# Presentation: 167359

## A Framework for the Assessment of High Latitude Satellite-Based 

**Monthly and Daily Precipitation Estimates using Ground Observations**

**D.T. Bolvin, R.F. Adler, G.J. Huffman, E.J. Nelkin**
**Laboratory for Atmospheres**
**NASA Goddard Space Flight Center**
**http://precip.gsfc.nasa.gov**

**Datasets**

**Goal**

**GPCP 1DD (Daily) vs. FMI at Large Spatial Scale**

**Motivation**

- Assess the quality of the current Global Precipitation Climatology Project (GPCP) satellite-based high-latitude land precipitation estimates by comparing with dense rain gauge observations as a precursor to evaluation of future high-resolution satellite-based precipitation datasets, i.e., GPM.

- Most ground validation analyses of satellite-based precipitation estimates have covered primarily the tropics and mid-latitudes
- Challenging to estimate precipitation from satellites at high latitudes as microwave and infrared sensors have difficulty distinguishing precipitation from cold land - _how well are we really doing?_
- Challenging to obtain dense, high-quality precipitation gauge observations at high latitudes
- Provide a framework for assessment and validation of the future GPM precipitation estimates

**GPCP 1DD (Daily) Satellite Estimates**
- Satellite-based precipitation estimates (1°x1°, UTC day resolution)
- Tropics: Geo-IR calibrated by coincident DMSP SSM/I microwave estimates
- High Latitudes: TOVS/AIRS estimates calibrated by SSM/I occurrence and GPCP V2 SG monthly amount
- 1DD estimates scaled to sum to the 2.5°x2.5° GPCP V2 Satellite-Gauge (SG) monthly product (using bilinear interpolation) for product suite consistency

**FMI Gauge Observations**
- Official Finnish government gauge network observations
- Excellent quality control
- Daily observations ending 00 UTC
- Measures liquid _and_ solid (liquid equivalent) precipitation
- No wind-loss correction applied
- Data courtesy of the Finnish Meteorological Institute

**Conclusions**

**GPCP V2  Monthly  vs. FMI Results**

**GPCP 1DD (Daily) vs. FMI at Small Spatial Scale**

- _Uncorrected_ (for wind loss) GPCC gauge analysis agrees well (qualitatively) with FMI gauge observations
- Differences GPCP 1DD (daily) estimates (which incorporate _wind-loss corrected_ GPCC gauge analysis) and FMI observations are mostly due to wind-loss correction
- Large-Scale Comparison
- Percent bias difference follows a predictable annual cycle directly related to the wind-loss correction; GPCP 1DD (daily) exceeds FMI by 17%
- Daily correlation coefficient does not exhibit a predictable annual cycle; average is relatively high at 0.69
- Preliminary Small-Scale Comparison
- Occurrence of GPCP 1DD (daily) and FMI precipitation fairly well correlated, but day-to-day differences in bias can be large
- Best correlations occur in summer months and worst occur in winter months, which follows the magnitude of the wind-loss correction
- Skill of GPCP 1DD (daily) estimates provides strong evidence of success for high-resolution GPM-era precipitation estimates

**Future Work**

- This is really just the beginning of a longer term effort
- Continue investigating the quality of the GPCP 1DD (daily) at small spatial scales
- Analyze GPCP 1DD (daily) versus FMI observation correlations as a function of time and space averaging
- Investigate secondary sources of differences between the GPCP 1DD (daily) and FMI
- Obtain and use other high-latitude gauge data sets for comparison in other regions (Canada, for example)
- Investigate new GPCC event-by-event wind-loss corrections
- Develop techniques to refine the GPCP V2 SG and 1DD (daily) high-latitude estimates
- Develop techniques to perform the assessment using 3-hourly, 0.25-degree TRMM precipitation estimates

- Example GPCP 1DD (Daily) Image for August 15, 2005

- Typical FMI Gauge
- Coverage over the
- _Analysis Span of _
- _August 2005 - _
- _September 2006_

- Generate GPCP vs. FMI Monthly Time Series Plot for a **SINGLE 2.5°x2.5° **
**GREEN** for All Months in the Analysis Span

- FMI agrees well with the (not wind-loss corrected) GPCC gauge analysis
- GPCP products heavily weighted by wind-loss corrected GPCC gauge analysis
- 1DD (daily) vs. FMI bias will follow GPCP V2 vs. FMI bias, which is primarily the wind-loss correction

- GPCP 1DD (Daily) vs. FMI Daily Time Series Plots Over the Entire FMI Domain (Finland)

- August 2005

- February 2006

- GPCP 1DD (daily) vs. FMI Difference closely matches annual cycle of wind-loss correction

- Time-Series of Monthly Correlation Coefficient Between
- Daily GPCP 1DD and FMI (over the entire FMI area)

- Monthly Time-Series of Percent Bias Difference Between
- Monthly Average GPCP 1DD and FMI (over the entire FMI area)
- Percent Bias Difference  = ((GPCP 1DD - FMI) / FMI) * 100

- Average correlation coefficient relatively high at 0.69

- Scatterplot of all 8 Daily 2°x1° Precipitation Values for the month
- within the **4°x4° GREEN BOX**

- August 2005

- February 2006

- February 2006

- August 2005

- Time Series of Daily Precipitation for the **2°x1° RED BOX**