Report-back:
Elevation matters: How well do observations in your region cover local topography?

March 2024 - Led by Guo Yu and written by Sonia Nieminen

We set out to learn how Mountain Rain or Snow’s observations are distributed by elevation in order to get a better picture of how well these data describe their terrain.

In mountain regions, does our dataset cover local topography?

The reports of rain, snow, and mixed precipitation submitted by Mountain Rain or Snow’s observers are key to answering one of the central research questions behind Mountain Rain or Snow: How well do NASA’s satellite products estimate precipitation type from their vantage point in space, especially in mountain regions? To answer this question, team members on the Mountain Rain or Snow project compare ground-based precipitation observations submitted by community observers to the outputs of NASA’s satellite products. This results in a “report-card” on the success rates of those products (you can read more about how we assess one of NASA’s products, the GPM IMERG algorithm, in this report-back).

An important part of this work is making sure that the “report-card” offers the most comprehensive assessment of performance as possible. Imagine a student checking their grade for a class. If the student checks only the scores they’ve received for quizzes, but doesn’t check their scores for homework, the student may have a limited impression of their overall grade. In the same way, it’s important to take into account all aspects of Mountain Rain or Snow’s observations in order to do an unbiased assessment of the products' performance. In other words, we have to first understand our data before we can use it to answer our research questions.

Ecoregions and elevation gaps: The data tell a tale of highs and lows

With this in mind, we set out to learn how the reports submitted to Mountain Rain or Snow were distributed by elevation - a very important factor influencing precipitation phase. Elevation matters because just a few hundred feet can mean the difference between rain and snow.

First, we collected the elevation for each report and sorted reports into ecoregions, which group geographic areas by shared ecological features. Check out the map of EPA Level III ecoregions here. Then, we created plots of the number of observations compared to elevation for each ecoregion. Some examples are shown below, and all ecoregions’ charts are available at the bottom of the page.

A map of the United States with colored areas indicating ecoregion boundaries. A key on the outside of the map correlates each ecoregion to its title. Click on this image to open the PDF version of this EPA Level III ecoregions map.

Click the preview above to view the map of EPA Level III ecoregions (more info about ecoregions here).

A chart showing the amount of submitted observations of rain, mixed, and snow at each elevation in the Colorado Plateaus ecoregion.

Figure 1. The elevation distribution for observations submitted in the Colorado Plateaus. Notice how observations cover nearly all of this ecoregion's elevations.

A chart showing the amount of submitted observations of rain, mixed, and snow at each elevation in the Northeastern Highlands ecoregion.

Figure 2. The elevation distribution for observations submitted in the Northeastern Highlands. Notice how observations nearly all came from the lower 2,500 feet of this ecoregion.

We found that some ecoregions had observations distributed across all or nearly all elevations, and others had gaps. For example, reports in the Colorado Plateaus covered the elevational gradient quite comprehensively (see Figure 1 above). In contrast, in the Northeastern Highlands, where the highest elevations reach 4,000 feet, almost all observations were from below 2,500 feet (see Figure 2 above). Note that the elevation increments (the y-axis) are different for each ecoregion. This is because each area has its own unique terrain. Some ecoregions have high mountain peaks above 11,000 feet, and some regions don’t quite reach 1,000 feet.

Many ecoregions show a cluster of observations around one or two elevations - presumably, the location of population centers, or perhaps the preferred location from which a super-observer sends many observations. For example, many observations were submitted around 5,000 feet in the Central Basin and Range - right around the elevation of Reno, NV, which is at the western edge of this ecoregion (see Figure 3 below).

Understandably, lower coverage across elevations may be due to logistics like difficulty accessing certain elevations (e.g., reporting at 11,000 feet), or simply the fact that the majority of the ecoregion lies within a certain elevation range. Also, it is important to consider that areas with a lot of observers are more likely to be able to collectively access a range of elevations compared to areas where there are fewer observers.

A chart showing the amount of submitted observations of rain, mixed, and snow at each elevation in the Central Basin and Range ecoregion.

Figure 3. The elevation distribution for observations submitted in the Central Basin and Range.

Boots on the ground are needed to cover the terrain's diversity

When observations represent only a portion of the ecoregion’s elevational diversity, our analysis may not represent the overall performance of the success rates of NASA's satellite products. For example, if an ecoregion’s highest point is 7,000 feet, but nearly all observations come from below 3,000 feet, our analysis might show that the product performed well in the ecoregion when in reality it was poorly predicting rain vs. snow on the mountaintops. This would be like saying that a student has an A in the class because of their high quiz scores, without realizing that the student’s homework grade is very low. When we have gaps in our data’s elevational distribution, we have less certainty in the accuracy of our assessments.

Now that we understand how the observations are distributed by elevation in each ecoregion, we’re on our way to a more comprehensive understanding of how well these data can answer our research questions. Improving elevational diversity would improve our science, and we need your help to do this – what ideas do you have to help fill your ecoregion’s elevational gaps?

Here’s a way to get started: Access your ecoregion’s chart by using the drop-down tabs below, and find the elevations with few or no observations. Can you seek out ridges, mountain passes, and trails at high elevations with few reports? Or, if your region is missing reports from low or middle elevations, try hiking through a canyon or tagging along while a friend drives to a lower area. If you live, work, and recreate at elevations that already have high amounts of observations, keep sending reports from those locations and share the project with a friend or coworker who could submit observations from those underrepresented elevations. (Send them this link: rainorsnow.org/signup).

Where are your ecoregion's gaps?

Check out each ecoregion’s chart using the collapsible tabs below. These names of each region match the titles that the EPA uses to denote their Level III ecoregions, so their names may be different from our project’s keyword regions.