Use of GPS RO in Operations at NCEP

Lidia Cucurull

NOAA

Joint Center for Satellite Data Assimilation

Outline

Introduction to COSMIC mission and GPS RO technique.

Status of the assimilation of GPS RO data into the NCEP’s Global Data Assimilation System.

Pre-implementation impact studies with COSMIC. Summary and future plans.

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

6 Satellites launched in April 15 2006

Three instruments:GPS receiver, TIP, Tri-band beacon

Demonstrate quasi-operational GPS limb sounding with global coverage in near-real time

Climate Monitoring web page:

www.cosmic.ucar.edu

GPS Occultation

Basic measurement principle:

Deduce atmospheric properties based on precise measurement of phase delay and amplitude.

Characteristics of GPS RO Data

Limb sounding geometry complementary to ground and space nadir viewing instruments– High vertical resolution (0.1 km surface - 1km tropopause)– Lower horizontal resolution (~300 km)

High accuracy (equivalent to < 1 deg K from 5-25 km) All weather-minimally affected by aerosols, clouds or precipitation Independent of radiosonde calibration No instrument drift No satellite-to-satellite observational bias

GPS radio occultation measurements & processing

s1, s2, a1, a2

s1, s2

1, 2

N

T, e, P

Raw measurements of phase and amplitude of L1 and L2

Raw measurements of phase of L1 and L2

Bending angles of L1 and L2

Bending angle

Refractivity

Single path

Spherical symmetry &Satellites orbits.

Ionospheric effect cancellation

climatology

Auxiliary meteorological data

Radio holographic method, Multi path

Forward Models:

Refractivity:

€

N = 77.6P

T+ 3.73×10−5 Pw

T 2

Bending angle:

)(

)(

ln2)(

2/122

nrx

dxaxdxnd

aaa

=

∫ −−=

∞

The JCSDA developed, tested and incorporated into the new generation of NCEP’s Global Data Assimilation System (GSI/GFS) the necessary components to assimilate two different type of GPS RO observations (refractivity and bending angle). These components include:– complex forward models to simulate the observations

(refractivity and bending angles) from analysis variables and associated tangent linear and adjoint models

– Quality control algorithms– Error characterization models– Data handling and decoding procedures– Verification and impact evaluation procedures

Milestones accomplished

COSMIC became operationally assimilated at NCEP on May 1st 2007, along with the implementation of the new NCEP’s Global Data Assimilation System (GSI/GFS).

Profiles of refractivity were selected for implementation in operations, while the tuning of the assimilation of bending angles is currently being analyzed at NCEP.

Several impact studies for selected periods show a positive impact in model skill when COSMIC profiles are assimilated on top of the conventional/satellite observations.

Milestones accomplished (cont)

GSI/GFS Impact study with COSMIC

Anomaly correlation (the higher the better) as a function of forecast day for two different experiments: – PRYnc (assimilation of

operational obs ), – PRYc (PRYnc + COSMIC

refractivity) We assimilated around 1,000

COSMIC profiles per day In general, the impact of the

COSMIC data will depend on the meteorological situation, model performance, location of the observations, etc.

November 2006

Fit to rawinsonde (November 2006)

•Dashed lines: PRYnc•Solid lines: PRYc (with COSMIC)

•Red: 6-hour forecast•Black: analysis

Summary

COSMIC became operationally assimilated at NCEP on May 1st 2007, along with the implementation of the new NCEP’s Global Data Assimilation System (GSI/GFS).

Profiles of refractivity were selected for implementation in operations, while the tuning of the assimilation of bending angles is currently being analyzed at NCEP.

Several impact studies for selected periods show a positive impact in model skill when COSMIC profiles are assimilated on top of the conventional/satellite observations. We have recently improved the assimilation of GPSRO profiles over complex topography.

What’s next?

More evaluation of the assimilation of GPSRO profiles operationally.

Update QC checks and obs error for refractivity. Improve the performance of the assimilation of observations of

bending angle (switch to BA in operations?). Testing, tuning and assimilation of GSPRO from

CHAMP/GRACE (in pre-operational mode) and MetOp/A GRAS (when available).

Assimilation of GPSRO observations into the regional model. Explore more complex FO to take into account horizontal

gradients of refractivity.