressure or on pressure surfaces. More specifically, we use pressure instead of height. This is common practice in meteorology. As we have seen when we examined the hydrostatic balance, pressure decreases with height. The decrease is not "linear" (that is the pressure change is not proportional to the height change) but "exponential". Moreover, it also depends on the density (or temperature) of the air: the denser (or colder) the air, the faster is the change of pressure with height. This is precisely why it is convenient for meteorologists to use pressure as a vertical coordinate rather than use height. When we do so, we do not need to continue and use density in our equations. The difference between two pressure surfaces is directly related to the mass of air between them and this is important when we look at fluid motion and need to consider balances of forces, such as the geostrophic balance. When we use pressure surfaces, the isobars are replaces by lines of constant height (in other words, we measure the changes in the height of the pressure surface at different points) and it is the height gradient that is driving the motion on a pressure surface. On a pressure surface air will initially flow from the regions where height is large (equivalent to high pressure) to where it's low (equivalent to low pressure). Coriolis force will deflect the flow such that it is parallel to the height contours moving in a clockwise (anti-clockwise) around low height centers in the Northern (Southern) Hemisphere and opposite around high height centers, just as with pressure on a height surface. Standard pressure surfaces in meteorology are 1000, 850, 700, 500, 300, and 200 millibars. In midlatitudes this is roughly equivalent to 100, 1500, 3000, 5000, 9000, and 12000 meters above the surface.
The purposes of this lab are to:
The data that you will view were compiled from all available radiosonde (weather balloon) profiles, ship and weather station measurements made over the fifteen year period 1958-1973. The horizontal coverage is global, with 2.5 degree resolution. Vertical measurements extend from the surface (1000mb) to the lower stratosphere (50mb), with measurements every 50mb. More information about this data set can be obtained from the OORT dataset documentation.
The OORT data set is different from the NCEP/NCAR Reanalysis set that we used in last week's lab. In the latter, a global climate model was used to assimilate the data while here we are looking at the actual observations. This direct averaging is faithful to the data but has some drawbacks. The data set contains numbers at all pressure levels and latitude bands because at least some observations have been made at all latitudes over this long a period of time. But the time average hides some important differences between one part of the world and another. In northern midlatitudes, where much of the latitude circle is covered by land and many of the nations have long been industrialized, the radiosonde network is dense and data are plentiful. But near the equator and in the Southern Hemisphere, where ocean covers a greater part of most latitude circles and many nations are just now developing, the radiosonde network is sparse and the averages are less reliable indicators of the true mean state of the atmosphere. In fact, the "Roaring 40's" of the Southern Hemisphere are almost entirely ocean, almost perpetually stormy, and are therefore rarely visited by humans. This part of the world is virtually unobserved by radiosondes. Thus, some parts of the climatology are more reliable than others. This sampling problem is the stimulus for the development of Earth-orbiting satellites that can observe every location on the planet and give us a true global picture.
Task1: Use the viewer to examine the zonal mean January temperature. Which is colder in January: the North Pole or the South Pole? Why? Looking at all pressure levels, where is the coldest spot located? Now make a time-latitude plot to display the seasonal cycle of temperature as a function of latitude. Select "Time" from the horizontal axis pop-up menu, and select "Latitude" from the vertical axis pop-up menu; click "Redraw".
Answer the following questions:
Transfer the January temperature and January humidity tables to Excel. Open the January temperature Excel table. Each column of data corresponds to a different latitude band, in 2.5 degree latitude increments, and each row gives the temperature (in degrees Celsius) at a different pressure level. The rows are arranged in order of decreasing pressure (increasing altitude) because that is the sequence in which data are recorded by a rising weather balloon; thus the first row of data at the top of the table corresponds to the Earth's surface. We are used to having the surface at the bottom, so sort the data according to pressure in ascending order.
