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Objectives: By completing this lab, the student will learn to:
visualize current, past, and forecast weather data for the entire world using Earth Nullschool (developed by Cameron Beccario)
perform calculations using atmospheric data derived from Nullschool.
Background: In this lab you will examine forecast data and perform calculations to derive: wind power density (???), pressure heights (Δ?), and pressure gradient force (???). Earth Nullschool allows you to visualize many different atmospheric parameters at different pressure levels in the atmosphere (e.g., temperature, wind speed and direction, CO2 concentration, misery index, precipitation, etc.). It allows you to examine global patterns of current, past, and future or forecast conditions (4 days out).
Wind power density is related to the kinetic energy of wind, and is very useful for planning for wind turbine construction. Typically, wind power is measured as power per unit area, or W m-2. The same unit is used for solar irradiance. Wind speed is simply the distance a parcel of air travels in a unit of time, like meters per second (m s-1). Wind power density (???) is related to the cube of wind speed by:
where ? is air density in kg m-3, ? is wind speed in m s-1, and ? is cross-sectional area in m-2. If we divide ??? by ? Eq.1 becomes:
The resulting value from Eq.2 is written in W m-2 based the units outlined above. Earth Nullschool provides an estimate of ??? in kW m-2, but you will also perform the calculation to see how the value is derived using atmospheric parameters.
Next, you will calculate the elevations of different pressure surfaces relative to the sea level pressure using the following equation:
where Δ? is the height difference between ?1 and ?2 in meters, ?? is the gas constant (287 J kg-1 K-1), ?̅ is the average temperature between ?1 and ?2 (notice we are not using virtual temperature in this case because we don’t know the mixing ratio), ? is gravity (9.81 m s-2), and ?1 and ?2 are the different pressure levels in hectopascals (hPa).
Finally, for pressure gradient force (???) you will examine a deep low-pressure systemin the north Atlantic on 2021-10-25, 9:00 local time.
Recall that we can use the following equation to estimate the ???:
where ?1 and ?2 are different pressure level locations in Pascals (Pa), ? is air density in kg m-3, and ? is the distance between pressure the pressure locations in meters. The calculation yields a value in units of acceleration, m s-2.
Task 1: Before performing calculations, do some exploring using nullschool at: https://earth.nullschool.net/. Do the following tasks:
Spin the globe around, and have a look for interesting features. Try to find the following:
-A cyclone in the Northern hemisphere, and in the southern hemisphere
-An anti-cyclone in the northern hemisphere, and in the southern hemisphere
Explore the different overlays by clicking the “earth” button in the lower left-hand corner:
-Switch to “Particulates Mode”, use the “DUex” overlay, and look for regions of the world that have high and low amounts of aerosols. Think about how this might relate to our discussions on cloud formation.
-Have a look at “ocean” and “space” mode, just for fun.
-Make sure you switch back to “air” mode… This is what we’ll be working with for the rest of the lab
-In “air” mode, switch the overlay to “temperature”, and have a look at air temperature across North America
-Use the “Control” menu to navigate through time (use << to go back a day, >> to go forward a day). Have a look at weather over the past week, and forecast weather for the coming week. Think about how this relates to what you’ve observed in Calgary over the past several weeks, focusing particularly on wind direction.
Task 2: Open the MS Excel spreadsheet (Lab4_student.xlsx). Cells highlighted in yellow require input data derived from Nullschool. Cells in blue require embedded calculations or functions. To derive values from Nullschool simply click on a region of the globe with your mouse, and then select different Modes, Heights, and Overlays to see how the values change.
Figure 1: Nullschool image centered on the Belcher Islands in Hudson Bay. Oct 21, 2021 @ 1400 hrs Calgary time
In Nullschool, navigate to Hudson Bay, and find Sanikiluaq (https://en.wikipedia.org/wiki/Belcher_Islands). Change the Date to 2021-10-25, 0:900 UTC (Note: you can change the date from local to UTC by clicking on “UTC” in the first row of data in the Earth dialogue window. Since our students are spread all over the place this year, it’s important to do that! Figure 1 shows what you should be seeing on your screen).
Click to place your cursor in the centre of the islands (see Figure 1), and select:
A small dialog box should appear on the screen above the control panel. This box shows you the geographic coordinates of the location, the wind speed and direction at the surface, and the sea level pressure on the specified date.
You will derive data for this date and location and fill in the yellow highlighted cells in the Excel spreadsheet. Enter the surface pressure in cell B4 of the spreadsheet and enter data for the remaining cells highlighted in yellow by changing the Heights and Overlays in the control panel. Notice that Nullschool does not provide a value for ??? at the surface. With the values entered, you will now calculate the values in the blue cells. First, you will convert wind speed in km h-1 to m s-1. Second, calculate the air density based on the temperature and pressure using the equation below (see also Lecture 4, note that we won’t use virtual temperature here, but think about how you could potentially determine the virtual temperature using information available in Nullschool):
Be sure to convert the Nullschool values of pressure to Pascals and temperature to Kelvin. Third, calculate ???/A using density in kg m-3 and wind speed in m s-1 (equation 2). The calculation will yield a value in W m-2; convert to kW m-2. Your calculated values of ???/A should be equivalent to those from Nullschool. If not, review your calculations and conversions and try again.
Task 3: Now you will determine the heights of different pressure surfaces, at the same location (try not to move your cursor, but if you move it a bit, it will be okay). You already entered the sea level pressure for the specified date and location. That pressure now serves as the reference to determine the heights of all other pressure surfaces above.
First, for cell N4 enter the temperature value of the surface. For all other heights calculate the average temperature between each pressure level and the surface.
Second, enter Eq. 3 in cell L5. Be sure to use the average temperature in Kelvin. ?1 refers to the pressure at the surface and ?2 refers to the pressure at the level for the calculation.
Plot WPD (x-axis) vs. elevation (y-axis), and wind speed (x-axis) vs elevation (y-axis). Compare the patterns you see in the two graphs.
Figure 2: Oct 25, 2021 wind/pressure centered on East Coast of Canada
ask 4: Keep the Null School date and time the same, but zoom out and pan to the East, towards the East Maritimes. You should find a low-pressure system just off the south-east coast of Newfoundland, and a high-pressure system off the west coast of Spain/Portugal (check your settings: Mode: Air; Height: Sfc; Overlay: MSLP). Click the cursor somewhere near the centre of low-pressure, and the centre of the high-pressure, and observe the MSLP.
Use Eq.4 to calculate ???. You will calculate ??? between the centre of the low-pressure system off Newfoundland, and the high-pressure system west of Portugal (location specified in Figure 2).
You will need to write down the coordinates of both locations and the MSLP values. To determine the distance (?) between the two locations, use the following web-based calculator: https://www.movable-type.co.uk/scripts/latlong.html. Enter the coordinates and this calculator will determine the distance in km. For ? use 1.1 kg m-3 and ensure ? is in meters.
Now, find another location on the Earth to calculate another surface level PGF. Try to find something a little different… Maybe an area where you think the PGF might be significantly higher or lower. Be sure to fill out the table with the lat/long for the location you choose.
By finishing this lab, you should have the following:
1. A table showing all values for ???
2. Graphs of ??? and wind velocity vs. height.
3. A table showing all values for Δ?.
4. Values for the input parameters used to calculate ???, as well as the calculated value in m s-2, based on the scenario outlined in Step 4 and the one you chose.
You will be asked to submit some of these values to the Module 4 assessment (D2L quiz).