# Presentation: 506956

## Contributions of GPS Data                    to Severe Weather Forecasting                in California

**Seth I. Gutman                                                                                                                       NOAA Earth System Research Laboratory                                                                       Boulder, CO  USA   80305**

**Notes:**
A NASA SeaWifs image of a large extratropical cyclone approaching the northern California coast in April, 2003.  The impact of storms like this and ways to improve heavy precipitation forecasts using systems like NDGPS will be discussed later.

## Outline

- Background
- What is GPS Meteorology?
- The storm of 4-5 January 2008
- Advances in monitoring and prediction
- Other applications
- Conclusions

## Acknowledgements

- 1 NOAA Earth System Research Laboratory, Boulder, CO 80305 USA
- 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, CO 80305 USA

## Background

- It’s also reasonable to assume that the techniques developed by geodesists to estimate atmospherically induced signal delays as nuisance parameters and (2) remove them to improve survey accuracy, were not developed to improve weather forecasts.
- So the recognition that ground based GNSS observations could be used to study the atmosphere can rightfully be called serendipitous.
- This is how GPS Meteorology works and how NOAA is using it  to improve its severe weather forecasts and warnings in California.

**Notes:**
One person’s noise is another person’s signal.  High accuracy NDGPS (HA-NDGPS) requires corrections for the signal delays caused by temperature, pressure and water vapor in the lower atmosphere. 

## Background

- It is the source of clouds and precipitation, and an ingredient in most major weather events.
- PW moves rapidly through the atmosphere, redistributing energy (latent heat) through evaporation and condensation.
- One of the most valuable attributes of GNSS is its ability to provide accurate water vapor estimates under all weather conditions, including thick cloud cover and precipitation.

**Notes:**
Assimilating GPS data into numerical weather prediction models results in significant improvements in weather forecast accuracy over periods ranging from 0h – 12 h.

## Background

- It is by far the most plentiful greenhouse gas;
- It absorbs and radiates energy from the sun;
- It affects the formation of clouds and aerosols and the chemistry of the lower atmosphere;
- So understanding & monitoring water vapor, and the effects it has on atmospheric radiation and circulation, is vital to the diagnosis and prognosis of  long-term changes in climate including droughts & severe weather.

**Notes:**
Warm air can sustain a higher concentration of water vapor than cooler air without becoming saturated. Consequently, as air warms, for whatever reason, more evaporation may take place and the concentration of water vapor may increase. An increase in water vapor enhances the greenhouse effect and gives rise to further warming. This positive feedback, warming from increased greenhouse gases leading to an increase of water vapor and therefore even more warming, is a feature of climate models used for estimating the effect of increased greenhouse gases. According to the 1990 Intergovernmental Panel on Climate Change (IPCC) report, Climate Change: The IPCC Scientific Assessment [Houghton et al., 1990], this feedback could amplify the temperature change due to a doubling of carbon dioxide by some 60%. The IPCC updates released in 1996 and 2007 do not change this conclusion. Thus an understanding of the mechanisms distributing water vapor through the atmosphere and of water vapor's effects on atmospheric radiation and circulation is vital to estimating long-term changes in climate. 

## Overview of GPS Meteorology

**GPS Signal in Ionosphere                                           **
- Refractivity associated with changes in electron plasma density or TEC between 50 and 400 km AGL.
- Signal delays in dispersive media are inversely proportional to frequency.
- Ionospheric delays are estimated (or removed) using dual frequency receivers.

**GPS Signal in Troposphere**
- Refractivity associated with changes in T,P,WV in neutral atmosphere.
- Signal delays are unrelated to frequency below 30 GHz.
- Delays must be modeled using assumptions about the structure and length-scale variability of these parameters.

**Notes:**
As the GPS signal travels from the satellite to a receiver near the ground it is slowed and bent (refracted) as it passes through the ionosphere and troposphere.  High accuracy GNSS positioning requires corrections for ionospheric and tropospheric refractivity.

## GPS Water Vapor Measurements

***Point Loma, CA (PLO5)******La Jolla, CA (SIO3)******San Diego, CA (NKXB)***

**Notes:**
This is a time series of water vapor retrieved from continuous GPS observations at the N/MDGPS site at Point Loma and the Scripps Institution of Oceanography in La Jolla just up the coast.  The fuchsia squares indicate the total amount of water vapor in the atmosphere calculated from weather balloons launched twice daily at the NWS upper-air facility near Miramar Naval Air Station, not too far away from Scripps.  What is the cause of the “dry” sounding at 12Z on January 5?  If we didn’t have some independent moisture observation to compare with the radiosonde, how could we tell this sounding was questionable?  What is the impact of a “dry” sounding on weather forecast accuracy?

## Major West Coast Storm 4-5 January 2008

-   Blizzard warning
-   Heavy snow warning
-   Winter storm warning
-   Flash flood watch
-   Flood warnings
-   Coastal flood watch
-   Coastal flood warning
-   Gale warning
-   Heavy surf warning
-   High wind warning
-   many others....

**Notes:**
The west coast storm of 4-5 January 2007 was a whopper.  NOAA’s forecasts of the timing and amounts of precipitation all along the coast, and estimates of rain at low elevations and snow fall in the mountains north of San Francisco and Sacramento were off by hours and as much as 60% in some cases.  These kinds of errors  are not atypical.  The keys to improving severe winter storm warnings are improved observations feeding advanced weather models, and providing forecasters with the diagnostic and prognostic tools they need to improve forecast skill over longer time spans.

