Scientific Objectives of Global Surface Radar Networks for GRP
February 13, 2007

Ground based radars and radar networks play an important role in many national weather services. Because these radar networks offer the ability to monitor precipitation with a unique combination of high spatial and temporal resolution, the GEWEX radiation panel sees much value in working together with the various national data providers to answer some important climate questions.

While the value of national radar network data is high, we recognize that quality control (QC) and quantitative Precipitation Estimation (QPE) issues are fundamental and cannot be disregarded. We further recognize that there are activities, some sponsored by WMO intended to improve some of the fundamental QC and QPE concerns and strongly endorse such activities for both improved monitoring of climate variability and trends as well as for global satellite rainfall product validation. The Global Precipitation Measurement (GPM) Mission sees the latter as a vital part of its validation strategy for passive microwave radiometer rainfall estimates over land. Improved QC and QPE thus represent not only a positive step forward for any national agency, it strongly couples regional precipitation information to the global water and energy balance that can only be derived from satellites. A comprehensive global monitoring strategy must involve such a strongly coupled system of checks and balances between surface and satellite observations.

Despite shortcomings with current QC and QPE capabilities, the GRP nonetheless sees immediate value in collecting regional radar network data for a number of important studies. The immediate scientific objectives of regional surface radar network data encompass:

• To investigate the extent to which satellite observed radiances vary with the life cycle of storms. ISCCP already provides a storm history product. Precipitation retrievals that use ice scattering signals (e.g. land), will most likely need life-cycle information to improve their skill in relating ice scattering signals to surface precipitation. Having a suitable dataset for such studies in diverse regions will greatly benefit the global algorithm development work.

• To investigate scale dependent characteristics of precipitation. Regional surface radar networks span nearly the full range of scales from “micro” to “synoptic” and thus offer an ideal tool to study the basic phenomenology of these systems in various regimes. Data from the main climate regimes, including tropical, midlatitude and polar would be needed.

• To assess, in conjunction with other GRP products, the horizontal scales at which local energy flux imbalances translate into an accelerated or decelerated hydrologic cycle.

• To investigate the effects of the interaction of precipitation system with the orography. In particular, spaceborne systems are known to produce poor results in the presence of orographic lifting. Improving the current status will require more knowledge about the nature of clouds and precipitation as they approach and cross an elevated region.

• To address the relative importance of 1) surface characteristics, 2) land use, and 3) pollution on precipitation formation/suppression.

• As a prototype for a uniform global high resolution product that merges satellite and in-situ observations, taking into account the strengths of each. Having surface radar network data available for diverse regions around the globe would greatly expedite such merged analysis.

Working Group on Precipitation Radar Networks