Identifying dry/wet periods in Slovakia using satellites

By Ján Kaňák, Ľuboslav Okon, Peter Kajaba

In the frame of the H SAF validation cluster, the evaluation of precipitation products is a very important task. The validation is based on the comparison of satellite data against ground references (rain gauge or radar measurements). Until recently, comparisons of instantaneous precipitation or 24 hour accumulations were performed to evaluate H SAF precipitation products quality and performance.

The aim of this case study is to compare long-term monthly accumulated rainfall data from the H SAF accumulate precipitation product H05-A/B and rain gauge measurements to evaluate rain characteristics over Slovakia; using a record of 10 years of accumulated H SAF precipitation data, in comparison with monthly rain gauge data from 2012-2022.

The H SAF H05A and H05B products are available from July 2012. For this work we used the operationally-generated and archived data from July 2012–June 2022.

We defined three different categories of rainfall anomalies:

1. DRY — <10%–49% of long-term monthly average
2. NORMAL — 50%–150% of long-term monthly average
3. WET — 151%–>190% of long-term monthly average

For the study SHMÚ collected data from the Slovak rain gauge network, which were quality checked by the climatological department. From these data monthly sums (mm/month) were calculated for each station. The station data were interpolated using a 500m digital elevation model, to account for the influence of orography on the spatial distribution of rainfall (Figure 1). From this, we calculated the 10-year monthly averages of the monthly precipitation sums.

Figure 2 shows the coverage of the H SAF precipitation product used here. However, only Slovak territory can be covered by the SHMÚ rain gauge network.

Figure 3 shows the 10-year average of monthly precipitation sums based on H05A/B (satellite) product (right) and based on interpolated gauge measurements (left). Differences between the two data sources are clearly visible. Looking at the average annual cycle, the large scale patterns in the autumn months (3rd row) compare best. Larger differences between the rain gauge-based maps and the satellite-based maps are observed especially during winter (1st row).

Monthly precipitation

H SAF Rain gauge

 

Figure 4 shows the monthly relative anomalies (percentage of difference between monthly-accumulated precipitation and monthly long-term mean of accumulated precipitation).

From 120 evaluated months in the period July 2012 to June 2022, we found:

• 13 months (10.8%) with underestimated monthly precipitation sums by H05 against RG
• 93 months (77.5%) with agreement of monthly precipitation sums by H05 against RG
• 14 months (11.7%) with overestimated monthly precipitation sums by H05 against RG

Comparison table for H05 versus RG estimation of rain characteristics showing agreement, under or over estimation of the monthly cumulative rainfall by satellite in comparison to ground-based measurements:

Legend to the comparison table:

The H SAF H05 product might also be considered for the evaluation of rainfall characteristics in other European or African regions, considering the extended full disc coverage of the H05B (P-AC-SEVIRI) product.

Additional content

H SAF precipitation products
H05B product