Performance Assessment of Satellite and DACCIWA Optical Gauge Rainfall Products for the Analysis of Trends and Drivers of Rainfall Extremes in Ghana

Rainfall plays an important role in the socio-economic development of any nation. Below or above normal rainfall conditions have serious consequences on key socioeconomic sectors such as agriculture. This is particularly true for countries where rain-fed agriculture is a predominant practice as such, rainfall impact studies are important. In this thesis, rainfall impact studies in Ghana were examined in three different areas. The first part of the study evaluated the performance of satellites and merged rainfall products over Ghana in order to examine their reliability as surrogate ground-based gauge measurements for areas where gauge networks are sparse. Satellite products (TAMSAT, TRMM, ARC2, CMAP, GPCP and CHIRPs) and gauge-only products (CRU and GPCC) were validated with gridded rainfall data from the Ghana Meteorological Agency (GMet) on monthly to annual time scales using a suite of statistical analysis. The results showed that, the performance of the satellites and merged rainfall products is a function of the scales and locations used in the validation. While ARC2 showed large biases on both monthly and annual time scales, all other products, especially, TRMM and TAMSAT showed relatively good skills on the monthly scale (r > 0.90) than the annual. On the countrywide basis, CHIRPs and CRU markedly revealed better skills on both time scales. In all the four agro-ecological zones of the country, the products were able to capture the respective rainfall patterns, onset, cessation and spells (wet and dry) of each zone. Thus, TRMM and TAMSAT will serve as better surrogates at monthly time scale whereas CHIRPS and CRU would be better substitutes to ground-based gauge data for both time scales over the entire country and in the four ecological zones. In the second part of the study, the performance of the new Dynamic Aerosol Chemistry-Cloud Interactions in West Africa (DACCIWA) optical rain gauge (DOG) measurements over the Ashanti region of Ghana were assessed. The study further examined the validities of subdaily rainfall data from IMERG and TRMM over the region for their exploits in the key socio-economic activities of the country such as agriculture, hydrology and water resource management. Daily rainfall data for nine co-located stations from the Ghana Meteorological agency (GMet) were used to assess the validity of the DOGs. A point-point assessment of IMERG and TRMM from 2016–2017 for the nine stations were then performed using the DOGs. The outcome of the assessment revealed that the DOGs were able to capture the diurnal and monthly rainfall patterns in the region. IMERG revealed a better skill in capturing the daily rainfall than TRMM although both products showed some similarities in the June–August (JJA) season. Both products slightly over-estimated gauge measurements (<2 mm/day) in all selected stations. The third part of the study examined the climatologies and trends of extreme rainfall events in Ghana. The study further probed the links between the rainfall extremes and SST anomalies at the oceanic basin. The trend analysis revealed that CDD, CWD, R10mm and R20mm were in the range of 5–140, 5–14, 25–60 and 2–26 days per year respectively. Moreover, PRCPTOT, R95p, R99p, SDII, RX1day and RX5day were in the range of 800-1800 mm, 150–450 mm, 30–130 mm, 6–14 mm, 30–100 mm and 60–190 mm per year respectively. The maximum of temporally averaged intensity rainfall indices have shown to cover southwestern Ghana while the minimum of these indices covered northwestern and eastern coasts of Ghana. Significant decreasing trends in wet indices were observed over the Volta lake and central Ghana while low positive trends were dominant over the northern sectors of the country. In most cases, with the exception of the CDD and CWD indices, there were significant positive (negative) correlations with the Atlantic (Pacific and Indian) basins SSTs. IOD had a dipole effect on rainfall indices with the central and southern sectors generally covered by negative correlations while northern and coastal Ghana have shown positive correlations. The R95p index over central Ghana had negative correlations with AMO while positive correlations dominated some isolated parts of the country in the south and northeast. The impacts of AMO on PRCPTOT index were insignificant over most parts of the country with a few exceptions over southeastern and northern boarder regions. These results have implications on the improvement of monthly-annual forecasts of the Ghanaian rainfall and its extremes, and also provides prior knowledge for better understanding of multidecadal modulations of global inter-annual teleconnections

Source: KNUST