# Presentation: 440832

## Application of GPS Radio Occultation Data to Weather Analysis and Prediction

- Bill Kuo
- NCAR/MMM Division
- and
- UCAR/COSMIC Office

## GPS Radio Occultation

- The 24 GPS satellites are distributed roughly in six circular orbital planes at ~55o inclination, 20,200 km altitude and ~12 hour periods.
- Each GPS satellite continuously transmits signals at two L-band frequencies, L1 at 1.57542 GHz (~19 cm) and L2 at 1.227 GHz (~24.4 cm).
- A ray passing through the atmosphere is refracted due to the vertical gradient of density.

- GPS
- satellite

- Low Earth Orbiting
- (LEO) satellite

## GPS Occultation

- Basic measurement principle:
- Deduce atmospheric properties based on
- precise measurement of phase delay and amplitude.

## GPS/MET Experiment

- UCAR, JPL, and U. of Arizona, collaborated on the GPS/MET program in 1993, with a goal to demonstrate the radio occultation sounding technique for Earth’s atmosphere.
- The satellite was launched on April 13, 1995, and data were collected for two years.

**The GPS/MET Team**

## Two Recent GPS Occultation Missions

- SAC-C
- Argentine spacecraft carrying a multispectral imager and magnetometer
- Launched: Nov. 21, 2000
- 98.3o inclination
- 700 km orbit
- Both rising and setting occultation

- CHAMP
- a German mission for magnetometry and gravity mapping
- Launched: July 15, 2000
- 87o inclination
- 435 km orbit
- 5 year mission life

## Examples of GPS RO soundings

## A GPS/MET sounding through an intense upper-level front

- 0055 UTC October 22, 1995 over China

## Location of a CHAMP sounding

**Location of a CHAMP sounding**

## CHAMP compared to two radiosondes in Typhoon

**CHAMP compared to two radiosondes in Typhoon**

- CHAMP (red/blue lines): 2008 UTC 29 July 2001

-     18UTC 7/29/01	    Ishigakijima		00UTC 7/30/01

## CHAMP Comparison with Hualien radiosonde

- The Hualien station
- is affected by cloud and
- precipitation

## Detection of gravity waves with GPS

- From T. Tsuda

- Dry GPS/MET
- retrieval

## Characteristics of GPS RO Data

**Limb sounding geometry complementary to ground and space nadir viewing instruments**
**High accuracy (equivalent to < 1 deg K from 5-25 km)**
**High vertical resolution (0.1 km surface - 1km tropopause)**
**All weather-minimally affected by aerosols, clouds or precipitation**
**Independent height and pressure**
**Requires no first guess sounding**
**Independent of radiosonde calibration**
**No instrument drift**
**No satellite-to-satellite bias**

## How accurate is GPS/MET sounding?

- CHAMP and SAC-C Missions provide opportunity for Cross-comparison

## Cross-comparison between CHAMP and SAC-C

- From
- Hajj et al.
- (2003)

- Paris of SAC-C
- and CHAMP
- soundings
- within 30
- min of each
- other

## Comparison with Radiosondes

## Radiosonde Stations and Manufacturers

- Vaisala/Australia            IM-MK3/India Shang/China MEISEI/Japan Mars/MRZ          VIZ                           AIR                     Others

- From Junhong Wang

## Quantifying Regional Differences 

- Calculate the mean absolute difference in refractivity between CHAMP and Radiosondes (NCR)
- Calculate the mean of the absolute value of the difference in refractivity between CHAMP and the ECMWF (NCE)
- Compute the ratio (D = NCR /NCE)
- Perform calculation using radiosonde data from different regions of the world from June 2001 to present.

- From D.Rossiter (UCAR Summer Student)

## Regional Results 

| Region | Sonde Type | Average # of matches |  |  | Ratio
NCR/NCE |
| --- | --- | --- | --- | --- | --- |
| India  | IM-MK3 | 72 | 0.88% | 0.35% | 2.49 |
| Japan | MEISEI | 77 | 0.29% | 0.15% | 1.88 |
| China | Shang | 213 | 0.31% | 0.24% | 1.27 |
| Australia | Vaisala | 213 | 0.27% | 0.26% | 1.03 |

## India: Statistical Plots

- ECMWF - CHAMP

- Mean                      Standard                 Deviation                   Count

- CHAMP-Sonde

- ECMWF-Sonde

- Regional Measurements                      June 2001 to present

## Comparison of radiosonde performance

- IM-MK3 (India)

- Vaisala (Australia)

##  Preliminary Conclusions

- The RO technique is accurate enough to differentiate differences in performance of various types of radiosonde systems.
- IM-MK3 (used by India) appears to have the poorest performance.
- Vaisala (used by Australia and others) gives the best performance.
- These results are consistent with independent assessment performed by Wang et al. (2003).

