Abstract. This paper compares state-of-the-art atmospheric moisture tracking models. Such models are typically used to study the water component of coupled land and atmosphere models, in particular quantifying moisture recycling and the source-sink relations between evaporation and precipitation. There are several atmospheric moisture tracking methods in use. However, depending on the level of aggregation, the assumptions made and the level of detail, the performance of these methods may differ substantially. In this paper, we compare three methods. The RCM-tag method uses highly accurate 3-D water tracking (including phase transitions) directly within a regional climate model (online), while the other two methods (WAM and 3D-T) use a posteriori (offline) water vapour tracking. The original version of WAM is a single-layer model, while 3D-T is a multi-layer model, but both make use the "well-mixed" assumption for evaporation and precipitation. The a posteriori models are faster and more flexible, but less accurate than online moisture tracking with RCM-tag. In order to evaluate the accuracy of the a posteriori models, we tagged evaporated water from Lake Volta in West Africa and traced it to where it precipitates. It is found that the strong wind shear in West Africa is the main cause of errors in the a posteriori models. The number of vertical layers and the initial release height of tagged water in the model are found to have the most significant influences on the results. With this knowledge small improvements have been made to the a posteriori models. It appeared that expanding WAM to a 2-layer model, or a lower release height in 3D-T, led to significantly better results. Finally, we introduced a simple metric to assess wind shear globally and give recommendations about when to use which model. The "best" method, however, very much depends on the research question, the spatial extent under investigation, as well as the available computational power.

[1] One of the central questions in hydrological research is where and to what extent evaporated water of a region returns as precipitation in another region. This study addresses this question and presents a detailed process-based approach implemented into a regional climate model. It allows tagging and tracking of the moisture evaporating from a given region into the atmosphere until it returns to the land surface as precipitation. Our approach is fully three-dimensional and enables the detailed consideration of vertical transport mechanisms for tagged water. We present a case study for the region around Lake Volta in West Africa. The simulation demonstrates the performance of the regional model and the implemented tagging mechanism. It shows the evolution of the tagged moisture field and reveals details of the transport: Moisture evaporated from Lake Volta is initially transported predominantly to the east and north, lifted by convective processes and then transported in upper layers to the west far away from the source of evaporation. The results indicate that the coupling between boundary layer and higher levels through convective processes can be essential for the fate of tagged water substances. Detailed analysis for a 2 month period in the rainy season 1998 shows that locally up to 6% of precipitating water originates from the Lake Volta region. Less than 2% of the evaporated water is locally recycled as precipitation in the source area. A further 10% precipitates in the rest of the Volta Basin.Citation: Knoche, H. R., and H. Kunstmann (2013), Tracking atmospheric water pathways by direct evaporation tagging: A case study for West Africa,

Abstract. This paper presents a new Copula-based method for further downscaling regional climate simulations. It is developed, applied and evaluated for selected stations in the alpine region of Germany. Apart from the common way to use Copulas to model the extreme values, a strategy is proposed which allows to model continuous time series. As the concept of Copulas requires independent and identically distributed (iid) random variables, meteorological fields are transformed using an ARMA-GARCH time series model. In this paper, we focus on the positive pairs of observed and modelled (RCM) precipitation. According to the empirical copulas, significant upper and lower tail dependence between observed and modelled precipitation can be observed. These dependence structures are further conditioned on the prevailing large-scale weather situation. Based on the derived theoretical Copula models, stochastic rainfall simulations are performed, finally allowing for bias corrected and locally refined RCM simulations.

This paper presents a new Copula-based method for further downscaling regional climate simulations. It is developed, applied and evaluated for selected stations in the alpine region of Germany. Apart from the common way to use Copulas to model the extreme values, a strategy is proposed which allows to model continous time series. In this paper, we focus on the positive pairs of observed and modelled (RCM) precipitation. As the concept of Copulas requires <i>independent and identically distributed</i> (<i>iid</i>) random variables, meteorological fields are transformed using an ARMA-GARCH time series model. The dependence structures between modelled and observed precipitation are conditioned on the prevailing large-scale weather situation. The impact of the altitude of the stations and their distance to the surrounding modelled grid cells is analyzed. Based on the derived theoretical Copula models, stochastic rainfall simulations are performed, finally allowing for bias corrected and locally refined RCM simulations

In this study, a regional climate model-based evapotranspiration tagging (ET-Tagging) algorithm has been applied for the first time over Southeast China. Fifteen month simulations (October 2004 to December 2005 were performed to investigate where and to which extent the tagged evapotranspired water from the Poyang Lake region returns to the land surface as precipitation. The contributions of direct evaporation and transpiration were estimated separately using an extended ET-Tagging partitioning algorithm. In 2005, the contribution of moisture originating from the Poyang Lake region to the local annual precipitation in Southeast China reaches a value of up to 1.2%. A maximum contribution of 6% is found near the Poyang Lake region in August. In 2005, 69% of total tagged precipitation originates from direct evaporation of water whereas 31% from transpiration. In winter, precipitation originating from transpired moisture only accounts for around 10% of the total tagged precipitation, but in the summer season the contribution of transpiration increases up to 50%. To explore the source-target relations under consideration of the respective precipitation regime, we introduce source-specific precipitation efficiencies. For the period under investigation, the efficiency for direct evaporation generally dominates, except during the comparatively dry August and in the winter months. Our study shows that the location and the magnitude of tagged precipitation show large spatial and temporal variations. The comprehensive interactions between land surface characteristics and synoptic weather conditions control the annual cycle of the individual contributions to precipitation, emphasizing the important impacts of vegetation cover and land use on the atmospheric hydrological cycle.