New Use Case: Vegetation-Atmosphere Coupling (VAC) index as a land-atmosphere coupling diagnostic

Question

New Use Case: Vegetation-Atmosphere Coupling (VAC) index as a land-atmosphere coupling diagnostic

Opened this issue 6 months ago · 1 comments

Describe the New Use Case

The Vegetation-Atmosphere Coupling (VAC) index as described in Zscheischler et al. (2015) and summarized by Paul Dirmeyer here.

The original VAC index is computed using anomalies of fraction of absorbed photosynthetically active radiation (FPAR) or evapotranspiration (ET), and 2 m temperature with the long-term mean seasonal cycle removed. FPAR and ET were from satellite (FPAR) or model/model-obs fusion products (ET). For this use case, we will focus on ET as FPAR is often prescribed in current and likely next generation land models for forecast applications.

The long-term mean data should be required to be user provided, but for the use case, we'll have to compute those ourselves. For observations, we can use ERA5 for 2 m temperature, and possibly FLUXCOM or GLEAM for 'observed' ET.

For the models, we will likely not have access to long-term datasets except for long-term climate simulation datasets. For operational forecasts, there are often only short test periods to draw from. The user will likely need to be tasked to provide what they consider an appropriate model long-term seasonal mean value to compute anomalies from.

Further, the original VAC index estimates percentile anomalies empirically from the long-term anomaly time series. The user may need to provide the observed and model percentile values (or fitted Gaussian distribution parameters?). An alternative would be to use just the anomalies in anomaly units, not percentiles, but that often has more noise than percentile based anomalies.

Finally, the visualization of this metric is often done in a 2-d scatter plot, with varying levels of color coding depending on user preference. We will likely need to coordinate with forthcoming MetPlotPy enhancements across TBD issues.

Use Case Name and Category

Vegetation-Atmosphere Coupling (VAC) index as a land-atmosphere coupling diagnostic

Input Data

Observed ET and 2 m temperature for evaluation period
Modeled ET and 2 m temperature for evaluation period
User provided long-term seasonal means for both obs and model
User provided long-term percentiles for both obs and model

Acceptance Testing

Describe tests required for new functionality.
As use case develops, provide a run time here

Time Estimate

3-4 days

Sub-Issues

Consider breaking the new feature down into sub-issues.

Add a checkbox for each sub-issue here.

Relevant Deadlines

Approximately 6/30/2024

Funding Source

2785031 (Dyn Veg Drought)

Define the Metadata

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Projects and Milestone

Select Repository and/or Organization level Project(s) or add alert: NEED CYCLE ASSIGNMENT label
Select Milestone as the next official version or Future Versions

Define Related Issue(s)

There will be coordination with MetPlotPy and possibly MetCalcPy for VAC plotting and computation respectively.

New Use Case Checklist

See the METplus Workflow for details.

Answer 1 · 2024-04-05T20:55:48.000Z

Note that we may want to also add logic to METcalcpy to handle longterm climatology-based statistics.
Would need a separate METcalcpy issue to do so. This was discussed with @anewman89 on April 5, 2024 at a Land Project meeting.