## Using GPS soundings to evaluate global analyses

## ECMWF – NCEP/NCAR reanalysis

- 100 mb
- Temperature

## GPS – NCEP/NCAR Reanalysis

- 100 mb
- Temperature

## GPS – ECMWF analysis

- 100 mb
- Temperature

## Comparison between SAC-C and global analyses – Dec 2001

- 40 S – 90S

## Assimilation of GPS RO data

## Assimilation of CHAMP data

- Zou et al. (2003) performed an assimilation of CHAMP GPS RO data during the two-week period of 16-31 July 2002.
- Assimilation of bending angles using NCEP 3DVAR system with 6-h update cycle
- Data provided by UCAR COSMIC Data Analysis and Archival Center (CDAAC)
- Out of 3030 GPS RO soundings, 1427 penetrated below 850 mb
- NO-GPS experiment: Include all data currently used by NCEP
- GPS experiment: Include all data currently used by NCEP + CHAMP data

## Assimilation of CHAMP data

- Southern Hemisphere

- 500 mb height

- 500 mb height

- 850 mb q

- 850 mb q

- From Zou et al. (2003)

## Assimilation of CHAMP data

- 500 mb height

- 500 mb height

- 850 mb q

- 850 mb q

- From Zou et al. (2003)

- Northern Hemisphere - tropics

## Assimilation of CHAMP data

- From Zou et al. (2003)

- Southern
- Hemisphere

- 500 mb height

## A forecast impact trial with CHAMP radio occultation measurementsSean Healy, Adrian Jupp and Christian Marquardt

## Trial Run from UK Met Office

- We are assimilating CHAMP refractivity profiles provided by GFZ,
- between May 26, 2001 - June 11, 2001.
- Each profile contains ~120 refractivity values (150 max), with a vertical separation of 200m.
**We do not assimilate refractivity below 4km because of the well known biases. **
- Observation errors are based on Kursinski’s 1997 estimates, but we have inflated them to 2% at the surface, falling linearly to 0.2% at 10km. “QC” based on a 1D-Var calculation.
**Note, we only obtain ~40 measurements per assimilation cycle. **

## NWP forecast fit to radiosondes at 50hPa(Trop)

## NWP forecast fit to radiosonde 250hPa (SH) 

## Summary of UK Met Office Study (Healy et al. 2003)

- We have performed the 1st impact study with RO data at the Met
- Office. Given the small number of observations, results are
- very encouraging.
- PMSL in tropics improved, but largest impact seen in the southern
- hemisphere.
- Upper-troposphere and lower stratosphere show improvements
- in the  250hPa Temp (SH) and 50hPa(all) .
- The results would support the case for assimilating RO measurements
- operationally.
- Prospects of obtaining measurements from a constellation
- (e.g., COSMIC, ACE+) are very exciting in the light of these results.

## Case / model domain / GPS occs

**Case / model domain / GPS occs**

## Observations

** ****GPS refractivity **
** ****Conventional observations**
** ****QuickScat  surface wind vector**
** ****SSM/I  Precipitable water**
** ****SSM/I  Rainfall rate**
** ****SSM/I  vertically integrated cloud liquid water**
** ****SSM/I  surface wind speed **

**Observations**

##  Differences between GPS and No-GPS Experiments in MSLP

** ****Differences between GPS and No-GPS Experiments in MSLP**

##  Differences between GPS and No-GPS Experiments in MSLP

** ****Differences between GPS and No-GPS Experiments in MSLP**

## MM5 4DVAR assimilation of 47 GPS RO over a one-day period

- Prediction of an Antarctic cyclone

## Slide 43

## RO observational operator (1)

- How well does the Abel-retrieved *N(z)* represent the local atmospheric refractivity
- at the estimated ray tangent point in the presence of strong horizontal gradients?

- From Sergey Sokolovskiy

## RO observational operator (2)

- 1) Modeling of RO observable in straight-line approximation (Syndergaard et al., 2002).
-     Since the same approximation is used in forward and inverse modeling, the errors are
-     partially cancelled out.

