California

How ground-based GPS stations help weather forecasters predict heavy rain and flooding

Beachgoers flock to Avila to beat the heat in SLO County

Record-breaking temperatures in San Luis Obispo County cause people to escape the heat at Avila Beach on Saturday, September 2, 2017.
Up Next
Record-breaking temperatures in San Luis Obispo County cause people to escape the heat at Avila Beach on Saturday, September 2, 2017.

Geodesy is the study of Earth’s shape, gravity field and rotation.

An excellent method to study the Earth is by use of high-precision Continuous Global Positioning System (CGPS) stations that are firmly mounted on bedrock and can measure the slow, persistent ground motion of Earth’s crustal plates down to a few millimeters over time.

In the western United States, there are more than 1,200 CGPS stations, including more than 25 on the Central Coast. A few of these stations are classified as Global Positioning System Meteorology (GPS-Met), such as the ones located in Cambria, Los Osos and Point Sal.

Most of these stations are operated by UNAVCO (a nonprofit university-governed consortium), the Southern California Integrated GPS Network operated by the U.S. Geological Survey, Scripps Institution of Oceanography and other groups such as Caltrans. Data from these stations are readily available for all at www.unavco.org.

Information from these stations is used to monitor motions caused by plate tectonics, but also for other applications, like subsidence of the valley floor in the San Joaquin Valley.

It was discovered that these stations can also monitor the amount of water vapor in the atmosphere, which is critical in determining the potential for flooding, especially during atmospheric river (AR) and North American monsoon events. Here’s how:

Angelyn Moore is a scientist in the Geodynamics and Space Geodesy group at Caltech’s Jet Propulsion Laboratory (JPL) in Pasadena. Her Ph.D. dissertation was in time and frequency. She is currently coordinating an international federation of geodetic GPS agencies, and GPS data analysis for earth science applications.

She told me, “The GPS positioning technique is based upon measuring signal travel time from the GPS satellites to the receiving antenna of the CGPS stations to estimate the geometric distance between them. However, the signal is also subject to delays due to the total electron content in the ionosphere and the amount of moisture and total density of the troposphere. In estimating the position of the ground station to an accuracy better than two millimeters, it is necessary to account for these atmospheric delays.”

In other words, if you know the delay of the GPS radio signal caused by the ionospheric (the highest part of our atmosphere) most of the additional delay is caused by the amount of water vapor in the tropospheric. The tropospheric is the lowest layer of our atmosphere, which starts at sea level and extends upward to approximately 33,000 feet, depending on your latitude and the atmospheric conditions; Pretty much all the weather we experience on Earth occurs in the troposphere, which contains roughly 99 percent of the atmosphere’s water vapor.

The amount of water vapor is used to determine precipitable water (PW). If you took all the water vapor in the atmosphere above a fixed location at a specific time, condensed it and then deposited it on the Earth’s surface, that would be the PW value. In our area, PW is typically measured in inches. However, a PW of one inch does not indicate it will rain 1 inch. The amount of water vapor condensed is determined by many atmospheric factors, like dew point temperature and orthographic enhancement.

The National Weather Service (NWS), PG&E and other forecasters use radiosonde data from weather balloons typically launched from Vandenberg Air Force Base and other sources to determine PW values for the Central Coast.

The higher number of the GPS-Met stations over the years has dramatically increased the amount of PW data available to meteorologists, allowing them to more readily detect rapid increases in moisture over a fixed location.

This additional PW information allows NWS forecasters to issue more timely flood watches and warnings.

As the atmosphere continues to warm, it will be able to hold more water vapor, producing more intense rainfall events; consequently, these flood watches and warnings will become more critical.

Along the Central Coast, atmospheric rivers will become more of a concern.

These ARs can stretch for thousands of miles across the world’s oceans, but they are only a few hundred miles wide. They can draw vast amounts of water vapor into narrow bands ahead of cold fronts and transport ridiculous amounts of water across vast expanses. In fact, they can carry more fresh water than the Amazon River.

Along the West Coast, they are informally called the “Pineapple Express.” The Pineapple Express is a subset of an atmospheric river event that originates in the tropical waters near Hawaii; hence the pineapples. In the past, meteorologists simply referred to these as “the hose.”

In March 1995, an AR produced incredible 24-hour rainfall totals with Diablo Canyon reporting 8.4 inches of rain. San Simeon saw sustained wind speeds of 70 mph with gusts reaching 88 mph. Locations in the hills above Cambria reported rainfall amounts exceeding 12 inches over 24 hours.

PG&E is reaching out to about 150,000 customers, including 3,822 in San Luis Obispo and 2,393 in northern Santa Barbara counties, encouraging them to update their home phone number, mobile number, email and other key information so the company can contact them in advance should it become necessary to temporarily turn off power for safety this fire season. Please visit www.pge.com/mywildfirealerts or call the PG&E contact center at 1-866-743-6589 to update your information.

Related stories from Fresno Bee

  Comments