Task 2: Make charts (line graphs) of temperature vs. latitude at the 200 mb, 500 mb, and 1000 mb pressure levels. When using Excel, always select the "Scatter" plot option rather than the "Line" plot option, even when you are asked to make a line graph. You can easily connect the dots on a scatter plot to make it a line graph. Notice that the latitude values in row 1 contain both letters and numbers (i.e., T(-90)). Excel does not recognize these as numerical values, so you will need to add a row containing only the latitude values. Do this by inserting a blank row above the temperature data and entering "-90" in the cell corresponding to the temperature measurements at -90. Then click on the next cell in the row, type the appropriate formula in the formula bar, and then use autofill to paste the formula into the remaining cells in the row. To make all 3 graphs at the same time on one chart, select the row you created which contains the latitude numbers for the x-axis, and then, while holding down the Control key, select each of the rows for the three pressure levels, before clicking on the Chart Wizard. Based on your graph, answer the following questions:
Task 3: Make new line graphs showing temperature vs. pressure at three latitudes: One near the equator, a second in northern midlatitudes, and a third at the North Pole. (As in the previous step, put all three graphs on a single chart to make them easy to compare; but this time you have to select columns rather than rows.) On these graphs, altitude increases toward the left, i.e., Earth's surface is on the right and the lower stratosphere is on the left. Locate the tropopause (defined as the pressure level of minimum temperature) near the equator and in the midlatitudes. Focus now on the vertical temperature structure near the surface. At which latitude is near-surface air warmest relative to air at slightly higher altitudes? What part of the world should therefore be most prone to thunderstorms? Do you notice anything qualitatively different about the near-surface vertical temperature structure at the North Pole compared to the other two latitudes?
Task 4: Now make a chart in which you plot temperature vs. pressure at two locations (again on the same chart for ease of comparison): One at a Northern Hemisphere midlatitude location, and the other at the identical latitude but in the Southern Hemisphere. Comparing the vertical temperature structure in the two hemispheres, in which season are thunderstorms most likely to occur?
Task 5: Select and copy the equatorial temperature profile. Now open the January humidity Excel table; this shows water vapor mixing ratios (in grams of vapor per kilogram of dry air) in the same format as for the temperature table. Sort the data into ascending order as you did for temperature. Paste the equatorial temperature profile into the humidity table and make a scatter plot of temperature vs. humidity at the equator. Be sure that both the temperature and humidity profiles are sorted in ascending order; both variables should decrease with height.
Answer the following questions:
Task 6: Return to the viewer and display the January Zonal wind field. At what pressure level and latitude do the maximum mean zonal winds (the jet streams) occur? Does the latitude of the jet stream correspond more closely to that of the maximum or minimum temperature, or the latitude of maximum or minimum latitudinal temperature gradient? Consider the tropics (equator to 30 degrees latitude). What is the direction of the zonal wind near the surface? Zonal winds are positive if they move west-to-east (also called "westerlies" because the wind is coming from the west) and negative if they move east-to-west ("easterlies"). Similarly, meridional winds are positive for south-to-north motion and negative when moving north-to-south.
Task 7: To make the zonal wind direction easier to see, display the data as a black-and-white contour plot; click and hold on the pop-up menu currently displaying "colors"; select "contours"; click "Redraw". The thick zero contour separates positive (eastward) zonal winds on one side of the contour line from negative (westward) zonal winds on the other side. Poleward of about 10 degrees latitude, what is the direction of the zonal wind near the tropical tropopause? The zonal wind in the tropics is produced primarily by the Coriolis force acting on air parcels moving toward higher or lower latitudes. Remember that the Coriolis force deflects parcels to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. For air moving poleward, in what direction does the Coriolis force deflect it? What about for air moving equatorward? Based on this, infer the meridional wind direction in the upper and lower tropical troposphere from the observed zonal wind direction at these altitudes. In other words, in what direction would the wind have been moving in order for the Coriolis force to have produced the observed zonal winds? Try using a globe to help you visualize the movement of the winds.
Task 8: Now return to "colors," choose "Time" and "Latitude" for a time-latitude plot, and click "Redraw." Now type 200 (ie. 200mb) in the pressure selection box and click "Redraw" again. This will display the seasonal cycle of the zonal wind at the tropopause in color. In what season do the strongest jet stream winds occur? Given that weather systems are carried by the jet stream, do you expect that weather changes faster in general in winter or summer? Is the strength of the jet stream correlated primarily with maximum/minimum temperature or temperature gradient?
Task 9: Use the viewer to display the January Meridional wind field. Are mean meridional (south-north) winds weaker, stronger, or about the same strength as mean zonal (west-east) winds? Compare the meridional and zonal wind speed at 20N at a pressure level of 200 mb. Which component is larger?
Task 10: The height gradient at the 200 mb level is largely oriented in the meridional (south-north) direction. You can convince yourself by looking at a map which shows the height of the 200 mb pressure surface in geopotential meter. When the contours are close together that means a large pressure gradient. Remember that the gradient is at a right angle to the contours and thus most everywhere in meridional. Which two places depart mostly from the meridional direction?
Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation. Incorporate your answers to the following questions into your lab report:
July 9, 2007
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