Retrieving 2D water vapor fields from multiple microwave radiometers

Fig.1 Illustration of the tomographic measurement technique where the dark linesFig.1 Illustration of the tomographic measurement technique where dark lines are different elevation angles and dark circles are the locations of the instruments. Colored background represents the retrieved 2D water vapor field in g/kg

Since water vapor plays an important role in the atmosphere, many different instruments capable of measuring quantities of vapor are available to atmospheric scientists. One such instrument is the microwave radiometer, a passive instrument that can measure low-resolution profiles of water vapor. However, in the case of boundary layer and convective initiation studies, the resolution requirements by the scientific community are more demanding than what the instrumentation is currently capable of providing. Therefore, measurements techniques have been developed to help bridge this gap. One of these measurement techniques is tomography. The goal of this technique is to combine several similar instruments providing measurements at low ranging resolution or of integrated values, in such a way that they provide information on crossing paths, thereby making it possible to provide measurements at higher resolution (Fig. 1). This technique has been used in meteorology to measure field of water vapor and clouds with seemingly good results. Here, the performance of the tomographic retrieval of a 2D water vapor field using ground-based microwave radiometry was evaluated in order to assess the information content and the accuracy of the retrieval.

As seen in Fig. 2, increasing the number of instruments measuring the water vapor field does lead to a qualitatively better solution field. This also leads to an increase in the information content of each of the solution fields. However, there are limits to the tomographic retrieval. The individual warm and cold blobs present at the top of the boundary layer in the truth field are not individually resolved in the solutions. The constant boundary layer values in the truth field are also not well captured in the different solution fields. When examined more quantitatively, the variability of different solution fields is also reduced compared to the truth field. This is caused by the passive nature of the microwave radiometer measurements, which cannot resolve the exact location of the water vapor in the field.

Fig. 2 Different 2D water vapor solution fields using increasing number of microFig. 2 Different 2D water vapor solution fields using increasing number of microwave radiometers. All fields are in g/kg of water vapor. Black ticks are location of the radiometers. The left most field is the water vapor field that should measured.

About the author

Véronique Meunier is finishing her Ph.D. under the supervision of Prof. Pavlos Kollias. Her research interest is on developing new instruments and instrument based techniques related to ground-based microwave radiometry. Her thesis is about the evaluation of the use of ground-base microwave radiometer tomographic measurements to derive 2D water vapor fields.

She previously completed a B.Sc. and a M.Sc. in Atmospheric Sciences also at McGill University. Her M.Sc. was done under the supervision of Prof. Frédéric Fabry.