Conceptualisation and Design of a "Mesoscale Radiometer"

Where we describe the development and functionality of a new type of instrument, a mesoscale radiometer, designed to obtain crude but much needed measurements of fields of temperature and humidity over mesoscale areas around the instrument. 1. DECLARATION OF INTENT Whereas the meteorological research community wishes to measure or at least constrain better fields of humidity and temperature for storm studies. Whereas the operational weather community wishes to better predict storms, a task that requires improved constraints on the initial conditions of thermodynamic variables. Whereas studies focusing on mesoscale events have shown that lack of knowledge of midlevel moisture limits the most the accuracy of weather forecasts [1] and that lack of knowledge of temperature fields limits the most convection initiation forecasts [2]. Whereas technology exists to measure winds and precipitation over large areas but is lagging behind for temperature and humidity, except for satellite-borne sensors that do not provide, in fine, data with the necessary temporal resolution in cloudy environments. Whereas 3-D fields of thermodynamic parameters are required to advance mesoscale meteorology research, making simple ground-based profiling instruments insufficient for the task. Whereas the operational and research communities have been looking for [3] and demanding [4] new approaches to obtain better humidity measurements at all scales for many years. Whereas, prima facie, there did not appear to have been significant technological developments undertaken to fill this gap in our observing system capabilities at the time this project was instigated. It was hereby resolved to develop of a new type of ground-based sensor that would attempt to measure the said humidity and temperature fields over mesoscale areas. This is its story. 2. APPROACH CONSIDERATIONS If one decides to design an instrument to make thermodynamic measurements at the mesoscale, one is rapidly faced with a few physical and technical constraints: 1. Dwell time is limited. If one wants to measure fields of temperature and humidity at the mesoscale, the instrument must sample the region in an appropriately short time. A full volume scan of the atmosphere, say with several elevation angles, must therefore be completed in the order of 15 min or less, limiting the dwell time in any (azimuth, elevation) direction to much less than 1 s. 2. Active acoustic sensing is too slow for mesoscale distances, and passive sensing of acoustic waves and of electric, magnetic, and gravity fields did not seem to hold much potential for sensors on a ground-based platform. This leaves only EM waves. That being written, we may easily be wrong here; this issue should therefore be revisited before we completely abandon more unusual avenues. 3. If EM waves are used, they must include microwaves: since measurements must be taken by sampling the atmosphere over paths many tens of kilometres long, the wavelength(s) chosen must be in an electromagnetic region where the aerosolloaded and occasionally cloudy atmosphere is relatively transparent. Yet, at the same time, the atmosphere must cause changes in one or more EM properties (velocity/delay, emission, or scattering) as temperature and/or humidity changes. These seemingly contradictory constraints made us focus our attention on the peak and both tails of the 22 GHz water vapour band. 4. With long sampling paths, the instrument’s beam width must therefore be narrow and ideally similar at all wavelengths of operation. Given this and the previous constraints, the instrument should visually look like a radar, with a large antenna on a scanning pedestal. Two different approaches met the constraints: a tripleor quadruple-frequency radar operating at K-band, making differential attenuation measurements where targets from insects to precipitation are available; and a narrow-beam multi-frequency radiometer operating also K-band. Budget considerations made us go for the second option because the first author could not add another zero to his request for funding. For reasons that should now be clear, we started calling this instrument a “mesoscale radiometer”. © Proceedings of the 8th International Symposium on Tropospheric Profiling, ISBN 978-90-6960-233-2 Delft, The Netherlands, October 2009. Editors, A. Apituley, H.W.J. Russchenberg, W.A.A. Monna S06 P03 1