Water Vapor Profiles:Evaluating COSMIC Capabilities and Limitations Related to Climate Needs

Marty Ralph1, Dian Seidel2, Gary Wick1

1NOAA/OAR/Earth System Research Laboratory2NOAA/OAR/Air Resources Laboratory

22 January 2008, New Orleans

Water Vapor: A critical variable for Climate

• Role as a potent greenhouse gas– Trend detection– Reference atmospheric profiling

• Role and trends in global water cycle– Extreme events (floods and droughts)– Mean annual precipitation– Part of forcing for hydrology– Water supply– Ecosystems

• Role in earth’s albedo via clouds and aerosols– Impacts on global temperature

Climate issues that COSMIC might address (not an exhaustive list)

• Reference Atmospheric Profile Observations: need for highly accurate, long-term upper-air temperature and humidity (separable from each other) profiles, from the surface through the lower stratosphere (or even higher)– GCOS reference upper-air network (GRUAN) has noted the promising potential

of GPS/RO methods to make an important (but still not fully demonstrated???) contribution

• UTLS (upper troposphere/lower stratosphere) Water Vapor: – radiative effects of UTLS WV – cloud microphysical processes – WV feedback processes 

• Tropopause Structure and Variability: – Strat/trop coupling and exchange– Multiple tropopause structures– Tropopause as a climate change indicator

Selected technical assessments are needed

• Much of existing work explores ability of COSMIC to help with correction for inter-satellite biases

• Need to assess the precision and stability of COSMIC water vapor and temperature profiles in the lowest few kilometers– (Still seems to be concerns about the accuracy of COSMIC at

the lowest levels – can COSMIC consistently provide the required accuracy over time?)

– Confirm there is not a significant dependence on model profiles used to initialize the retrieval

– Provides motivation for a comparison of COSMIC soundings and dropsondes

• With necessary precision, it would be possible (in future) to evaluate small changes in the distribution of water vapor in the atmosphere

Water Vapor: A critical variable for Climate

• Trends detected in vertically integrated water vapor

• Tropical broadening

• Water vapor transport and atmospheric rivers

• Reducing uncertainty in climate change projections

• Downscaling to watershed scales

An exampleRegarding the Water Cycle

Extreme events and the role of Atmospheric rivers

IPCC model runs show evidence of increased extreme rainfall in west coast of North America at mid latitudes (Hegerl et al. 2004, J. Clim.)

Change in wettest day of the year by time of CO2 doubling in two models.

Color scale shows %change over present-day value. Values are plotted only if statistically significant.

Climate Change projections for extreme rainfall

ECMWF Model diagnostic study by Zhu & Newell 1998 (MWR)

35N

river is 95%of total flux

river <10% ofzonal circumference

35S

Observational studies by Ralph et al. (2004, 2005, 2006) extend model results:1) Long, narrow plumes of IWV >2 cm measured by SSM/I satellites considered proxies for ARs.2) These plumes are typically situated near the leading edge of polar cold fronts.3) P-3 aircraft documented strong water vapor flux in a narrow (400 km-wide) AR (along AA’).4) Airborne data also showed 75% of the vapor flux was below 2.5 km MSL.5) Moist-neutral stratification <2.8 km MSL, conducive to orographic precip. boost & floods.

400 km

Enhanced vapor flux in Atmos. river

Rain >10 mm/h:>12.5 m/s; >2 cm

Ralph et al. (2006), Geophys. Res. Lett.

• SSM/I satellite image shows atmospheric river

• Stream gauge data showregional extent of high stream flow(roughly 500 km of coast)

4217 mm1859mm (44% of 4-winter rainfall)

• Four winters of data recorded at CZD (NW of San Francisco) between 2001 – 2006.

• 9548 points of hourly data, with 1853 hours of rainfall totaling 4217 mm.

• IWV plumes >2 cm tagged by SSM/I satellites.

• GPS IWV >2 cm at BBY for at least 8 consecutive hours.

• Wind speed >13 m s-1 (~25 kts) at controlling layer (850-1150 m MSL) at BBY.

• 31 Atmospheric rivers produced 44% of the observed rainfall.

