Case description

In this exercise we will use Metview to produce the plots shown above:

• the plot at the top shows a map with mean sea level pressure forecast fields overlayed with the track of Hurricane Sandy.
• the plot at the bottom contains a graph chart showing the evolution of the minimum of the mean sea level pressure forecast along the storm track.

You will need to work with icons interactively and write Macro code, as well. First, you will create the two plots independently then align them together in the same page.

Preparations

XXX Download data

Verify that the data are as expected.

We will prepare the plot interactively using icons. Then, at the end, we will put it all together into a macro. Remember to give your icons useful names!

Creating the map plot

Setting the map View

With a new Geographical View icon, set up a cylindrical projection with its area defined as

South/West/North/East: 17/-97/51/-45

Set up a new Coastlines icon with the following:

• the land coloured in grey
• the sea coloured as #dcf0ff

Plotting the Mean Sea Level Pressure field

Plot the GRIB file sandy_msl.grib into this view using a new Contouring icon. Plot black isolines with an interval of 5 hPa between them. Animate through the fields to see how the forecast evolving.

Plotting the storm track

The storm track data is stored in the CSV file called sandy_track.txt. If you open this file you will see that it contains the date, time and geographical coordinates of the track points.

Create a new Table Visualiser icon and set it to visualise your CSV file:

• set the plot type to geo points
• select the columns holding the latitude and longitude by their index
• carefully specify the table delimiter and header information by setting
  • table_delimiter to whitespace ()
  • table_combine_delimiters to on
  • table header_row to 0

Now drag your Table Visualiser icon the plot to overlay the mean sea level forecast with the track.

Customising the storm track

The storm track in its current form does not look great so you need to customise it with a Graph Plotting icon by setting the

• the track line to black and thick
• the track points to be white filled circles (their marker index is 15) with red outline.

Plotting date/time labels onto the track

To finalise the track plot you need to add the date/time labels to the track points. This can be done with a Symbol Plotting icon by specifying the list of labels you want to plot into the map. Since it would require too much editing you will learn a better (programmatic) way of doing it by using Metview Macro.

Create new Macro and edit it. First, read the CSV file in with the Table Reader (see its documentation here):

tbl = read_table(
	table_delimiter	:	" ",
	table_combine_delimiters	:	"on",
	table_header_row	:	0,
	table_filename	:	"sandy_track.txt"
	)

Here the object referenced by variable tbl contains all the columns from the CSV file. Now read the date and time (from the first two columns) into separate vectors:

val_date=values(tbl,1)
val_time=values(tbl,2)

Next, you need to build the list of labels. Each label is made up from a day and an hour part separated by a slash. Use this loop to construct the list:

labels=nil

for i=1 to count(val_date) do
	labels = labels & ["   " & substring(string(val_date[i]),7,8) & "/" & val_time[i] ]
end for

Finally, define a Symbol Plotting visual definition and return it.

The code you need to add is like this:

sym = msymb(
	symbol_type	:	"text",
	symbol_text_font_colour : "black",
	symbol_text_font_size: "0.3",
	symbol_text_font_style: "bold",
	symbol_text_list	:	labels	
	)

return [sym]

By returning the visual definition your Macro behaves as if it were a real Symbol Plotting icon. So save the Macro and drag it into the plot to see the labels appearing along the track.

Creating the graph plot

Setting the View

With a new Cartesian View icon, set up a view to cater for the graph showing the mean sea level pressure values in hPa by setting

• the x-axis type to date
• the y-axis label to hPa
• the y-axis minimum value to 940 and its maximum is 1000

Computing the minimum pressure along the track

Since this task is fairly complex you will use a Macro for it. The idea goes like this:

• you read the track points from the CSV file
• define a lat-lon box around each point
• read the forecast mean sea level data for the box for the corresponding time
• compute the minimum of the pressure in the box
• from these minimum values you can build the curve data to plot.

Create new Macro and edit it. First, read the CSV file in the very same way as before but this time, on top of date and time, you also need to read latitude and longitude into vectors:

val_lat=values(tbl,3)
val_lon=values(tbl,4)

Next, read in the GRIB file containing the mean sea level forecast:

g=read("sandy_mslp.grib")

The curve data requires two lists: one for the dates and one for the values. First you need to initialise these lists:

trVal=nil
trDate = nil

Now the main part of the macro follows: you need to loop through the track points and build the curve dataset. You can use a loop like this:

for i=1 to count(val_date) do
...
end for

Within the loop first construct an area of e.g. 10 degrees wide centred on the current track point.

Next, read the forecast data for the current forecast step and the area you defined (supposing your area is called wbox):

p=read(
		data: g,
		par: "msl",
		step: i*24,
		area : wbox
		)

Here we used the fact the forecasts steps are stored in hours units in the GRIB file.

Next, compute the minimum of the field you read in using the minval() macro function:

pmin=minval(p)

Finally, build the lists for the values (scaling Pa units stored in the GRIB to hPa):

trVal= trVal & [v/100]

and for the dates:

trDate = trDate & [date(val_date(i)) + hour(val_time(i))]

Having finished the body of the loop the last step is to define an Input Visualiser and return it. The code you need to add is like this:

vis = input_visualiser (
    input_x_type        : "date",
    input_date_x_values : trDate,
    input_y_values      : trVal
)   

return [vis] 

By returning the visual definition you built a Macro behaves as if it were a real Input VisualiserSymbol Plottingicon.

Customising the storm track

Customise the graph with a Graph Plotting icon by setting the

• the line thicker
• the points to be blue filled circles (their marker index is 15) with a reasonable size.

Putting it all together

With a new Display Window icon design an A4 portrait layout with two views: your Geographical View icon should go top and your Cartesian View icon into the bottom. Now visualise your icon and populate the view with the data.

Extra Work

Try the following if you have time.

Ensuring the title has the correct date and time

There are various ways we can ensure that the title has the date and time according to the actual data. The default title in fact contains the date and time, but in this exercise we want more control over it.

Construct the second line of the title by extracting the date and time from the MSLP field and converting into an appropriate string - do this in the Macro code.

Hints:

• if you have a fieldset variable called msl_grib, the following line will extract the date at which the field is valid:
  • msl_date = grib_get_long(msl_grib, 'validityDate')
• we can do something similar for the validity time
• these are extracted as integer numbers, but can be combined into a proper date variable in Macro:

  • full_date = date(msl_date) + hour(msl_time)

• use the string() function to construct a date string similar to the one used in the current title
  • see String Functions for details of how to use it
• insert this into the mtext() function instead of the current title
• it is now more robust - if you use data from a different date or time, the title will still be correct
• note that this method will not work directly if you want to generate an animation from different time steps of data

Experiment with different backgrounds

Modify the Coastlines icon, for example:

• plot the US state boundaries
• try different land or sea shading colours
• change the frequency of the grid lines