Monitoring the Quality of Operational and Semi-Operational Satellite Precipitation Estimates – The IPWG Validation / Intercomparison Study Beth Ebert Bureau of Meteorology Research Center Melbourne, Australia 2 nd IPWG Meeting, Monterey, October 2004

Motivation – provide information to...  Me...! fill the blank spot  Algorithm developers How well is my algorithm performing? Where/when is it having difficulties? How does it compare to the other guys?  Climate researchers Do the satellite rainfall products give the correct rain amount by region, season, etc?  Hydrologists Are the estimated rain volumes correct?  NWP modelers Do the satellite products put the precipitation in the right place? Is it the right type of precipitation?  Forecasters and emergency managers Are the timing, location, and maximum intensities correct?

Web page for Australia – home

Earlier studies GPCP Algorithm Intercomparison Programs (AIPs) and WetNet Precipitation Intercomparison Programs (PIPs) found:  Performance varied with sensor Passive microwave estimates more accurate than IR and VIS/IR estimates for instantaneous rain rates IR and VIS/IR slightly more accurate for daily and monthly rainfall due to better space/time sampling  Performance varied with region and season Tropics better than mid- and high latitudes Summer better than winter (convective better than stratiform)  Model reanalyses performed poorer than satellite algorithms for monthly rainfall in tropics, but competitively in mid-latitudes (PIP-3)

More recent studies  Combination of microwave and IR gives further improvement at all time scales Good accuracy of microwave rain rates Good space/time sampling from IR (geostationary)  Strategies Weighted combination of estimates Using match-ups of microwave and geostationary estimates  Get a field of multiplicative correction factors  Tune parameters of IR algorithm  Map IR T B onto microwave rain rates Morphing of successive microwave estimates using time evolution from geostationary imagery  Paradigm for GPM?

Focus of IPWG validation / intercomparison study 1. Updated evaluation of satellite rainfall algorithms

Quantitative Precipitation Forecasts (QPFs) from Numerical Weather Prediction (NWP)  WCRP Working Group on Numerical Experimentation (WGNE) has been validating / intercomparing model QPFs since 1995  Results Performance varies with region and season  Mid-latitudes better than tropics  Winter better than summer (stratiform better than convective) NWP performance is complementary to satellite performance! NWP performance over Germany

Foci of IPWG validation / intercomparison study 1. Updated evaluation of satellite rainfall algorithms 2. Where, when, under which circumstances is NWP rainfall better than satellite rainfall, and visa versa?

Related studies

Related studies O bserved P recipitation V alidation

Parameters of study  Evaluate estimates for at least one year to get seasonal variations in performance  As many different regions (climate regimes) as possible So far:  Australia  United States  Western Europe  Any volunteers for Asia? Elsewhere?  Focus on daily rainfall  Rain gauge and radar rainfall analyses used as reference data  Focus on relative accuracy  Global estimates archived at U. Maryland

Algorithms  Operational and semi-operational algorithms Run every day Available to public via web or FTP Experimental algorithms OK  Sorted by sensor type Microwave IR or VIS/IR Microwave + IR  Blending strategy NWP models  Global models (ECMWF, US) Lower spatial resolution, global coverage  Regional models Higher spatial resolution, limited coverage

Evaluation methodology  Daily rainfall estimates of Rain occurrence Rain amount  Spatial resolution Finest possible resolution (typically 0.25° lat/lon) Coarser resolution (1° lat/lon) for comparison with NWP  Stratify by Season Region Algorithm type  Algorithm Rain amount threshold

Verification methods  Rain occurrence Frequency bias Probability of detection and false alarm ratio Equitable threat score  Rain amount Multiplicative bias RMS error Correlation coefficient Probability of exceedance  Properties of rain systems Contiguous Rain Area (CRA) validation method (Ebert and McBride, 2000)  Rain area, volume, maximum amount  Spatial correlation  Error decomposition into volume vs. pattern

Some results for Australia...

User page  Targeted to external users of satellite rainfall products

Developer page  Targeted to algorithm developers – contains more algorithms, some of which aren't publicly available (at least not easily)

Multi-algorithm maps  All algorithms and NWP models for 30 September 2004 over Australia

Basic daily validation product  Maps and statistics

Daily CRA validation  Properties of rain system Area Mean and maximum rain accumulation Rain volume Spatial correlation Error decomposition into volume and pattern error components

Monthly and seasonal summaries  Variety of statistical plots Time series Scatter plots Table of statistics Binary (categorical) scores as a function of rain threshold Error as a function of estimated (observed) rain rate

Intercomparison of algorithm types Australian Tropics Australian Mid-latitudes Multiplicative bias December September ° grid summer autumn winter spring

Intercomparison of algorithms Australian Tropics Australian Mid-latitudes POD December September ° grid

Caveats  Reference data (gauge and radar analyses) are not as accurate as targeted ground validation sites Performance results more meaningful in a relative sense than in an absolute sense  No ocean validation Microwave algorithms are expected to have better performance over ocean because emission signal is used Therefore microwave+IR algorithms should also perform better over ocean NWP QPFs perform better over land than over ocean since more observations used in model initialization  Not all algorithms cover the same period (some missing data)

Future of this study  Results so far will be examined closely and written up for publication  Satellite precipitation validation / intercomparison will continue into the future...  Algorithm developers Keep making your results available Good opportunity to check new or updated algorithms  Reference data providers Thanks for data currently provided More is better! Can you assist in the validation itself?  Users of validation results Are we giving you the information you need? Please provide feedback and suggestions for improvement