Key Question Addressed Recently

• How well does COSMIC occultation data represent water vapor profile information?– Recent studies in the Pacific indicate COSMIC

adds information over the model first guesses used in the retrievals (see Neiman et al. presentation this Wed AM in IOAS conference)

• 12 COSMIC soundings used to construct X-section along NW-SE axis through the AR• The COSMIC soundings yield cross-sectional thermodynamic structures comparable in

character and detail to previous aircraft-based dropsonde surveys.

COSMIC-Derived Cross Section of an Atmospheric River

Tropopause

Polar cold

front

Tradewindinversion

Atmosphericriver

Reverse thermalgradient - LLJ

From Neiman et al., Diagnosis of an Intense Atmospheric River Impacting the Pacific Northwest: Storm Summary and Offshore Vertical Structure Observed with COSMIC Satellite Retrievals, submitted to MWR, 2008.

COSMIC Data Impact on Soundings of an Atmospheric River

• 12 soundings transecting an atmospheric river were composited

• Differences between COSMIC soundings initialized with GFS and ECMWF models were compared with initial model differences

• In lowest 2.5 km MSL (i.e., where ¾ of vapor flux occurs in ARs), the COSMIC solutions converge relative to their model counterparts... especially the moisture

• Results show that COSMIC-based water vapor profiles are an improved representation of the true water vapor profile in this atmospheric river environment where these distributions are critical.

75% of vaporflux below

2.5 km in ARs

From Neiman et al., Diagnosis of an Intense Atmospheric River Impacting the Pacific Northwest: Storm Summary and Offshore Vertical Structure Observed with COSMIC Satellite Retrievals, submitted to MWR, 2008.

Major Gaps Associated with the Global Water Cycle

• Vertical profiles of water vapor with global spatial coverage, and good temporal resolution

• The boundary layer contains the most water vapor, but satellite sounders typically perform poorly at low-altitudes

• Water vapor transport is not monitored well (and not at all in remote areas), and yet will likely change in a changing climate; water vapor profiles are a key to this

• Downscaling of climate projections involving the water cycle will require accurate reanalysis data with documented uncertainties

• Rich structure in IWV!

• But, what is the vertical structure?

PW estimated from COSMIC GPS RO data

Punch Line

• Water vapor profile information is needed for a variety of questions, especially regarding the water cycle

• COSMIC is showing some promise on this, and should be explored

From: Wee and Kuo (2008)

PW retrieved from COSMIC GPS RO data using NCEP or ECMWF analysis as first guess

PW derived from NCEP or ECMWF analyses

Comparison of PW data from COSMIC and global analyses

From Wick et al., Intercomparison of Integrated Water Vapor Retrievals from SSM/I and COSMIC, submitted to GRL, 2007.

Comparison of SSM/I and COSMIC Retrievals of Integrated Water Vapor

• COSMIC and SSM/I retrievals of IWV collocated for 2 months in the Eastern Pacific

• COSMIC data exhibits strong overall agreement with 4 different SSM/I IWV retrieval algorithms

• Results suggest COSMIC-derived IWV retrievals are a valuable new validation source for SSM/I IWV products

From Wick et al., Intercomparison of Integrated Water Vapor Retrievals from SSM/I and COSMIC, submitted to GRL, 2007.

Dependences of SSM/I and COSMIC IWV Differences

• Differences found to depend on position, cloud liquid water content, precipitation, and resolution

• Results suggest new methodology for quantifying uncertainty in IWV retrievals based on coincident environmental parameters

Latitude Cloud Liquid Water

HAWAII NOAA P-3 campaign: Mar-Apr 2005

*A composite of descending (p.m.) SSMI satellite overpasses on threeconsecutive days.

*NOAA P-3 observations were made through a developing atmos.river on two consecutive days just prior to flooding rains hit the drought-stricken Pacific NW

*NOAA/ETL wind profiler at Astoria, OR recorded 70-kt LLJ within the atmos. river on 26 March

*Nearby rain gage recorded 7.5 inches of rain during theatmos. river event

Integrated water vapor (cm)

24 Mar 05

25 Mar 05

26 Mar 05

South Fork & Astoria, OR

Flooding rainsmitigate drought

NW

SE

Strong southerly component to the flow in area of large water vapor content indicates entrainment of tropical water vapor into Atmos River

IWV and horizontal waterVapor transport are well correlated