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Giovanni User Manual 4.21

GIovanni is an on-line environment for the direct statistical intercomparison of geophysical parameters in which the provenance (data lineage) can easily be accessed. This user manual provides assistance on how to use Giovanni and information on Giovanni's data products and services.

User Manual Sections:

 

Quick Start

Using the Giovanni user interface, it is possible to easily find and display selected data on a plot.  It is also possible to download the plot source files in netCDF format.  While this user interface does not require you to select criteria in any particular order, below is a common sequence of steps you can follow to obtain a plot and data.

Step 1: Select Plot: 

We use "Time Series" for the example.

Step 2: Select Area:

You can either type in longitude and latitude of the edges of your desired box as West, South, East, North, or click on the "Show Map" button and select an area with a click and drag movement. Alternatively, you can click on the "Show Shapes" button and select a shape from a pre-defined list of polygons.

Step 3: Select Variable(s):

To select a variable, you can begin either by:

(a) selecting checkboxes of the desired attributes on the left hand side, or

(b) typing a term into the Search box and pressing Search.  In this latter case, you can then narrow the results by selecting desired attributes on the left hand side.

Note that if you select attributes and then type in a search term, the search will be against the whole collection, not just those matching the selected attributes.

Also, note that all variables have a valid date range and selecting a variable will constrain the valid date ranges presented by the calendar date selector.

Step 3a:  Select "Vertical Slice"

This is the level or layer in a 3-dimensional variable that you want plotted on a map, histogram or time series. This is not necessary for vertical plots.

Step 3b:  Select "Destination Units"

Some variables can be converted into alternate units.  See the section on Units Conversion for details on how this works.

Step 4: Select Date Range:

Please note the relationship between date range and selected variables. If you select a date range that does not intersect the union of the date ranges of the currently selected variables, the user interface will display an error message.

 

General Plot/Service Guidelines

Using Shapes to Specify Areas

Shapes are treated differently depending on whether or not Giovanni needs to average over the region or simply subset the region.

When Giovanni averages over an area in services such as Area-Averaged Time Series, the shape is rasterized at a resolution four times higher than that of the data. The high resolution raster array is then regridded to an array at the data’s resolution, with weights proportional to the amount of shape coverage in each cell. The shape coverage weights are then further weighted by the cosine of the latitude, as described in the section on calculating statistics. These weights are used in the area averaging computation to enforce that cells with lower shape coverage have a smaller influence on the resultant average.

When Giovanni uses the shape to subset a region in services such a Time-Averaged Map, the shape is used to mask the data and keep all grid cells that at least partially overlap the shape.

Land-Sea mask data are included as two shapefiles, “Sea Only” and “Land Only”.  These shapefiles enable land & sea masking capability. For example, if a bounding box is defined and the “Land Only” shape file is selected, then the defined region is Land-Only within the bounding box. Currently two plot types, Time Averaged Map and Area-Averaged Time Series, are enabled for land sea masking capability.

In Giovanni, we derived a new sea mask from the original by defining the water surface as greater than or equal to 75% water. For data sets with different grid resolutions, a regridding algorithm is applied to this mask to derive a compatible mask for the corresponding grid resolution of the data.

 

Calculating Weighted Statistics 

Most Giovanni services calculate aggregate statistics of some sort along one or more dimensions in time and space.

Mean and Standard Deviation

With the exception of statistics for the Histogram plot, both these statistics use weights. Weights are the cosine of the center latitude of the data point being weighed. In the case of shapes, weights are further adjusted as described in "Using Shapes to Specify Areas."

The formula used to calculate the mean is:

Weighted Mean
Formula for weighted mean.

where w represents the weights, d represents the data points, and i is an index over all the data points being averaged. The formula used to calculate the standard deviation is:

Weighted Standard Deviation
Formula for weighted standard deviation.

Count, Minimum, and Maximum

These statistics are calculated over all data points in the user's selected area that are not fill values. For areas specified as bounding boxes, all data points whose grid center is within the bounding box are included. For shapes, all data points that at least partially overlap the shape are included. The count is the total number of included data points, the minimum is the smallest of the included data points, and the maximum is the largest of the included data points.

Regridding

Some of our services, such as correlation, have to pair two variables in time and space. If the two variables have different spatial resolutions, the finer resolution is regridded to the coarser resolution using the lats4d application.

 

Plot/Service Types

Maps

Time-Averaged Map

The time averaged map shows data values for each grid cell within the user-specified area, averaged (linearly) over the user-specified time range as a map layer. Fill values do not contribute to the time averages. The map can be zoomed and panned.  Plot options include setting minimum and maximum values for the color scale, and in some cases selecting other palettes.

