Water Vapor Layers in STORM-FEST Rawinsonde Observations

Abstract

The STORM-FEST (Fronts Experiment Systems Test) rawinsonde data were analyzed to determine the abundance and characteristics of moist layers within the troposphere. A moist layer was defined as a local maximum in relative humidity with lower relative humidity air above and below. Moist layers under the criteria occur in over half the soundings with an average location between 600 and 500 mb and an average thickness of approximately 120 mb. The layers also appeared to be more nearly aligned with isentropic, rather than isobaric, surfaces. Compositing of relative humidity profiles with a layer at approximately the same level showed an increase in lapse rate at the top of moist layers indicating that the layers are contained by dynamic mechanisms. In addition, there was no diurnal cycle to the characteristics of the layers. These factors suggest a close relationship between the layers and large-scale dynamics. An examination of spatial continuity suggests a horizontal scale of a few hundred kilometers. Their appearance poses a challenge for numerical modeling of atmospheric water vapor. Furthermore, limitations of the two types of rawinsonde instruments used in STORM-FEST are apparent in some characteristics of the layers, thus indicating instrumentation challenges posed by these structures for observing the atmospheric branch of the hydrological cycle.