Constraining Temperature and Humidity for Convection Forecasting: On the Necessity of Having Scanning Ground-Based Instruments

Because the forecasting of convection is extremely sensitive to initial conditions, the mesoscale community is considering the deployment of a dense (~ 150-km spacing) network of profiling instrumentation to achieve the accuracy in temperature (0.5 C) and humidity (0.5 g/kg) it needs in the lower troposphere.

Using analyses over one month from the Rapid Refresh model as input, we first computed the instrument spacing that would be required for a near-perfect, non-existing profiling instrument to achieve the stated accuracy. We found that in summer conditions, 40-km spacing between these idealized instruments would not achieve the required accuracy need. This is a simple consequence of the spatial variability of the temperature and humidity fields, especially in the lower troposphere. And while satellite-borne instruments may reach the required horizontal resolution, it is not clear whether they can retrieve the needed information low in the atmosphere over land, especially if the scene is partly cloudy.

If instead we use the information retrieved by relatively simple scanning instruments such as scanning microwave radiometers that can obtain information close to the horizon, we show that we can actually approach the needed accuracy requirement. This is possible because even though the measurement accuracy and capabilities of microwave radiometers are far from those of the ideal profiler mentioned above, their ability to observe in many directions in azimuth and elevation constrains the temperature and especially the humidity fields more effectively. The consequences of the result from this simulation exercise will be discussed.