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- Author or Editor: Daniel Ramond x
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Abstract
Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discussed with respect to the conceptual approach based on the frontogenetic forcing in highly idealized two-dimensional flow configurations.
Abstract
Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discussed with respect to the conceptual approach based on the frontogenetic forcing in highly idealized two-dimensional flow configurations.
Abstract
A stratocumulus cloud located above a cold front in a midlatitude depression has been observed simultaneously by a K-band radar, instrumented aircraft and radiosondes. The radar recording suggests that convective generating cells near the cloud top present a quasi-periodic organization. This is confirmed by a spectral analysis of reflectivity fluctuations within the generating layer.
Aircraft data collected in the precipitation streamers under this layer exhibit systematic fluctuations both in temperature and hydrometeor number density fields. A positive correlation is shown to exist between these fluctuations. A simple numerical model is used to estimate the temperature increase resulting from the release of latent heat by deposition of water vapor on the precipitating crystals. Satisfactory agreement is obtained with the observed temperature and radar reflectivity data.
Abstract
A stratocumulus cloud located above a cold front in a midlatitude depression has been observed simultaneously by a K-band radar, instrumented aircraft and radiosondes. The radar recording suggests that convective generating cells near the cloud top present a quasi-periodic organization. This is confirmed by a spectral analysis of reflectivity fluctuations within the generating layer.
Aircraft data collected in the precipitation streamers under this layer exhibit systematic fluctuations both in temperature and hydrometeor number density fields. A positive correlation is shown to exist between these fluctuations. A simple numerical model is used to estimate the temperature increase resulting from the release of latent heat by deposition of water vapor on the precipitating crystals. Satisfactory agreement is obtained with the observed temperature and radar reflectivity data.
Abstract
Multiple parameter radar measurements (dual frequency hail signal DFR-HS or differential reflectivity Z DR) can be strongly affected by the mismatch of the antenna illumination functions at the two wavelengths or polarizations. The resulting instrumental error can be estimated, with some approximations, from the independently measured reflectivity at one wavelength or polarization by using the actual radar illumination functions. This objective estimation of the antenna-induced error is made in some real cases and its possible conseqences on the data analysis are discussed.
Abstract
Multiple parameter radar measurements (dual frequency hail signal DFR-HS or differential reflectivity Z DR) can be strongly affected by the mismatch of the antenna illumination functions at the two wavelengths or polarizations. The resulting instrumental error can be estimated, with some approximations, from the independently measured reflectivity at one wavelength or polarization by using the actual radar illumination functions. This objective estimation of the antenna-induced error is made in some real cases and its possible conseqences on the data analysis are discussed.
