Verification of modeled shortwave radiation using LSA SAF MDSSFTD data
Verification of a nowcasting model output over Slovenia
The aim of nowcasting is to gain the best available estimates for various meteorological fields in a very short forecast lead time (0-3h). Nowcasting Models (NM) provide hourly updated gridded forecasts of shortwave radiation. The NM used at the Slovenian Environment Agency (ARSO) is based on the in-house limited area numerical weather prediction (NWP) model output, real-time surface weather station data and satellite cloud type classification. The NM system adds value to the classical NWP forecast by providing high-resolution analyses, nowcasts, and improved forecasts within the nowcasting range.
Often the in-situ measurements are used for validation. Additionally, satellite measurements complement or replace information on the incoming shortwave flux over the areas with limited measurements. We will show below how LSA SAF Total and Diffuse Downward Surface Shortwave Flux (MDSSFTD, LSA-207) product was included in the validation of NM forecasts. Satellite measurements are based on the SEVIRI instrument on-board the MSG satellite.
The comparison of the model output against MDSSFTD is displayed in Fig. 1. Verification was prepared for the extended region of Slovenia (marked in Fig. 1) over the period of twelve months from March 2022 to February 2023. Hourly averages of short wave radiation were prepared from instantaneous MDSSFTD measurements that are available every 15 minutes.
Mean absolute difference (MAD) between NM output and MDSSFTD was calculated for all of the available model forecasts (NM was run hourly preparing forecasts up to 4 hours in advance).
Fig. 1. A comparison across the area of interest between short wave radiation from NM (right) against LSA SAF MDSSFTD (left) for 15 August 2022 13:00 UTC
Fig. 2. Mean absolute difference between model and MDSSFTD measurements for the first 4 hours of forecast. Comparison is made for all cloud conditions (left panel) and clear sky only (right panel).
Fig. 2 shows MAD in W/m2 for the 4 hour forecast horizon including all cloud cover conditions and for clear-sky only. Overall, MAD values are lower for the clear sky category compared to all conditions. Also, with greater forecast time the MAD is increasing as well as its spread which is expected for nowcast performance. This increase may be related to difficulties in the forecast of cloud cover. Fig. 3 shows the daily cycle of MAD values. Values are shown for both the analysis and the first hour of the forecast. With increasing solar radiation in the course of the day, the MAD and its spread increases. The difference between the analysis and first hour of forecast is more pronounced before and after solar peak (in the morning and late afternoon).
We showed that LSA SAF MDSSFTD product is a high-quality independent data source for a quality assessment of shortwave radiation forecasts provided by nowcasting models. It is especially useful in areas with limited availability of in-situ measurements.
Fig. 3. Hourly MAD between NM and LSA SAF MDSSFTD for the analysis (left panel) and the first hour of the forecast (right panel)