## What Happened

- An exceptionally strong Extratropical Cyclone (a low pressure system) traversed much of the Pacific Ocean before the first main wave arrived late on January 3.
- The storm entrained moisture (water vapor) from the subtropics in a long narrow region called an “Atmospheric River.”
- The storm (actually a series of three storms) impacted the Pacific Coast from British Columbia to northern Mexico.

## What Happened

- Atmospheric rivers (AR’s) are formed as part of the "warm conveyor belt", a hemispheric cycle by which the cold dry air of the Arctic flows southward, and warm humid air from the tropics is forced northward.
- This "conveyor belt" plays a key role in the formation of Pacific storms.
- Modeling studies suggest that more than 90% of the total water vapor transported from the equator toward the poles are concentrated in these rivers.

## What Do AR’s Look Like?

- Atmospheric rivers (AR’s) represent the “business end” of west coast winter storms.

- Due to lack of observations, AR’s are poorly defined, monitored, and forecast.

**Notes:**
Atmospheric rivers, and how they are related to heavy precipitation events along the west coast were first described in detail in 2004.  Satellite and aircraft observations provided important information offshore, while weather radars and GPS onshore allowed us to monitor the evolution of these storms after they make landfall.

## The storm at time of major impact

- Note that major impacts were focused >500 miles south of the Low pressure center in this storm.
- This differs significantly from hurricanes, but the impacts are enormous and spread over a large area

***Many major impacts are associated with the landfall of the “atmospheric river” element of the storm, the precise characteristics of which are not currently monitored operationally offshore or onshore.***

- GOES IR image of major West Coast storm
-   Time = 0030 UTC 5 January 2008
-   Low pressure center is off WA coast

***L***

***Atmospheric river***

## L

- Pt. Piedras Blancas (**PPB**; 11 m MSL) + Three Peaks (Mountain site; 1021 m MSL)

***L***

- Three experimental observing sites monitored AR conditions upon land-fall

- Bodega Bay (**BBY**; 12 m MSL) + Cazadero (Mountain site; 475 m MSL)

**BBY**

**PPB**

- Goleta (**GLA**; 3 m MSL) + San Marcos Pass (Mountain site; 701 m MSL)

**GLA**

***Distance from BBY to GLA ~500 km, i.e. ***
***the average width of an atmospheric river***

**Notes:**
Here we’ll concentrate on data from the Bodega Bay and Cazadero sites.

## Typical AR Monitoring Site Bodega Bay (BBY)

- Typical AR Monitoring Site Bodega Bay (**BBY**)
- Radar Wind Profiler (1)                            Surface Met Sensors (2)                  GPS Receiver and Antenna (3)

- (1)

- (2) & (3)

**Notes:**
These are the typical elements of an atmospheric River monitoring site.  A Doppler radar wind profiler provides continuous information about the vertical wind profile and the height of the freezing layer.  A GPS receiver provides continuous information about the amount of moisture in the atmosphere above the monitoring site.  Surface meteorological sensors provide information about surface conditions including wind speed and direction, temperature, pressure, relative humidity and precipitation.

## GPS Water Vapor Measurements

***Bodega Bay, CA (BBY5)***

**Notes:**
The span of the AR is clearly evident in this GPS water vapor time series measured at Bodega Bay.  Precipitation started when integrated precipitable water vapor (IPW) exceeded 20 mm and ended soon after IPW fell below this level.  Why 20 mm?  We’re not sure yet, but the likely answer is going to be determined through careful modeling of these important events.

## Observations at Bodega Bay (upslope orientation = 230 deg)

**Observations at Bodega Bay** (upslope orientation = 230 deg)
- Max AR bulk flux: 115 units (total); 100 units (upslope component)
- Time of max AR flux: 1500-1600 UTC 4 January 2008
- Max wind in controlling layer (hourly avg): 39.4 m s-1; 194 deg
- Max IPW: 31.1 mm
- Max hourly rain rate: 0.8 in h-1 (20 mm h-1)
- Storm total rainfall at nearby mountain site: 10.39 in (264 mm)

- Wind speed, direction & return signal power from wind profiler
- Upslope wind speed and IPW from wind profiler and GPS
- IPW flux and rain rate from profiler, GPS & rain gauge

**Notes:**
This is how the Doppler radar wind profiler is combined with the GPS and surface observations to give a more complete picture of the dynamics and kinematics to the storm, especially with respect to the interaction of the AR with terrain as it makes landfall.

## Observations + 48-h forecasts

**Notes:**
This is a prototype of a weather forecaster’s decision support tool to help her improve forecast skill at time scales out to 48 hours.

## Conclusions

- A new forecast model moisture flux verification tool has been developed by NOAA’s Earth System Research Laboratory.
- The tool combines observations and NWP model output to maintain situational awareness and evaluate weather model performance.
- Plans are being made to transition the observing systems and moisture flux tool from NOAA Research into National Weather Service operations.

## Contact Information:

- Seth I. Gutman, Physical Scientist                                           NOAA Earth System Research Laboratory (ESRL)                325 Broadway R/GSD7                                                             Boulder, CO 80305-3328
- Phone: (303) 497-7031                                                                 FAX: (303) 497-6014
- Email:  Seth.I.Gutman@noaa.gov                                          Web: http://gpsmet_test.fsl.noaa.gov.

**Notes:**
If you want more information about this research or the contributions of the Nationwide Differential GPS Service to NOAA weather forecasting and climate monitoring, feel free to contact Seth Gutman at any time.

## Thanks for your Attention!

- Any Questions?

**Notes:**
Direct them to Seth... I’m not a weather guy.