Blending CHIRPS Data And GEFS Forecasts For An Enhanced Rainfall Forecast Product

This article originally produced for the UCSB Climate Hazards Center Blog by Martin Landsfeld, Laura Harrison, Chris Funk, and Juliet Way-Henthorne.  

Background – CHIRPS    
The Climate Hazards Group Infrared Precipitation with Stations data set (CHIRPS) is a land-based, quasi-global (latitude 50°N-50°S), 0.05 degree resolution precipitation data set. It has a relatively long-term period of record (1981 – near present). CHIRPS is based on a well-developed climatology and perturbed with infrared satellite measurements and in-situ observations to estimate gridded precipitation in near-real time. A final monthly precipitation product is created about 2 weeks after the end of the month when all the station data (from over 10 different sources) has been collected and blended with the satellite estimates. A preliminary product, CHIRPS Prelim, is available on the dekadal time scale and available 2-3 days after each dekad and month. It is blended with only 2 sources of station data— WMO GTS and CONAGUA.

Figure 1.  CHIRPS rainfall estimates product.

Background – GEFS

NCEP’s Global Ensemble Forecast System (GEFS) is a weather forecast system that provides daily forecasts out to 16 days at 1 X 1 degree resolution at 6-hour intervals.

Figure 2. GEFS rainfall estimates product.

This forecast product can be very useful to early warning famine and hydrological monitoring efforts, so the Climate Hazards Group (CHG) creates forecast precipitation fields at the dekadal (10 day) time scale and makes those available to researchers and decision makers in the EWX data viewer.

The data is also available for download in GeoTIFF format through CHG's FTP service.

Precipitation is accumulated to 10-day intervals, every day. The precip_mean, anom_mean, and zscore_mean directories contain the means of the ensemble runs, and the file names are comprised of the dates of the first day of the forecast, followed by the last day of the forecast.

Blending GEFS Forecasts with CHIRPS – “Early Estimate” product

The USGS/CHC has recently started producing an “Early Estimate” product that combines observed CHIRPS data for the past 10/30/60 days with CHIRPS-compatible 10-day GEFS rainfall forecasts. Graphics are current (through January 20, 2019 as of this publication) and updated every five days. Images from this product can be found here.

To make GEFS forecasts interoperable with CHIRPS, we bias-correct and downscale GEFS with respect to CHIRPS. The method that is followed is similar to the standard cumulative density function matching process of bias-correction. For a given target 10 day period’s GEFS forecast, its rank with respect to its climatology (1985 – present) is identified, and then a value of the same rank from the CHIRPS climatology is used to replace GEFS forecasts for each ~5km pixel. A comparison is shown below.

Figure 3. Comparison of GEFS 1 degree resolution forecast (left) with the blended CHIRPS-GEFS 0.05 degree forecast (right). 

Accuracy assessment:

GEFS and CHIRPS-GEFS 10-day accumulations were compared to historical Ethiopian station measurements. Ethiopia maintains many stations through the time period since 1985 and has a varied topographic relief. Significant improvements were made, as seen below (Figure 4).

The correlation, mean bias, and root mean squared error are shown in Table 1. Significant improvements in the correlation to station values were improved, and the mean bias and RMSE were reduced, demonstrating the increased accuracy when the GEFS forecasts are blended with the CHIRPS climatology and long-term time series.

                             Corr. Bias RMSE

               GEFS: 0.51 1.45     63.54

CHIRPS-GEFS:   0.68 1.05 37.26

Table 1. Comparison coefficients of GEFS and CHIRPS-GEFS estimates with rain gauge stations.

Spatial correlations where calculated between CHIRPS-GEFS forecasts and CHIRPS Final estimates for dekadal time periods over the entire time series. High correlations exist over significant portions of Africa for the first dekad of April (Figure 5). Other correlations, biases, and mean-biased errors for other time periods can be found here.

Figure 5. Correlation between CHIRPS-GEFS and CHIRPS Final for the 1st dekad of April (1985-2016). 

Using CHIRPS-GEFS

Real-time precipitation forecasts are critical for predicting flooding events and protecting property and lives. When torrential rains hit Kenya in mid to late April 2018, major flooding occurred, overwhelming drainage systems and collapsing dams. The Kenya Meteorological Department wrote:

Heavy rain has been affecting the central, the south-west and south-east areas of the country, including the capital Nairobi, since the beginning of the month, causing floods, flash floods, and casualties. According to media, as of 20 March, the death toll has reached at least 15 people in the provinces of Central, Nyanza, and Eastern. They also reported that around 1000 people were evacuated in the counties of Makueni (Eastern province), Kilifi and Tana (Coast province). Over the next 24 hours, more heavy rain with local thunderstorms is forecast for the affected areas. 

The impacts of this flooding can be seen below (Figures 6 and 7).

Figure 6. A passenger is rescued from his submerging vehicle following heavy downpour in Nairobi on April 15, 2018. 

Figure 7. Submerged vehicle in Nairobi, April 24th, 2018. 

The CHIRPS-GEFS 10-day forecast, made on April 10th, 2018, could have been used to help predict these events and warn residents of impending flooding. As an example, consider the left image in Figure 8 below. This map, obtained from the CHG experimental Early Warning Explorer using the CHIRPS GEFS latest under Global Datasets, shows a downscaled GEFS forecast for the 10th – 20th of April. Below is a comparison of  the anomalies of the CHIRPS-GEFS forecast with the subsequent CHIRPS Final for the same time period. The right image shows the observed CHIRPS data for that same time period. While the agreement is not perfect, the CHIRPS GEF forecasts did a very good job of capturing the potential high rainfall amounts. Figure 9 shows forecasts and observations for the next 10 days April 21-30.

As these maps illustrate, the CHIRPS GEFS forecasts can be very useful tools for flood prediction. Current efforts at the CHG are setting up daily updates of these predictions. These results will be available via ftp, and within the EWX.

CHIRPS-GEFS Forecast, 2018/4, dekad 1       CHIRPS Estimates, 2018/4, dekad 1

Figure 8. CHIRPS-GEFS forecast for dekad 2 of April, 2018 (left) and the CHIRPS Final estimates (right) for the same time period. 

The period of the 21st -30th of April, 2018 shows a similarly good comparison between the prediction and the CHIRPS estimate.