An accurate prediction of warm season precipitation amounts has remained an elusive goal for the atmospheric sciences despite steady advances in the ability of numerical weather prediction models to forecast many other atmospheric variables. One necessary condition for an accurate prediction of convective rainfall is a good forecast of where and when convection will initially develop. Current understanding of convection initiation processes are impeded by a lack of high-resolution, high-accuracy water vapor measurements.

It is known that existing observational techniques for mapping the three-dimensional distribution of water vapor on the mesoscale are lacking. Radiosondes, the traditional means of obtaining water vapor measurements, are insufficient because they provide only vertical profile information, are only available twice a day at most locations and have significant errors and biases. Additionally, there is a general absence of operational ground-based water vapor remote sensing systems and many satellite techniques have relative difficulty in obtaining high resolution and accuracy water vapor measurements in the lower troposphere. Thus, while water vapor measurements are critically linked to convective processes, measurements are simply not available to derive high quality initial conditions for the next generation of numerical weather prediction models or to provide guidance for bench forecasters seeking to nowcast convective development and evolution. Adequate means for measuring water vapor are only now beginning to emerge.

The International H₂O Project (IHOP_2002) brought together many of the existing water vapor sensors in the world to address whether improved water vapor measurements can enhance the understanding of moist convective processes and improve quantitative precipitation forecast accuracy. The field phase took place over the Southern Great Plains of the United States from 13 May to 25 June 2002. The IHOP_2002 investigators will focus on four coordinated and overlapping research components: i) The QPF research component seeks to determine the degree of improvement in forecast skill that occurs through improved characterization of the water vapor field. This work includes a variety of research and operational numerical modeling, data assimilation and expert systems; ii) The Convection Initiation (CI) research component seeks to further understand and eventually predict the processes that determine where and when convection forms; iii) The Atmospheric Boundary Layer (ABL) Processes research component seeks to improve understanding of the relationship between atmospheric water vapor and surface and boundary layer processes and their impact on convective development; and iv) The Instrumentation research component seeks to determine the optimal mix of water vapor measurement strategies to better predict warm season rainfall. This group will also work toward better quantification of measurement accuracy, precision and performance limitations as they relate to using water vapor measurements in warm season forecasts and data assimilation systems. An overview of IHOP and preliminary results will be presented at the conference.