Vector magnitude maps, such as those for wind speed magnitude, compute the magnitude of the vector at each time step first before averaging the magnitudes of each grid cell together across time. In contrast, vector maps compute the averages of the latitudinal and longitudinal components of each grid cell across time and display the resulting vectors.

Animation

The Animation service shows individual time slice maps of a data variable in an animated sequence.

Time-Averaged Overlay Map


To use the Time-Averaged Overlay Map, first select a variable or a pair of variables for plotting.  If a single variable is selected, this variable will be used for both the shaded (color) map and contour plot.
 
Select the time period of interest with Begin and End Dates, and the region-of-interest (shapefiles can be selected).   Then click “Plot Data”.
 
When the plot has completed, the options for the plot can be adjusted using the “Layers” menu.  Click on the icon, and the display will show the data variables selected.  If two variables are selected, the initial variable shown by the color map and the variable shown by the contour plot will be indicated by the colored radio buttons.  These can be switched by clicking the respective buttons under Contour and Shaded.
 
Under each variable for either the Contour Plot or Shaded map, there is an Options icon.  Clicking this will provide the plotting options.   For the Contour Plot, the maximum and minimum values can be changed, and the numerical spacing between contour lines can be adjusted.  Click “Replot” once the desired values have been entered.
 
For the Shaded Map, the maximum and minimum values can be changed, a different palette can be selected, “Smoothing” (eliminating pixel borders) can be chosen, and the Linear or Log value display can be selected.   Once the desired values and options have been selected, click “Replot”.
 
Also note that the Title and Subtitle, Caption, and Legend can be removed.  The country and state name labels, borders, and grid lines may also be removed as necessary.
 
Once the Overlay Map has been plotted according to the user custom specifications, it can be downloaded as either a KMZ, GeoTIFF, or PNG format image file.

User-Defined Climatology Map

The User-Defined Climatology maps compute averages for either a specific month or a 3-month group corresponding to the meteorological seasons (DJF=Dec,Jan,Feb, MAM=March,April,May, JJA=June, July, August, SON=Sep, Oct, Nov).  The average is computed over the years specified in the selection screen and displayed in a map. More than one month or season can be selected. We refer to this as a quasi-climatology because a "true" climatology is typically computed over many (e.g., 30) years.  Available only for monthly data.

Accumulation Map

A few variables are available for the Accumulation Map, in which instead of averaging over time, a total is computed over time for a given grid cell. These are typically precipitation-related variables, and are restricted to data variables that are continuous, with few or no gaps. (The reason is that gaps are treated the same as values of 0, resulting in a possibly significant low bias in data with gaps.)

Smoothing option for Maps

Giovanni uses matplotlib's filled contour algorithm to create smoothed images.

Comparisons

Correlation Map (and other comparisons)

The correlation map calculates correlation coefficient using simple linear regression between two variables over time within each grid cell, producing two maps: one showing the correlation coefficient (R) and the other displaying the number of contributing (matching) samples in each grid cell.  (Note that the values from both variables must be non-fill in order to contribute to the correlation computation.)  Any grid cell that contains fewer than three matched pairs over time will be assigned a fill value.

An additional product of the correlation computation is an average at each grid cell of the differences between the two variables at each timestep for that grid cell. This map may contain more values than the correlation map, as the differences will be computed for as few as one non-fill matched time step in a grid cell.

Static Scatter Plot

The scatter plot produces a (static) scatter plot of all data pairs from two selected variables.  The data pairs are matched in both space (grid cell) and time. The plot shows both the scatter and the parameters of the simple linear regression, i.e., slope, offset and correlation coefficient (R). Caveat:  the averaging that occurs within regridding may produce an artificially high correlation coefficient; interpret with care!

Interactive Scatter Plot

The interactive scatter plot produces a scatter plot and a map showing the location of data pairs in the scatter plot. Users can select data pairs of interest by selecting data pairs (click and drag on the scatter plot). Users can also select locations of interest by selecting region of interest in the map.

Time-Averaged Scatter Plot

The Time-Averaged Scatter Plot produces a scatter plot of all co-located points averaged over time and a map showing the location of data pairs in the scatter plot. Only values that are non-fill for both data fields at a given time-step are used in the computation of the averages over time for each grid cell.  

Users can select data pairs of interest by selecting data pairs (click and drag on the scatter plot). Users can also select locations of interest by selecting region of interest in the map.

