A new bulk microphysical parameterization (BMP) has been developed for use with the Weather Research and Forecasting (WRF) Model or other mesoscale models. As compared with earlier single-moment BMPs, the new scheme incorporates a large number of improvements to both physical processes and computer coding, and it employs many techniques found in far more sophisticated spectral/bin schemes using lookup tables. Unlike any other BMP, the assumed snow size distribution depends on both ice water content and temperature and is represented as a sum of exponential and gamma distributions. Furthermore, snow assumes a nonspherical shape with a bulk density that varies inversely with diameter as found in observations and in contrast to nearly all other BMPs that assume spherical snow with constant density. The new scheme’s snow category was readily modified to match previous research in sensitivity experiments designed to test the sphericity and distribution shape characteristics. From analysis of four idealized sensitivity experiments, it was determined that the sphericity and constant density assumptions play a major role in producing supercooled liquid water whereas the assumed distribution shape plays a lesser, but nonnegligible, role. Further testing using numerous case studies and comparing model results with in situ and other observations confirmed the results of the idealized experiments and are briefly mentioned herein, but more detailed, microphysical comparisons with observations are found in a companion paper in this series (Part III, forthcoming).

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[1] Deep intrusions of tropospheric air into the lower stratosphere above the subtropical jet are investigated using new observations and meteorological analyses. These intrusions are characterized by low ozone concentration and low static stability. The low-ozone layer is consistently observed from ozonesonde profiles and satellite remote sensing data from Aura/HIRDLS. The intruding layer occurs along and under the poleward extending tropical tropopause, which becomes the secondary tropopause in middle to high latitudes. The association of the ozone and the thermal structure provides evidence for the physical significance of the subtropical tropopause break and the secondary tropopause. The core of the intruding layer is typically between 370 and 400 K potential temperature ($15 km), but the vertical extent of the intrusion can impact ozone above 400 K, the lower boundary of the overworld. Two intrusion events over the continental United States in the spring of 2007 are analyzed to show the spatial extent and the temporal evolution of the intruding air mass. These examples demonstrate the effectiveness of potential temperature lapse rate, i.e., static stability, as a diagnostic for the intrusion event.Comparison with the potential vorticity field is made to show the complementarity of the two dynamical fields. The static stability diagnostic provides a tool to map out the horizontal extent of the intruding layer and to investigate its evolution. Furthermore, the diagnostic makes it possible to forecast the intrusion event for field studies.

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[1] Model simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by wind fields of the National Center for Environmental Prediction (NCEP) were performed in the midlatitude tropopause region in April 2008 to study two research flights conducted during the START08 campaign. One flight targeted a deep tropospheric intrusion and another flight targeted a deep stratospheric intrusion event, both of them in the vicinity of the subtropical and polar jet. Air masses with strong signatures of mixing between stratospheric and tropospheric air masses were identified from measured CO-O 3 correlations, and the characteristics were reproduced by CLaMS model simulations. CLaMS simulations in turn complement the observations and provide a broader view of the mixed region in physical space. Using artificial tracers of air mass origin within CLaMS yields unique information about the transport pathways and their contribution to the composition in the mixed region from different transport origins. Three different regions are examined to categorize dominant transport processes: (1) on the cyclonic side of the polar jet within tropopause folds where air from the lowermost stratosphere and the cyclonic side of the jet is transported downward into the troposphere, (2) on the anticyclonic side of the polar jet around the 2 PVU surface air masses, where signatures of mixing between the troposphere and lowermost stratosphere were found with large contributions of air masses from low latitudes, and (3) in the lower stratosphere associated with a deep tropospheric intrusion originating in the tropical tropopause layer (TTL). Moreover, the time scale of transport from the TTL into the lowermost stratosphere is in the range of weeks whereas the stratospheric intrusions occur on a time scale of days.

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