CEE 1331 SMU Hmw#3 Atmospheric Soundings and Severe Weather paper

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Question Description

In this homework we will analyze the temperature and moisture profile of the atmosphere and how it

relates to the intensity of thunderstorms. An atmospheric profile is simply a way of displaying how any

weather parameter (such as temperature and moisture) changes with height.

We will be plotting the weather data on a Stüve diagram. A Stüve diagram is a type of graph that has

height on the vertical axis and temperature across the horizontal axis. Since air pressure ALWAYS

decreases with height, we can use air pressure as a height measurement (using millibars) on the left side

of the chart as well as the standard method of measuring altitude (using kilometers above sea level) on

the right side of the chart.

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CEE 1331 Homework #3: Atmospheric Soundings and Severe Weather In this homework we will analyze the temperature and moisture profile of the atmosphere and how it relates to the intensity of thunderstorms. An atmospheric profile is simply a way of displaying how any weather parameter (such as temperature and moisture) changes with height. We will be plotting the weather data on a Stüve diagram. A Stüve diagram is a type of graph that has height on the vertical axis and temperature across the horizontal axis. Since air pressure ALWAYS decreases with height, we can use air pressure as a height measurement (using millibars) on the left side of the chart as well as the standard method of measuring altitude (using kilometers above sea level) on the right side of the chart. Note that at an altitude of 0 km (which is sea level), the air pressure is indicated to be near 1000 mb. At an altitude of approximately 2 km above sea level, the air pressure drops to around 800 mb. Likewise, when you get up to around 8 km above sea level, the air pressure drops to around 350 mb. Keep in mind that these must be understood as averages because the values do change from day to day! As we go higher into the atmosphere the air pressure will decrease until it eventually reaches NEAR zero millibars at the edge of outer space. In this homework we will not be going quite that high. We will only be going up to the level where the air pressure drops to 100 mb (which is approximately 16 kilometers or about 10 miles above sea level). You can see this by looking at the 100 mb level at the top left side of the Stüve chart and how it corresponds to 16 km on the top right of the chart. When you look at the Stüve graph you will note that in addition to the standard vertical and horizontal lines denoting height and temperature, respectively, we also have diagonal lines and curves. The diagonal lines are called DRY ADIABATS. The dashed curves are call WET ADIABATS. You can think of these as the paths that air bubbles take as they rise into the atmosphere. And just like their names suggest, a DRY air bubble (i.e., one that is not saturated and thus whose relative humidity is less than 100%) will follow the DRY ADIABATS as it rises into the sky. Similarly, a SATURATED air bubble (i.e., one whose relative humidity is 100%) will follow the curved WET ADIABATS as it rises. As you follow these curves, you will see how the temperature of rising air bubbles change with height. Let’s do an example. Let’s say you have an air bubble located at 1000 mb, and the temperature of the air bubble is 38°C. You would plot this air bubble by finding the 1000 mb horizontal line, and mark where it intersects the 38°C vertical temperature line. Of course, not every single possible temperature is going to have a vertical line. But you can see the vertical lines for 30°C and for 40°C. Therefore, you can ESTIMATE where the 38°C line would be and make your plot. OK, now let’s assume for this example that our air bubble at 1000 mb and 38°C is NOT saturated. In other words, its relative humidity is less than 100%. We are going to lift this air bubble up to the 800 mb level. Since it is not saturated, we will follow the DRY ADIABAT. In this example, it just so happens that we have a DRY ADIABAT already on the chart that passes through our point of 38°C and 1000 mb. This is NOT the case for every possible scenario. If you do not have a visible dry adiabat passing through your point of interest, simply use the DRY ADIABAT that would pass through your point by drawing one for yourself that is parallel to the DRY ADIABAT on either side of your point! But for now, we can just use the visible DRY ADIABAT that happens to conveniently pass through our example point. Now lift your bubble to the 800 mb level following the DRY ADIABAT and stop. What is the temperature of your air bubble now that it is at 800 mb? By dropping a vertical line down to the temperature axis, you can see that your air bubble has cooled to about 19°C. As long as the air bubble stays dry, that is to say, as long as its relative humidity is less than 100%, you would follow the DRY ADIABAT to see what the temperature of your air bubble would be at any level in the atmosphere. But we know by now that an air bubble that is rising will probably eventually cool to the point to where it can no longer “hold” all of the water vapor that may be in it. Once it reaches that point, the relative humidity will hit 100%, and condensation will begin. In other words, a cloud will form. But where will this happen? It happens at the level in the atmosphere called the LIFTING CONDENSATION LEVEL (LCL). This is where the base of a cloud is produced by our rising air bubble. So