Area-Averaged Scatter Plot

The Area-Averaged Scatter Plot computes an average over the selection area for each time step of two separate variables. The resulting values are matched up by time and plotted as an X-Y scatter plot. All cells whose center point falls within the selection box are included.

Difference of Time-Averaged Maps

The Difference of Time-Averaged Maps computes the time average for each grid cell for two variables being compared. The differences between the two resultant maps are then computed and plotted on a map.  Only variables with the same Measurement and Units can be differenced in this way.  Fill values in either resultant map are not included in the final difference.

Time Series

Area-Averaged Time Series

The standard Giovanni time-series plot is produced by computing spatial averages over the user-selected area of a given variable for each time step within the user's range.  Fill values do not contribute to the spatial averages. Each average value is then plotted against time to create the time-series output.

Seasonal (Interannual) Time Series

The Seasonal Time Series computes an area averaged time series for each year in the user's selection for a given month or 3-month meteorological season, To avoid biasing the results, partial seasons (i.e., missing one or two months) are not plotted. Meteorological winter (Dec-Jan-Feb) is labeled with the year in which January falls, so DJF for 2007 goes from Dec 2006 to Feb 2007.  This service is available for monthly data only. 

Hovmoller Plots

The Hovmoller plot averages over either latitude or longitude at each time step and creates a two-dimensional color slice plot for the remaining horizontal dimension vs. time.  Lat - time Hovmoller plots show latitude on the vertical axis.  Lon - time Hovmoller plots show longitude on the  horizontal axis.

Time Series of Area-Averaged Differences

This service compares two variables over time by first differencing the first variable from the second at each grid cell, and then computing the average difference over the user-selected area. The area averaged difference is computed over a geographic (Cartesian) map.

Vertical Plots

Cross Section (Latitude-Pressure)

This service creates a two-dimensional representation of data parameter values plotted in pressure or altitude (in the vertical dimension) vs. latitude or longitude (in the horizontal dimension). The data parameter values in the Cross-Section Plot are interpolated and displayed in log-scale. The data are averaged over longitude, and the service will generate a profile rather than a cross-section if a single latitude value is selected.

Cross Section (Longitude-Pressure)

This service creates a two-dimensional representation of data parameter values plotted in pressure or altitude (in the vertical dimension) vs. latitude or longitude (in the horizontal dimension). The data parameter values in the Cross-Section Plot are interpolated and displayed in log-scale. The data are averaged over latitude, and the service will generate a profile rather than a cross-section if a single longitude value is selected.

Cross Section (Time-Pressure)

This service creates a two-dimensional representation of data parameter values plotted in pressure or altitude (in the vertical dimension) vs. time (in the horizontal dimension). The data parameter values in the Cross-Section Plot are interpolated and displayed in log-scale. The data are averaged over longitude and latitude and will generate a profile rather than cross-section if the Begin Time and End Time are the same.

Vertical Profile (Time and Space Averaged)

Several of the variables in Giovanni have a vertical dimension in addition to the horizontal dimensions of Longitude and Latitude.  For example, The Atmospheric Infrared Sounder (AIRS) Temperature, water vapor, and relative humidity have vertical dimension of atmospheric pressure.  The vertical profile plot option displays a profile of the given variable which is first averaged over the user selected region and then over the selected period.

Other Plots

Zonal Mean

The Zonal Mean over a given area is the sequence of data generated by taking the average over a range of longitudes for each latitude. The Zonal Mean calculation only works with data in a single atmospheric layer (defined by pressure in hPA), such as 300 hPA as an example. The zonal mean cannot be calculated for a pressure range, i.e., for the range 300-400 hPA. The zonal mean operation works by exporting the data from the dataset and then averaging along the longitude axis for each latitude point.

Histogram

The histogram service computes a histogram over the values present in the given temporal and spatial selection. No averaging is done over any dimensions. Fill values in the data are dropped and not considered in the results. The unweighted sample mean, unweighted sample standard deviation, and median are also presented in a box in the top right hand corner.

 

Variables

 

Aerosol Optical Depth or Thickness or Aerosol Extinction

Aerosol optical depth or thickness is a measure of radiation extinction at the encounter of aerosol particles in the atmosphere.

The extinction or total aerosol optical depth is a measure of radiation extinction due to aerosol scattering and absorption. Aerosol Total Optical Depth is available through Giovanni at 550 nm from MODIS.
Read more Red Dice Information for Educators (DICCE Project)

Giovanni includes several measurements of Total Aerosol Optical Depth from different instruments and platforms, often measured at different wavelengths. 

