Precipitation

Reigning in the Rain with Satellite and Radar

By Marguerite Huber

When it rains, it pours!background_water_puddle

Actually, that phrase is not necessarily true. A rain shower can consist of just drizzle, a steady rain, a downpour, or even all three! Either way, accurate rain totals are the basis of watershed modeling for evaluating the water cycle.

Meteorological data (precipitation, temperature, humidity, etc.) required for watershed assessments have traditionally come from land-based weather gauge stations. They collect weather data from all over the country. Unfortunately, not all watersheds have meteorological stations. Some watersheds have too few, are too far away, or aren’t working properly to correctly represent precipitation totals or their distribution within the watershed. You can check the National Oceanic and Atmospheric Administration’s website to see how many weather gauge stations are in your watershed!

For accuracy, the best options for watershed modeling applications in the U.S. are rain gauges and weather radar data, but precipitation amounts can vary throughout the watershed. Where land-based stations are lacking, remote sensing and radar satellite data are increasingly being used to augment data in space and time.

EPA scientists were involved in a study aimed at providing options for watershed modelers. They did this by comparing precipitation data from radar-based stations to data from ground-based stations to see the effectiveness of using either one for watershed modeling, especially at locations where gauge stations were insufficient.

Because ground-based gauges are the norm, the scientists evaluated the efficacy of using radar or gauge precipitation data to support watershed modeling.

Researchers evaluated two areas in Wisconsin using hourly precipitation data from 2002-2011: the Manitowoc River Basin and Milwaukee area, which are approximately 84 miles apart.

National Climatic Data Center precipitation data from gauges on the ground were compared to two different types of satellite and radar data: North American Land Data Assimilation System and NEXt generation RADar Multi sensor Precipitation Estimates. Both were used to evaluate the reliability of radar and gauge precipitation data.

Results showed gauge and radar data at Milwaukee to be similar, while the Manitowoc River Basin had large differences in precipitation occurrence and totals, which strongly suggest radar data as being more reliable.The gauged precipitation at Manitowoc River Basin also poorly correlated with radar data, which can detect more frequent precipitation, drizzle, and small storms.

In the end, the researchers concluded that the use of radar precipitation data can be an acceptable alternative to the gauged data in Manitowoc River Basin. The results also show benefits from automating the collection process of radar data as an additional option in watershed modeling.

With options of using land-based or radar data, scientists will be able to conduct more accurate watershed assessments, providing important information for keeping our watersheds healthy.

About the Author: Marguerite Huber is a Student Contractor with EPA’s Science Communications Team

Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action.

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When It Rains, It Pours: The Climate Link Between Extreme Precipitation and Drought

By Allison Crimmins

flooding image with traffic light in foregroundFrom the photos my Colorado friends posted this summer, I wasn’t surprised to learn that 2013 has been the wettest on record for Boulder. However, Boulder also experienced drought, the most destructive wildfire in Colorado’s recorded history, and a week of record heat. How is it that Colorado can experience both extreme wetness and extreme dryness in one year?

These seemingly conflicting events fit a pattern that scientists expect to occur under global climate change—a pattern that has been developing for the past few decades over many parts of the United States. Precipitation (rain, snow, etc.) is increasing, and more of it is coming in the form of downpours, blizzards, and other intense bursts, with longer dry spells in between. If you add up the percentage of land in the U.S. where a greater-than-normal amount of total precipitation fell in the form of intense single-day events, eight of the top 10 years for extreme precipitation have occurred since 1990.

How does it work? Imagine our atmosphere as a sponge passing over the land’s surface, soaking up moisture through evaporation and occasionally wringing out the collected water through precipitation. Warmer air can hold more water vapor, so the sponge absorbs more in a warmer world, leaving the land drier than it used to be. Eventually, the sponge becomes waterlogged—and when it’s finally wrung out, often miles away from where it picked up the water, it releases a huge amount all at once. This pattern occurs now in many parts of the U.S. and around the globe. Warmer air causes more evaporation, which leaves dry areas drier, but also results in heavier rainfalls.

If current trends continue, we can expect more droughts as well as more devastating deluges, like the one that resulted in loss of life, massive property damage, and thousands of evacuations throughout Boulder and Larimer Counties. As my friends join the rest of Boulder in rebuilding, city planners and emergency managers will keep these growing risks in mind so the city is better prepared for future extreme weather.

About the Author: Allison Crimmins is an environmental scientist with EPA’s Climate Change Division, where she focuses on the impacts and risks associated with climate change. Prior to joining the EPA she studied oceanography, climate science, and public policy. She lives, works, and cooks a mean strawberry rhubarb pie in Washington, DC.

Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.

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Science Wednesday – Modeling Matters: It Was Supposed To Rain!

Each week we write about the science behind environmental protection. Previous Science Wednesdays.

By Tanya Otte

By early June, my yard was already parched. The drought-tolerant annuals planted to brighten things up were suffering, but relief was on the way. Yielding to the forecast and my shortage of time, I skipped watering the plants. When I got home, the rain gage was bone dry. Eyeing the wilted flowers, I muttered: “But it was supposed to rain today!”

Supposed to rain?!? As if the forecast was a guarantee.

Why are weather forecasts for rain so often wrong?

Forecasts for rain are seeded by weather models. While the science and computing power behind those models have advanced in recent years, rain prediction remains one of the most difficult tasks…even just a few hours ahead of time, and particularly in the summertime.

Precipitation is the result of extremely complex atmospheric processes, many of which are at time and space scales that are not well represented in the operational weather models. The models that provide insight into daily weather forecasts cover the forecast area (either the whole U.S. or some region) as tiles that are often about 7.5 miles on each side. Depending on the terrain elevation, land-water boundaries, and urban-rural distinctions, the weather conditions can be different even within each tile.

Predicting thunderstorm activity is challenging, even for the most experienced meteorologists. Models can tell us if the large-scale and the regional-scale weather conditions (e.g., low-pressure system, cold front, jet stream, sea breeze) would be conducive for thunderstorms to form in a certain area, but not exactly where and when. It’s like putting a pot of water on the stove with the heat on high. You’ve created conditions that will result in boiling (convective activity), but you won’t be able to predict where or when the first bubbles will form.

Rain is also sensitive to subtle changes in wind, moisture, temperature, and pressure in columns of air that extend from the ground upward. This can affect the rising and sinking of air and thus determine whether rainfall occurs. Slight errors in the predictions of any of these atmospheric characteristics will affect the accuracy of the precipitation forecast. Modeling precipitation is tricky business, and it can affect your air quality forecast, too!

Next time you think it’s supposed to rain, give your meteorologist a break!

About the author: Tanya Otte, a research physical scientist, has worked at EPA in atmospheric modeling and analysis since 1998.

Editor’s Note: The opinions expressed in Greenversations are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.

Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.

Please share this post. However, please don't change the title or the content. If you do make changes, don't attribute the edited title or content to EPA or the author.