how do we find this LCL? First you need to know what the DEW POINT TEMPERATURE of the air bubble is at the surface where it starts rising. Remember, the dew point temperature is just another way of measuring how much water vapor is in the air. The higher the dew point, the greater amount of water vapor (i.e., moisture) is in the air. So, let’s say the dew point temperature of our surface air bubble at 1000 mb is 24°C. To find the bubble’s approximate* LCL (i.e., where cloud base will form), you simply follow the dashed-curve WET ADIABAT that passes through the dew point temperature up to where it crosses the DRY ADIABAT line that passes through the temperature of the air bubble. When you plot 24°C at 1000 mb, you will quickly note that there is no WET ADIABAT that conveniently passes through that point. BUT…you will also note that there are WET ADIABATS on either side of your plotted dew point temperature. So, you simply make your own WET ADIABAT that parallels both of these, and then you follow it! Therefore, when you lift your “dry” air bubble on its DRY ADIABAT, and intersect that path with the WET ADIABAT that passes through your air bubble’s surface dew point temperature, you have found the approximate level in the atmosphere where it will saturate (i.e., RH=100%) and become a cloud. You have found its LCL. In our example above, the air bubble saturates at the 775 mb level. This is where the base of the cloud will be! And you can see this corresponds to a little more than 2 km above sea level. Any further lifting of our air bubble must now follow the WET ADIABAT because it will remain saturated for the rest of its journey into the sky. At any step of the way, we can compare the rising air bubble’s temperature to its surroundings. Thus, we can determine if the air bubble is warmer than its surroundings, or colder than its surroundings, or the same temperature as its surroundings. If the air bubble is warmer than its surroundings, then it will continue to rise on its own much like a hot air balloon rising into the relatively cool air around it. In this scenario, we say the atmosphere at this level is UNSTABLE relative to the air bubble. If, however, the air bubble is colder than its surroundings, then it will want to fall back to where it came from since it would be more dense than its surrounding environment. In this case, we would say the atmosphere at this level is STABLE relative to the air bubble. If the air bubble finds itself at the same temperature as its surrounding environment, then it would tend to stop rising because it would be equally as dense as its surroundings. We would say that the atmosphere at this level is NEUTRAL relative to the air bubble. RISING AIR BUBBLES and THUNDERSTORM POTENTIAL Thunderstorms form whenever the atmosphere is mostly unstable. This allows air bubbles to rise through great depths in the atmosphere. And the warmer these bubbles are compared to their surrounding environment, the faster they will rise and the stronger a thunderstorm will be. In this part of the homework, we are going to lift an air bubble in a specific environment and then determine whether thunderstorms are possible, and if they are, just how strong they will be. First, we need to define the atmospheric environment in which air bubbles will be rising. Plot the following MORNING temperatures at each pressure level on your Stüve diagram. You may skip plotting the dew point temperatures, except for the first one, for right now. We will get back to this parameter later in the homework. Pressure Level (mb) Temperature (°C) Dew Point Temperature (°C) 1000 850 700 22 20 2 16 19 -25 500 400 300 200 150 100 -20 -27 -40 -50 -50 -35 -45 -55 -65 -75 -80 -65 Once you have plotted your temperature points on the Stüve (called an atmospheric sounding), connect your points with straight line segments. (Do NOT include your dew point temperature with one of the straight line segments). You will then be able to see how the temperature is changing with height from the surface (1000 mb) up to 100 mb (approximately 16 kilometers above sea level). Now you are ready to lift surface air bubbles into this environment. Using the procedure discussed earlier in this homework, 1) 2) 3) 4) 5) Find the pressure level where the LCL is located. What happens to the air bubble at the LCL? What is the relative humidity of the air bubble at its LCL? What is the air bubble’s temperature at the LCL? Is the air bubble at the LCL (warmer, cooler) than its environment? _______________ mb _______________ _______________ % _______________ °C _______________ Let us now assume that we can lift our bubble even higher than its LCL. As mentioned earlier, any further lifting must be done on a WET ADIABAT since our air bubble is now saturated (i.e., RH= 100%) and will remain saturated for the rest of its trip into the atmosphere. Lift the air bubble until it crosses the environmental temperature line. When it crosses this line, any further lifting will result in the air bubble being warmer than its environment. This means once it crosses this line, it will freely rise on its own. This level is called the LEVEL OF FREE CONVECTION (LFC). 6) At what pressure level is the LFC for this air bubble? 7) What is the bubble’s relative humidity at the LFC? _______________ mb _______________ % Continue lifting the air bubble (although now it is doing it on its own because it needs no help). Lift it until once again it equals the environmental temperature. This is the point where the bubble will ultimately stop rising because any further lifting will result in the air bubble being colder than its environment. This level is called the EQUILIBRIUM LEVEL (EQL). 