 

Component Aerosol Optical Depth

In addition, the optical depth of several different species of aerosol is available from the GOCART model, and the Optical Depth related to Absorption only is available from the Ozone Monitoring Instrument.

 

Angstrom Exponent

The Angstrom Exponent describes the spectral dependence of aerosol optical thickness (τ) on the wavelength of incident light (λ).  This provides additional information on the particle size (larger the exponent, the smaller the particle size), aerosol phase function and the relative magnitude of aerosol radiances at different wavelengths.  The spectral dependence of aerosol optical thickness can be approximated (depending on size distribution) by, 

τa = β λα   where   α   is Angstrom exponent (β = aerosol optical thickness at 1 μm)

Angstrom exponent (computed from τ measurements on two different wavelengths) can be used to find τ on another wavelength using the relation.

equation

 

Pixel Count


Level 3 gridded products are often produced by averaging multiple pixels from the Level 2 orbital products in a given grid cell.  For such algorithms, it is sometimes useful to know how many level 2 pixels, or Pixel Count, were used in the average.  Note however, that while low pixel counts typically indicates a lack of representativeness, medium pixel counts are often obtained when pixels cluster into one portion of the cell, improving representativeness only marginally.

Latent Heat Flux

The heat/energy transfer involving evaporation of water at the sea surface, dependent on difference of sea and air surface specific humidity, wind speed, and sea surface roughness.

Sensible Heat Flux

The heat/energy transfer involving conduction and convection at the sea surface, dependent on difference of sea and air surface temperature, wind speed, and sea surface roughness.

Wind Stress

The momentum transfer (downward from atmosphere to ocean) involving shear stress exerted by the wind on the sea surface, depending on wind speed and sea surface roughness.

i)   Wind Stress Magnitude (scalar)

ii)  Wind Stress Vector (vector expressed via latitudinal and longitudinal components)

 

Source Data Products

AIRS Products

The Atmospheric Infrared Sounder (AIRS) is an infrared spectrometer with 2378 channels in the 3.7–15.4 micron spectral range.

It was launched in May 2002 aboard the Aqua spacecraft. The AIRS primary products include atmospheric profiles of temperature and humidity on global scales, day and night. The temperature profiles cover the atmospheric column from the surface to the stratosphere (0.1 mb) and the humidity profiles are accurate only in the troposphere, below about 200-300 mb. In addition, AIRS products include global surface temperatures, total column precipitable water vapor, cloud properties and outgoing long-wave radiances which are of significant value in studies of Earth's radiation budget and climate variability.

GOCART Products

The Goddard Chemistry Aerosol Radiation and Transport (GOCART) model simulation provides global daily and monthly aerosol optical depths data at seven wavelengths (350, 450, 550, 650, 900, 1000, and 1500 nm) for major tropospheric aerosol components, including sulfate, dust, black carbon (BC), organic carbon (OC), and sea-salt aerosols at a horizontal resolution of 2.5x2.0 deg. The GOCART model uses the assimilated meteorological fields of the Goddard Earth Observing System Data Assimilation System (GEOS DAS), generated by the Goddard Global Modeling and Assimilation Office (GMAO). The data provided from the GES DISC are from the GOCART Simulation Experiment ID G4P0.

GSSTF3 Products

The air-sea turbulent fluxes of GSSTF3, involving heat/energy and momentum transfer between the atmosphere and ocean facilitated by turbulent motion, consist of three major components: latent heat flux, sensible heat flux and wind stress.

MODIS Products

MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS view the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths (see MODIS Technical Specifications).

The aerosol variables in Giovanni are extracted from Level-3 gridded atmospheric daily products for both MODIS on Aqua and MODIS on Terra.  The MODIS Aerosol Product monitors the ambient aerosol optical thickness over the oceans globally and over a portion of the continents.

OMI Products

The Ozone Monitoring Instrument (OMI) measures primarily in the ultraviolet and near-UV part of the spectrum. OMI supplies an Aerosol Extinction Optical Depth (the same as Total Aerosol Optical Depth) as well as an Aerosol Optical Depth due only to radiation absorption (Aerosol Absorption Optical Depth).

