MAP and Surface Hydrology

Given the fact that one of the first primary objectives of MAP is concerned with moist processes over mountainous terrain, a link of the atmospheric aspects to surface hydrology appears natural since the earth surface as the interface between soil and atmosphere plays an important role as source or sink of moisture in the total water budget. First thoughts in this respect have already been raised at the initial MAP workshop in September 1994 in Zurich. The need and importance of a correct representation of the soil-atmosphere exchange processes has recently been highlighted by operational numerical weather prediction models running into problems with summer precipitation forecasts due to deficiencies in the soil water budget and evapo(transpi)ration parameterization.

The focus of MAP on moisture and precipitation aspects also attracted the attention of Prof. H. Grassl, director of the World Climate Research Programme (WCRP) who raised the specific question whether MAP could and should become part of the Global Energy and Water Cycle Experiment (GEWEX), which is one of the major subprogrammes of WMO's WCRP.

In this context the MAP-SSC decided at its second meeting in Bologna (January 1996) to invite hydrologists to a workshop in order to examine fields of cooperation within MAP and their potential interest. Consequently, the first MAP hydrology workshop was held in Zurich on 26 April 1996. 10 hydrologists (A(1), CH(2), D(1), F(2), I(3), CAN(1)) took part plus 7 members of the MAP SSC and CIG committees. It was well recognized that the atmospheric and hydrological communities would strongly benefit from each other if well coordinated projects could be set up. The expertise of the hydrologists in accurately modelling the soil water budget including the evaporation and transpiration processes at the surface provides valuable help to the atmospheric scientists in specifying the fluxes at the lower boundary of their modelling domain. On the other hand, accurate quantitative precipitation forecasts as input to hydrological models can greatly help to improve the river runoff calculations and therefore aid in flood forecasting. It was further recognized that the spatial and temporal scales which can be treated by hydrological and atmospheric models are currently converging, matching at grid distances between 1 km and 5 km. As a conclusion of the workshop, it was decided to complement the MAP objectives by synergetically sensible and valuable hydrological aspects which are included in the basic MAP document, the MAP Design Proposal, and by defining corresponding projects within MAP. The scientific objectives have been extended to read:

1. To improve the understanding of orographically influenced precipitation events and related flooding episodes, involving deep convection, frontal precipitation and runoff.
2. To improve the numerical prediction of moist processes over and in the vicinity of complex topography, including interactions with land-surface processes.
3. To improve the understanding and forecasting of the life cycle of Foehn-related phenomena, including their three-dimensional structure and associated boundary layer proces-ses.
4. To improve the understanding of three-dimensional gravity wave breaking and associated wave drag in order to improve the parameterization of gravity wave drag effects in numerical weather prediction and climate models.
5. To provide data sets for the validation and improvement of high-resolution numerical weather prediction, hydrological and coupled models in mountainous terrain.

A first iteration to complement the Design Proposal has already been effectuated (September 1996), a fully revised version will be available in January 1997.

The main research areas of the hydrological component, which is interleaved with the atmospheric part, are the following:

• the integration of high-resolution radar data with classical rain gauge observations for precipitation measurements,
• the improvement of methodologies for the estimation of return periods of heavy precipitation events in ungauged sites, typically in high mountain areas, where orographic effects are dominant,
• the adaptation of soil-vegetation-atmosphere transfer schemes and surface-layer formulations to mountainous terrain, and
• improvements in accounting for hydrological processes with view to the integration of hydrological and atmospheric numerical models to provide "coupled" flood forecasts.

After the decision to include hydrology a MAP representative was invited to present the programme to the GEWEX Hydrological Panel (GHP) at its meeting in Toronto (H.C. Davies, August 1996). It was recognized that consideration of orographic precipitation adds a valuable complement to the aspects treated in the various existing GEWEX subprogrammes. The result of this interaction with the GEWEX community is that MAP will continue as an independent programme in close collaboration with GEWEX.