Retrieval simulations of a spaceborne differential absorption radar near the 380 GHz water vapor line

Abstract. Differential Absorption Radar (DAR) is an emerging technique for high resolution humidity profiling inside clouds and precipitation. This study evaluates the potential of using a spaceborne DAR operating near the 380 GHz water vapor absorption line to profile water vapor in the mid and upper troposphere, particularly inside deep convective systems. To quantify the expected precision and accuracy of DAR and to define optimal channel selection, we modeled radar reflectivities from large-eddy simulation fields and then implemented retrievals using the simulated observations.

End-to-end retrieval simulations across the 350–380 GHz range were used to identify optimal radar frequency triplets, minimizing precision and biases, at each altitude. Each optimum triplet included the most transparent frequency available, with the other two radar tones varying with altitude. At higher altitudes the optimization identifies frequencies close to the line center and the optimum frequencies move progressively away from the line at lower altitudes. Results show that single-pixel (horizontal resolution ≃ 400 m and vertical resolution = 200 m) precision generally exceeds 100 % with biases typically below 10 %. Precision can be enhanced by averaging along-track. For instance, by optimizing the triplet selection, a precision of 0.01
gm⁻³ can be achieved by averaging over 50 km in anvil outflows with extensive cloud coverage. We note that the improvement may be less than expected in scenarios where cloud coverage is limited since the DAR technique only works in cloudy volumes.

Lastly, we use real world clouds observed by CloudSat to quantify global yield. Most radar tones examined here achieve a global sampling yield of over 95 % at their target altitude. When developing a DAR instrument, selecting the appropriate triplet is essential, taking into account the target altitude and cloud types intended for observation.