8) At what pressure level is the EQL? 9) What is the relative humidity of the air bubble at the EQL? _______________ mb _______________ % The EQL is often the height to which a cloud will grow. Using the altitude scale in kilometers on the right-hand side of the Stüve diagram, 10) To what height – in kilometers - will the cloud reach? _______________ km The amount of energy available to thunderstorms growing in any environment is given by what is called the POSITIVE area on the Stüve diagram. Specifically, the POSITIVE area on the graph is defined as the area TO THE RIGHT of the environmental temperature line and TO THE LEFT of the air bubble’s path between the LFC and the EQL. This area is known as Convective Available Potential Energy, or CAPE. The larger the CAPE area the more intense a storm can be. In the figure below, the heavy black line is the atmospheric temperature profile. The bubble’s path starts with the DRY ADIABAT at temperature T and switches to a WET ADIABAT after the LCL is reached. It then rises along the WET ADIABAT, crossing the environmental temperature line at the LFC. It continues on the WET ADIABAT until it crosses the environmental temperature line again at the EQL. The green shaded area between the LFC and the EQL is an example of CAPE. Note that there is also a portion of the bubble’s path – shaded in red - that is to the LEFT of the environmental temperature line between the SURFACE and the LFC. This is called the NEGATIVE area and is known as Convective Inhibition, or CINH. This is the amount of energy required to FORCE an air bubble to rise. The reason the bubble must be forced to rise is because as soon as it rises from the surface, it becomes colder than its surrounding environment, and thus it wants to return from where it came. Therefore, the atmosphere is INHIBITING the air bubble from rising. But if the bubble is sufficiently forced (i.e., such as along a frontal boundary or by being pushed up a mountain), it is possible to get an air bubble to rise to its LCL and then eventually on to its LFC. 11) Shade in the CINH and the CAPE areas on your Stüve chart using a “cool” color (i.e., green or blue) for the CAPE and a “warm” color (i.e., yellow, orange, or red) for CINH. So far, we have seen that the atmosphere – as depicted by our MORNING temperature profile – does have some energy available (CAPE) for thunderstorm development IF AND ONLY IF enough forcing can be provided to overcome the CINH and allow air bubbles to reach their LFC. Once there, the air bubbles will continue to rise on their own – being warmer than their surroundings - until they reach their EQL. Now let us jump ahead into the AFTERNOON and heat up the lower levels of the atmosphere and see how this changes the potential for thunderstorms and their intensity. Keep in mind that the mid- and upper-levels of the atmosphere do not change very much during the day, but near the surface the temperature can heat up dramatically. Let’s say the surface winds throughout the day are out of the SE bringing in warm, more humid air from the Gulf of Mexico. And therefore, by late afternoon, the surface temperature rises to 30°C, and the surface dew point rises to 22°C reflecting the rich, Gulf moisture flowing into the area. Let’s assume conditions aloft do not change. Plot this afternoon temperature profile on a second Stüve chart. Now, lift an air bubble with a temperature of 30°C and a dewpoint of 22°C, starting once again at 1000 mb. 12) What is the LCL of this afternoon rising air bubble? 13) What is the air bubble’s temperature at this LCL? 14) Is the air bubble at the LCL (warmer, cooler) than its environment? _______________ mb _______________ °C _______________ Continue lifting your air bubble until it first equals the temperature of the environment in order to find its LFC. Shade in the CINH area with a “warm” color (yellow, orange, or red). 15) At what level does the air bubble reach its LFC? _______________ mb 16) Is the amount of afternoon CINH (less, greater) than in the morning? _______________ 17) What does this amount of CINH tell you about the difficulty in producing thunderstorms in the afternoon compared to the difficulty you found in the morning? _________________________________________________________________________________ Continue lifting your air bubble until it reaches its EQL. 18) At what pressure level does the air bubble reach its EQL? _______________ mb 19) To what height – in kilometers - will the cloud reach? _______________ km 20) Shade in the afternoon CAPE area on your Stüve chart using a “cool” color (i.e., green or blue). 21) How does the amount of CAPE in the afternoon compare to the amount of CAPE you found in the morning? _________________________________________________________________ _____________________________________________________________________________ 22) What does this tell you about the intensity of afternoon thunderstorms compared to the intensity of thunderstorms that might form in the morning? _____________________________________________________________________________ *Using the WET ADIABAT that passes through the surface dew point temperature and finding its intersection with the DRY ADIABAT that passes through the surface temperature APPROXIMATES the location of the LCL. Ideally, one would use MIXING RATIO lines to determine the EXACT value for the LCL. But in the interest of saving time for this lab session and reducing the complexity of the Stüve chart, this approximation is introduced. ...
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