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John P. Iselin and William J. Gutowski Jr.

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.

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William J. Gutowski Jr., Zekai Ötles, and Yibin Chen

Abstract

A sensitivity study is performed to examine the potential effect of spatial variations in sea surface temperature (SST) that typically are not resolved in general climate models (GCMs). The study uses a single-column atmospheric model, representing a grid box of a GCM, that overlies a surface domain divided into many subgrid cells. The model is driven by boundary conditions representative of the Gulf Stream off the mid-Atlantic coast of the United States, for the year 1987. A heterogeneous simulation, which includes subgrid spatial variability in SST, is contrasted with a homogeneous simulation, which assigns spatial mean SST to all cells.

In summer, the presence of both stable and unstable surface layers in the heterogeneous domain causes heterogeneous–homogeneous differences in monthly, spatially averaged surface latent-heat flux of up to 47%. In contrast, in winter, the surface layer is unstable everywhere and heterogeneous–homogeneous differences in latent heat flux are smaller. Spatially averaged, surface sensible heat flux shows less influence of SST heterogeneity because this flux during summer is small. Further simulation suggests that a GCM can capture the effect of spatially varying boundary layer stability by resolving it just at the surface. The SST heterogeneity is also capable of driving sea-breeze-type circulations. Scale analysis suggests that typical resolution of contemporary climate GCMs will generally be insufficient to resolve these circulations.

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William J. Gutowski Jr., James W. Seidel, and Andrew B. Ervin

Abstract

A previous examination of water vapor layers in Project STORM-FEST is extended to include Project STORM-WAVE rawinsonde observations and assess the contribution of layers in these two datasets to atmospheric water transport. The observations indicate that the contribution of these layers to water transport climatology is only a few percent. However the analysis also shows that episodes occur fairly frequently where these layers contribute 20% or more of the horizontal transport. Instances when the layer’s moisture is an important part of the water transport tend to occur for relatively dry soundings. Numerical models that fail to resolve the layers during these episodes may thus miss condensation events leading to cloud formation and precipitation, and also give overly smooth vertical profiles of radiative heating and cooling. The layers thus appear to be important for numerical weather prediction.

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