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M. Zupanski and J. McGinley

Abstract

Recent diagnostic and numerical studies have been shown that cyclogensis events in the lee of the Alps occur when: 1) an upper-level trough is upstream: 2) a low-level frontal system impinges on the Alps and, 3) an upper-level streak on the west side of trough moves into the northern Mediterranean.

Three case studies focusing on the rapid development stage of Alpine lee cyclogensis are investigated by performing a set of numerical experiments, with emphasis on the above mentioned factors. In order to create slightly different initial fields, we have used a two layer smoothing technique, alternatively reducing low-level available potential energy testing (2)], or reducing an upper-level wind maxima [testing (3)]. Once this is done we readjust the mass and momentum fields using a variational initialization scheme with weak geostrophic constraints.

Based on the results of these cases of lee cyclogensis, the weaker lee developments were significantly reduced by decreasing the low-level frontal intensity (2), which implied a greater influence of the low level dynamical processes (frontal impingement) relative to jet streak processes (geostrophic adjustment). In the case of relatively strong cyclogensis, dynamical processes associated with the upper-level jet streak become more important factor than low forcing. An overall inhibitory effect of the Alps was obvious in all three cases. particularly in the case of strong cyclogensis. However, these experiments did show localization of development as manifested by a high-low dipole structure of the mountain induced pressure perturbations.

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G. G. Carrió, W. R. Cotton, D. Zupanski, and M. Zupanski

Abstract

A cloud-nucleating aerosol retrieval method was developed. It allows the estimation of ice-forming nuclei and cloud condensation nuclei (IFN and CCN) for regions in which boundary layer clouds prevail. The method is based on the assumption that the periodical assimilation of observations into a microscale model leads to an improved estimation of the model state vector (that contains the cloud-nucleating aerosol concentrations). The Colorado State University Cloud Resolving Model (CRM) version of the Regional Atmospheric Modeling System (RAMS@CSU) and the maximum likelihood ensemble filter algorithm (MLEF) were used as the forecast model and the assimilation algorithm, respectively. On the one hand, the microphysical modules of this CRM explicitly consider the nucleation of IFN, CCN, and giant CCN. On the other hand, the MLEF provides an important advantage because it is defined to address highly nonlinear problems, employing an iterative minimization of a cost function. This paper explores the possibility of using an assimilation technique with microscale models. These initial series of experiments focused on isolating the model response and showed that data assimilation enhanced its performance in simulating a mixed-phase Arctic boundary layer cloud. Moreover, the coupled model was successful in reproducing the presence of an observed polluted air mass above the inversion.

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Sara Q. Zhang, T. Matsui, S. Cheung, M. Zupanski, and C. Peters-Lidard

Abstract

This work assimilates multisensor precipitation-sensitive microwave radiance observations into a storm-scale NASA Unified Weather Research and Forecasting (NU-WRF) Model simulation of the West African monsoon. The analysis consists of a full description of the atmospheric states and a realistic cloud and precipitation distribution that is consistent with the observed dynamic and physical features. The analysis shows an improved representation of monsoon precipitation and its interaction with dynamics over West Africa. Most significantly, assimilation of precipitation-affected microwave radiance has a positive impact on the distribution of precipitation intensity and also modulates the propagation of cloud precipitation systems associated with the African easterly jet. Using an ensemble-based assimilation technique that allows state-dependent forecast error covariance among dynamical and microphysical variables, this work shows that the assimilation of precipitation-sensitive microwave radiances over the West African monsoon rainband enables initialization of storms. These storms show the characteristics of continental tropical convection that enhance the connection between tropical waves and organized convection systems.

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