Humidity observations using the NCAR dropsonde

The NCAR dropsondes are used extensively in hurricane surveillance, for targeted observations of severe systems and for a significant number of atmospheric basic research projects. These instruments are deployed from airborne platforms and measure pressure, temperature, water vapor and winds. In this presentation we focus on the observation of humidity by dropsondes. Although the sensor is the same used on the Vaisala RS92 radiosonde, the performance characteristics are significantly different due to the different method of deployment. Time lag and icing are a fundamental limit of the current technology. Furthermore, we recently identified a significant dry bias, which was caused by an improperly implemented algorithm within the sensor module, and which has been present for the last 7 years. This dry bias, which has a strong temperature dependence and impacts mostly observations below freezing, will be discussed in detail. A correction for this bias has already been implemented in the operational systems. Despite this dry bias these observations are the best humidity profiling observations currently possible within storms and under challenging open ocean conditions. We will discuss the overall uncertainty of humidity observations in light of fundamental limitations of the sensor and will provide an outlook on future developments to extend humidity observations to higher altitudes using dropsondes.