A “Radar” for Humidity, Clouds and Temperature: Introducing the Mesoscale Microwave Radiometer

Fabry and Sun (2010) have evaluated the atmospheric variables that are important in convective cloud formation and evolution in models. They have found that mid-level moisture is the most important quantity in storm evolution. This is followed by low-level moisture, low and mid-level temperature. Therefore, better measurements of these quantities and their assimilation in NWP model would greatly improve the possibility to correctly forecast convection. However, the water vapor field is extremely variable spatially and temporally. This makes it very difficult to measure accurately.

Themens and Fabry (2014) investigated the ability of different ground-based measurement strategies for constraining thermodynamic variables in the troposphere particularly at the mesoscale. Using analyses over one month from the Rapid Refresh model as input to an optimal estimation technique, it is shown that the horizontal density of a network of non-existing, ideal vertical profiling instruments must be greater than 30 km in order to achieve accuracies of 0.5 g kg^-1 for water vapor and 0.5 K for temperature. This illustrates the unfeasibility of such a network and call for another approach. The findings suggest that measurements from scanning radiometers will be needed to properly constrain the temperature and especially moisture fields to accuracies needed for mesoscale forecasting.

The mesoscale radiometer, a scanning microwave radiometer, was designed to bridge this gap in the measurement of water vapor. This instrument is capable of obtaining a full volume scan of the surrounding atmosphere at mesoscale in a time scale similar to that of operational radars. The prime utility of this instrument is to investigate the horizontal distribution of moisture in the atmospheric boundary layer. At the same time, it provides us with horizontal gradients of liquid water content in the atmosphere as well as some limited temperature constraint at low levels. The mesoscale radiometer has been developed at the McGill’s J.S. Marshall Weather Radar Observatory in Montreal, Quebec, Canada.

Initial measurements from the mesoscale radiometer were obtained on August 19-20^th 2009. These measurements were compared to surface and GPS stations as well as radar measurements. They demonstrate the ability of the mesoscale radiometer to provide humidity information as well as detect the presence of liquid clouds along long paths.

Fabry, Frédéric, and Juanzhen Sun, (2010): "For how long should what data be assimilated for the mesoscale forecasting of convection and why? Part I: On the propagation of initial condition errors and their implications for data assimilation." Monthly Weather Review, 138.1, 242-255.

Themens, David, and Frédéric Fabry. (2014):  "Why Scanning Instruments Are a Necessity for Constraining Temperatureand Humidity Fields in the Lower Atmosphere." Journal of Atmospheric and Oceanic Technology, 31.11, 2462-2481.