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Presentación realizada en: 2nd ACCORD ASW celebrado del 4 al 8 de abril de 2022 en Ljubljana, Eslovenia.

The status of CY43 HARMONIE-AROME 4D-Var is presented. Central to the HARMONIE-AROME 4D-Var implementation is the multi-incremental approach and the current setup comprises two outer loops at six and three times the main forecast grid resolution of 2.5 km. This setup is flexible though, as is the length of the observation window. Results will be given for 4D-Var in comparison to 3D-Var for three domains, centered around Scandinavia, the Iberian Peninsula and the Netherlands. The starting point here is that both 3D-Var and 4D-Var are using identical observations types. Generally speaking it can be concluded that 4DVar forecasts are superior. The paper also gives an extensive list of opportunities from which particularly HARMONIE-AROME 4D-Var can benefit. For example, increasing the observation set, improving the treatment of observations during the observation window and new algorithmic avenues.

Ponencia presentada en: 11th International Winds Workshop, Auckland, New Zealand (20-24 February 2012) The “High Resolution Winds product (HRW)”, developed inside the “Satellite Application Facility on support to Nowcasting and very short range forecasting (NWC SAF)” software package, provides a detailed calculation of Atmospheric Motion Vectors locally and in near real time. Several important changes have been implemented in its algorithm in the last two years, up to current HRW v2012 version. The AMVs extraction capabilities have been extended, tuned and validated for seven SEVIRI channels (HRVIS, VIS06, VIS08, WV062, WV073, IR108, IR120), considering both Cloudy AMVs and Clear air Water Vapour AMVs. “CCC method” proposed by Borde and Oyama (2008) has been implemented in the algorithm for the AMV height assignment, using NWC SAF/Cloud Top Temperature and Height product data. The results of the last version validation, showing significant improvements through smaller RMSVDs, are shown in this paper. Preliminary results of a study that aims to better understand the temporal and spatial scaling issues associated to the AMV extraction are also shown. In the framework of this study, HRW v2012 AMVs have been extracted from MSG-1 Rapid scan HRVIS and VIS08 channel data. Results are studied for different target sizes (8x8 to 40x40 pixels) and temporal gaps between images (5 to 90 minutes). The outputs have been validated against Radiosounding winds and NWP analysis wind fields.

We studied the use of IASI data to improve the forecasts of extreme weather events in the Arctic region. For this purpose, the HARMONIE/Norway regional model was used. A set of 366 IASI channels was initially chosen from the ECMWF archived database. Active channels showing the best fit with the analysis system were selected by applying a multi-step monitoring technique. The IASI data were assimilated together with most of the available conventional and operational satellite observations using a three-dimensional variational data assimilation system. Four experiments with cyclic assimilations and subsequent 48-hour forecasts were performed during the IPY-THORPEX campaign period to evaluate the impact of the IASI data and the campaign observations on the hydrostatic HARMONIE/Norway analyses and forecasts.

Nowcasting, a very short-term weather forecasting, has been more and more used in the later years, due to improved forecast models and more computing power. More frequent weather forecast updates depend on observations that can be collected frequently. An observation that can be collected every hour is the GNSS Zenith Total Delay (ZTD), which is the delay in a satellite signals ray to a receiver due to the composition of the atmosphere, converted to a humidity measurement. This dissertation studies the impact GNSS ZTD observations on a rapid refresh nowcasting system. Two experiments, with, and without GNSS ZTD observations, have been performed. The bias of the GNSS ZTD observations was corrected using a variational approach. Nowasting with seven hour forecasts was updated every third hour. A Rapid Refresh (RR) scheme – a non cycled data assimilation and forecast system – was used as nowcasting approach, using short-range forecasts from a three hour cycling system as initial state. The impact of the GNSS ZTD observations was evaluated by comparing the analyses and forecasts of the two above mentioned experiments against observations. Results from the two experiments showed that there is a difference between the two model runs. The most remarkable differences could be seen in relative and specific humidity, wind speed, wind direction, and one and six hour accumulated precipitation forecasts. Significant difference in root-mean-square error (RMSE) was observed between the two experiments, especially during the first two hours of the forecast at some of the nowcasting times. The bias correction of the observations was successful for some of the stations used to collect GNSS ZTD observations. But, some of the stations showed a clear spin-up observed during the first ten days. All the stations used to collect the GNSS ZTD observations was located in Sweden. A promising positive impact of GNSS ZTD was found, but further development is needed to get a stable improvement from this observation in a RR system. Furthermore, use of more observations from other countries inside the domain and more careful bias correction would be advantageous.

Satellite instruments currently provide the largest source of infor- mation to today's data assimilation (DA) systems for numerical weather predic- tion (NWP). With the development of high-resolution models, the efficient use of observations at high density is essential to improve small-scale information in the weather forecast. However, a large amount of satellite radiances has to be removed from DA by horizontal data thinning due to uncorrelated observation error assumptions. Moreover, satellite radiances include systematic errors (biases) that may be even larger than the observation signal itself, and must be properly removed prior to DA. Although the Variational Bias Correction (VarBC) scheme is widely used by global NWP centers, there are still open questions regarding its use in Limited-Area Models (LAMs). This thesis aims to tackle the obser- vation error difficulties in assimilating polar satellite radiances in the meso-scale ALADIN system. Firstly, we evaluate spatial- and inter-channel error correla- tions to enhance the positive effect of data thinning. Secondly, we study satellite radiance bias characteristics with the key aspects of the VarBC in LAMs, and we compare the different VarBC configurations with regards to forecast performance. This work is a step towards improving the...