- _Modeling of excess phase along straight line:_

- _Inversion of the straight-line excess phase:_

- 2) Curved trajectories (rays) corresponding to Abel-retrieved refractivity can be used,
-     instead of the straight lines, to improve modeling of RO observable (Ahmad, 1998)

## RO observational operator (3)

- Once the trajectories (straight or curved), used for modeling of RO observable, do not depend
- on the model refractivity, the modeled observable is linearly related to that refractivity

- where ***D*** is simply the length of ***j**** *ray inside ***k**** *voxel
- or contribution from ***k*** pixel (interpolation required).
- Index ***k*** (a 3-*D* set of indices) counts all voxels
- (pixels) contributing to excess phase along rays.

- _1) Forward linear operator:_
- _(straight-line excess phase path)_

- _2) Inverse linear operator:_
- _(Abel-inverted refractivity)_

- _Observational linear operator:_

## COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate)

-  6 Satellites launched in 2005
-  Three instruments:
- GPS receiver, TIP, Tri-band beacon
-  Weather + Space Weather data
-  Global observations of:
- Pressure, Temperature, Humidity
- Refractivity
- Ionospheric Electron Density
- Ionospheric Scintillation
-  Demonstrate quasi-operational GPS limb sounding with global
-       coverage in near-real time
-  Climate Monitoring

## Slide 48

## COSMIC Spacecraft Status

- GPS receiver

- Minotaur
- launch vehicle

- COSMIC
- spacecraft

## COSMIC Soundings in a Day

- Red dots are current radiosonde sites, green dots are 2500 COSMIC soundings in 24-h.

**Notes:**
Created by Bill Schreiner, 18 Oct. 2002.  6 S/C in 6 planes.  24 GPS satellites.  Azcut = 42 degrees (the Azcut angle is the cutoff for how far out of the LEO plane the occultations are allowed to happen.  The nominal value of Azcut is 45 degrees.

## GPS radio occultation missions

| Mission | Launch-Duration | # Soundings/day | Remarks |
| --- | --- | --- | --- |
| GPS-MET | 4/1995        2+ | ~125 | Proof of Concept |
| CHAMP | 11/2000      ~5 | ~250 | Improved receiver, tracking |
| SAC-C | 11/2000      ~3 | ~500 | Improved receiver, open loop tracking test |
| GRACE | 5/2002        ~5 | ~500 | RO data not yet available |
| COSMIC | 9/2005        ~5 | 2500-3000 | Real time-ops |
| TerraSAR-X | 7/2005        ~5 | ~400 | COSMIC RX & Antennas |
| EQUARS | 7/2006        ~3 | ~400 | COSMIC RX & CHAMP antennas |
| METOP | 5/2007        ~5 | ~500 | Real time - ops |
| NPOESS | 3/2008        ~10 | ~500 | Real time-ops. Ionosphere |

## COSMIC + EQUARS Soundings in a Day

**COSMIC**** **
**EQUARS **
**Radiosondes**

## Research Opportunities

- Develop cost-effective assimilation techniques for GPS RO data assimilation:
- Comparison of 3DVAR/4DVAR/EnKF assimilation approaches
- Comparison of different observation operator (ray tracing, refractivity,... etc)
- Optimal coupling between Nadir passive satellite measurements with GPS RO data
- Demonstrate the value of GPS RO in global and regional scale weather analysis and prediction.
- Use of GPS RO data to study atmospheric circulations where traditionally there are few or no soundings (e.g., oceans, polar regions, Tibetan plateau, ...etc)

- Use of GPS RO data (from on-going and future missions) to evaluate the performance of global climate models (e.g., CCSM)
- Geopotential height changes, refractivity climatology
- Use of GPS RO data to improve the understanding of global hydrological cycle (e.g., combining GPS RO with other satellite data).
- Study the exchange between upper troposphere and lower stratosphere (GPS RO is of highest accuracy in this region)
- Use of GPS RO data in field programs (location of an occultation can be predicted two weeks in advance)
- Many other possible research topics

## Operational Demonstration

- A major objective for COSMIC is to demonstrate the usefulness of GPS RO soundings in operational numerical weather prediction.
- Many operational NWP centers have expressed strongly interest in using the COSMIC data:
- NCEP
- ECMWF
- UK Met Office
- Met. Services of Canada
- Chinese Met. Administration
- Danish Met. Institute
- German Weather Service
- Japan Met. Agency
- Major impact in Op. NWP
- 	is expected!

- Canadian Met. Center – 7/14/03

## Summary

- GPS radio occultation technique is a very promising atmospheric observing system for both weather and climate:
- High vertical resolution
- High density (depending on the number of missions)
- High accuracy
- No instrument drift (important for climate detection)
- GPS RO technique is here to stay (COSMIC is coming, and more missions are being planned)
- GPS RO data will be valuable to:
- climate change analysis, validating of climate simulations
- operational weather prediction
- ionospheric research and space weather forecasting