A glance at the mesoscale structure of moisture: highlights of refractivity observations during IHOP_2002

During the International H2O Project (IHOP_2002), near-surface refractivity data were collected by the National Center for Atmospheric Research S-Pol radar in the Oklahoma Panhandle in May and June 2002. Since refractivity is a function of temperature, pressure, and especially moisture, its observation by radar gives us a unique glimpse at the 2-D structure of the field of surface moisture at the mesoscale. While most moisture measurements until now has been limited to point measurements and vertical profiles, refractivity measurements allow us to observe the time evolution of the moisture field in the same way that radar reflectivity made possible the study of the mesoscale structure of precipitation.

During IHOP_2002, 7 weeks of data were collected during which we observed a great variety of events: fronts, dry lines, wave phenomena, conditioning of the boundary layer by surface conditions, convection initiation... Highlights of different categories of events are presented here to illustrate the type of information and detail that can be seen in radar-derived near-surface refractivity fields.

In particular, a more in-depth analysis of a good convection initiation near the intersection between a cold front and the dry line is presented (11 May 2002). Initial results suggest that, in order to properly predict convection initiation, detection of weak convergence boundaries (or kilometer-scale dynamics) may be more important than mapping kilometer-scale moisture variability. That being said, knowledge of the moisture variability at scales of 10 km and larger are required to hope to properly predict where convection will occur. The irony resides in the fact that, in many cases, weak boundaries were only detected thanks to refractivity or moisture field measurements. Hence, kilometer-scale moisture or refractivity may be required solely to resolve the otherwise hidden small-scale dynamics.