Measurement of Precipitable Water Vapor
Using the MET4 or MET4A Meteorological Measurement System and Global
Positioning System Radio Signals
Water vapor is the
engine of the weather. The unusually large latent energy
associated with waters phase changes significantly affects
the vertical stability of the atmosphere, the structure and
evolution of storm systems, and the meridional energy balance of
the atmosphere. Hence, the distribution of water vapor plays a
crucial role in weather and global climate.
What is GPS Meteorology?
Positioning System (GPS) Meteorology is the application of GPS
data to the monitoring and analyses of atmospheric conditions.
Atmospheric monitoring can be done by both ground-based and
space-based GPS applications.
transmit radio signals that can be inverted to measure
atmospheric profiles of refractivity. The refractivity profile
can be transformed to profiles of tropospheric humidity given a
receivers at fixed locations can be used to gather data that can
be used to determine integrated Precipitable Water Vapor (PWV).
Atmospheric scientists have shown that GPS-determined PWV
observations can significantly improve weather forecasting
Why is the Measurement of PWV important?
Positioning System (GPS) tracking network was established to
provide high precision navigation and geodetic positioning. The
system consists of satellites in orbit around the earth and a
ground-based network of support stations to update the
ephemeredes and clocks.
of GPS satellites transmit atomic-clock controlled L-band
signals to receivers on the earth. Time delays of the signal
travel paths from multiple satellites to a single receiver are
used to establish the ground position of the receiver. Data
recorded by GPS receivers at fixed locations will show signal
path delays caused by a variety of effects.
One class of
GPS-signal delays can be directly attributed to the passage of
the signals through the Earths ionosphere and atmosphere.
delay is dispersive (frequency dependent) and can be determined
by observing both of the frequencies transmitted by the GPS
satellites (L1 & L2) using a dual-band GPS receiver. These
ionospheric delays can be eliminated without reference to
observations recorded by other GPS receivers ¹.
The remainder of
the atmospheric delay, or neutral delay, is not dispersive and
cannot be estimated from single station observations. However,
it is possible to parameterize the neutral delays affecting each
station in a network of GPS receivers and to estimate these
parameters jointly as part of the overall geodetic inversion for
the relative geometry of the network and the precise orbits of
the satellites.² (Hoffman-Wellenhof et al., 1993). The physical
basis for estimating the zenith delay parameters is that a GPS
receiver is typically tracking 4 to 8 satellites simultaneously
and over-determines the zenith delay parameters without regard
to azimuth or elevation.
The neutral delay
can be decomposed into a "hydrostatic delay"
associated with the "dry" atmosphere and a
"wet" delay associated with the permanent dipole
moment of water vapor.
frequent, and dense sampling of water vapor is needed for
operational weather forecasting as well as for weather and
climatic research. Given the present operational weather data
system, inadequate resolution of the temporal and spatial
variability of water vapor has been cited as the single greatest
obstacle to improved short range precipitation forecasts.
Mesoscale numerical model simulations have shown that when
model-predicted precipitable water is relaxed toward an observed
value, the model recovers the vertical structure of water vapor
with an accuracy much greater than that from statistical
retrieval based on climatology, leading to significantly
improved short range precipitation forecasts. One can infer that
GPS water vapor data will be similarly valuable to longer range
and global numerical simulations.
using the NCAR/PennState mesoscale weather model, Kuo et al.,
1995, have shown that the combination of PWV and surface
humidity data significantly benefits the numerical models. A 20%
improvement in the numerical weather forecasting was achieved
when PWV time-series constrained the model. An additional
improvement was achieved when the surface humidity was included.
How is PWV estimated from GPS Receiver Data?
hydrostatic delay (ZHD) has a magnitude (equivalent GPS phase
delay length) of about 2300 mm at sea level. It is possible to
predict the ZHD to better than 1 mm given surface pressure
measurements accurate to 0.3 hPa (millibar) or better.
The Zenith Wet
Delay (ZWD) can vary from a few millimeters in desert conditions
to more than 350 mm in very humid conditions. It is not possible
to predict the wet delay with any useful degree of accuracy from
surface measurements of pressure, temperature, and humidity.
However, once the ZHD parameters have been estimated from the
measured GPS neutral delay, it is possible to estimate ZWD by
subtracting ZHD from the ZND where the ZHD is derived from the
surface pressure readings.
The Zenith Wet
Delay can then be transformed into an estimate of Precipitable
Water Vapor by using either a numerical weather model or a
statistical/analytical model of the vertical temperature
distribution at the receiver site.
Calculation of PWV
The steps to
estimate PWV from MET3/GPS measurements are:
1) Determine the
total station delays (or zenith total delay integrated over all
azimuths and elevations) from the GPS station network with each
station tracking multiple GPS satellites.
Total Delay = Ionospheric
Delay + Neutral Delay.
2) Measure the
Ionospheric delay from comparison of the L1 and L2 GPS signals
recorded with a dual-band GPS receiver, and calculate the
Neutral Delay = Total Delay
- Ionospheric Delay.
3) Calculate the Zenith
Hydrostatic Delay from the barometric pressure, temperature, and
humidity as measured by the Paroscientific MET3.
Zenith Neutral Delay =
Zenith Hydrostatic Delay + Zenith Wet Delay.
4) Calculate the Zenith Wet
Zenith Wet Delay = Zenith
Neutral Delay - Zenith Hydrostatic Delay.
5) Estimate Precipitable Water
Vapor from the Zenith Wet Delay. PWV can be
estimated from either a numerical model or a
statistical/analytical temperature model.
represents a milestone improvement in environmental sensing
technology. More accurate prediction of storm systems will
improve surface, coastal, and air travel safety. Agriculture and
farming will greatly benefit from these models by improving crop
yields and better understanding micro-climates.
ground-based measurements of barometric pressure, temperature,
and humidity necessary to determine precipitable water vapor are
made with the Paroscientific MET4 or MET4A Meteorological Measurement
Systems. The MET4/4A is directly compatible with most GPS reference
stations. And the ease of installation, high accuracy, and excellent
reliability of both instruments makes then an ideal choice for critical
installations where high quality data is necessary.
Reference Station Monument
Paroscientific's MET4/4A are being deployed world-wide to supplement
GPS data and provide accurate precipitable water vapor
information. For more information about the MET4 and
Fan-Aspirated MET4A, contact your local Paroscientific
representative or our sales and applications engineers.
¹ J. Duan, M. Bevis, P. Fang, Y.
Bock, S. Chiswell, S. Businger, C. Rocken, F. Solheim, T. van
Hove, R. Ware, S. McClusky, T. Heering, and R. King, 1996: GPS
Meteorology: Direct Estimation of the Absolute Value of
2 S. Businger, S.
Chiswell, M. Bevis, J. Duan, R. Anthes, C. Rocken, R. Ware, M.
Exner, T. van Hove, and S. Solheim 1996 - The Promise of GPS
Meteorological Sensors Co-located with GPS Receivers for
Geophysical and Atmospheric Measurements