SeaWiFS Deep Blue Products

The Sea-viewing Wide-Field-of-view Sensor was designed primarily with ocean color in mind. However, it has several wavelengths that are similar to those of MODIS, making it possible to retrieve Aerosol Optical Depth using a variant of the MODIS Deep Blue Algorithm. Under NASA's MEaSUREs program. The Consistent Long Term Aerosol Data Records over Land and Ocean from SeaWiFS project has produced Total Aerosol Optical Depth at a variety of wavelengths and two spatial resolutions, 0.5° and 1°.

N.B.:  The Long Term Aerosol Data Records project plans to put out a release 4 as a final release.

 

Other Features

Web Coverage Service

The Open Geospatial Consortium (OGC) Web Coverage Service (WCS) interface standard allows users to access geospatial data over the internet. The GES DISC WCS interface complies with a standard protocol, enabling clients to access data both directly, and time-averaged data utilizing Giovanni-4 capabilities. For more information see the giovanni4 WCS documentation page.

Units Conversion

Many of the variables can be converted from the current units to different units, such as mm/hr to inch/day. This capability is indicated by an option menu in the Units column for that variable (coming soon...) For efficiency's sake, this conversion is usually applied to the output data from a given service.  However, there are two cases where the conversion must be done before the processing algorithm runs. The first is for comparison services that require identical units to be sensible, i.e., the services with the word "Difference" in them.  The second set of cases are those where the service algorithm aggregates (e.g., averages) the data over the time dimension.  In these case, if the destination units is a monthly rate (e.g., inch/month), then the conversion must be done before the algorithm runs in order to account for the varying number of days in each month.

Caveats 

Service requests for variables specified in monthly rate units (e.g., mm/month) will give plots bias relative to plots that use variables with daily or hourly rate units (e.g., inch/day).  Plots that display the time dimension will tend to show higher values for longer months (i.e., months with more days). For example, suppose a user requests a time series of precipitation in mm/month. The March data points will show the total precipitation for 31 days while the April data points will show the total precipitation for only 30 days.   Plots that average over the time dimension will have similar problems. Longer months will tend to have larger values, which will pull the average up. Shorter months will tend to have smaller values, which will pull the average down.  Histogram plots will be slightly skewed by monthly units conversion. Values from longer months will tend to end up in higher-valued bins and values from shorter months will end up in lower-valued bins.

Legends or Palettes

Under the Map Plotting service, users are offered a choice of all of the palettes available in Giovanni through the "View All Palettes" button on the Map Options Dialog.  The Map Options button is available in the upper right hand corner of the map area after the map is rendered.

FAQ

Why do I only get the header and footer when I try to load Giovanni?

The most likely reason you are unable to load Giovanni fully is a problem with javascript. In order to load Giovanni, javascript must be enabled on your browser and your browser must be able to access all the javascript libraries Giovanni needs.

 

1. First, check to make sure that JavaScript is enabled in your browser. Visit http://enable-javascript.com/. If the JavaScript is enabled, you should see a text that says "Javascript is enabled in your web browser. If you disable JavaScript, this text will change.” If javascript is disabled in your browser, consult your browser's documentation to reenable javascript.


2. If Giovanni still doesn't load once you've made sure JavaScript is enabled, try accessing Giovanni from a computer on a different machine, preferably on a different network. For example, if you are having trouble loading Giovanni on your work machine, try loading Giovanni on a machine at home or in a library. If Giovanni loads on this other machine, you will need to work with the system administrator for the machine having trouble to figure out why Giovanni is getting blocked.

3. If you've tried a different machine and Giovanni still doesn't load, then you need to verify your browser can access the Giovanni javascript that resides on our servers. Try accessing this URL in your browser: http://giovanni.gsfc.nasa.gov/giovanni/js/giovanni4.min.js. The browser will load the javascript as text, so if you see a lot of text with parentheses and semi-colons, you have successfully accessed one of our javascript libraries. If you can't load the library, please contact our help desk at gsfc-help-disc@lists.nasa.gov for further assistance. We will ask you to give us your IP address, which can can find by visiting http://whatismyipaddress.com/.

4. If Giovanni still doesn't load once you've made sure you can access our javascript libraries, you will need to try accessing the external libraries Giovanni depends on. Try accessing these URLs in your browser:

Each of these should load a javascript library as text with parentheses and semi-colons. If any of these libraries fail to load, you will need to work with your system administrator to figure out why you are being blocked. These libraries are not under our control, so we unfortunately cannot see what is causing the problem.

 
5. If you've verified that you can access all the external libraries Giovanni depends on, please contact our help desk at gsfc-help-disc@lists.nasa.gov for further assistance. We will ask you to load Giovanni in your browser with the developer console enabled. Each browser has its own developer console:

 
 
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