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 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; along the Central Coast there are over 25.
A few of those stations are classified as Global Positioning System Meteorology, or GPS-Met, including the ones located in Cambria and Los Osos and at 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 such other groups as Caltrans.
Data from those stations are readily available for all at www.unavco.org/.
Information from the stations is not only 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 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 in Pasadena.
Her doctoral 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.
“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,” Moore told me.
“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,” she continued.
“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 single 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, the lowest layer of our atmosphere, starts at sea level and extends upward to approximately 33,000 feet, depending on your latitude and the atmospheric conditions.
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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, or PW.
If you took all the water vapor in the atmosphere above a fixed location at a specific time, condensed all of 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.
National Weather Service, Pacific Gas and Electric Co. 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 rising number of 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.
That additional PW information allows National Weather Service 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, flood watches and warnings will become more critical.
Along the Central Coast, atmospheric rivers will become more of a concern. 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," 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.
The winds produced an enormous amount of orographic enhancement. Locations in the hills above Cambria reported rainfall amounts exceeding 12 inches over 24 hours.
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PG&E is reaching out to about 150,000 customers, including 2,393 in northern Santa Barbara County, encouraging them to update their home phone numbers, mobile numbers, email addresses 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.
Visit www.pge.com/mywildfirealerts or call the PG&E contact center toll-free at 866-743-6589 to update your information.
John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at firstname.lastname@example.org or follow him on Twitter @PGE_John.