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DIAMET was conducted during a period of particularly intense storm activity over the North Atlantic. As an example of an intense storm and to illustrate the scientific approach used in DIAMET, we present a more detailed study of Extratropical Cyclone Friedhelm on 8 December 2011. The strongest low-level winds in this storm occurred on its southern and southwestern flanks. We concentrate in this paper on the prominent cloud banding often found in the southern quadrant of intense storms ( Neiman et al
DIAMET was conducted during a period of particularly intense storm activity over the North Atlantic. As an example of an intense storm and to illustrate the scientific approach used in DIAMET, we present a more detailed study of Extratropical Cyclone Friedhelm on 8 December 2011. The strongest low-level winds in this storm occurred on its southern and southwestern flanks. We concentrate in this paper on the prominent cloud banding often found in the southern quadrant of intense storms ( Neiman et al
tropopause. Also, we show how convection in the southern part of the rainband was capped by the remnants of an upstream tropopause fold, adding to the evidence of Russell et al. (2008 , 2009) that thin stable layers of stratospheric origin are important agents of convective inhibition in synoptic weather systems. These results serve to challenge the perception that convection develops downstream of an upper-level trough as a direct response to forcing by the PV anomaly. The case study was chosen
tropopause. Also, we show how convection in the southern part of the rainband was capped by the remnants of an upstream tropopause fold, adding to the evidence of Russell et al. (2008 , 2009) that thin stable layers of stratospheric origin are important agents of convective inhibition in synoptic weather systems. These results serve to challenge the perception that convection develops downstream of an upper-level trough as a direct response to forcing by the PV anomaly. The case study was chosen
was discovered during the third author’s master’s thesis research ( Chiariello 2006 ). Therefore, the purpose of this article is to present this cold-type occluded front and to demonstrate its consistency with the static-stability rule. 2. Case study of a cold-type occluded front This case was found from looking at occluded fronts over the North Atlantic Ocean using model output from the European Centre for Medium-Range Weather Forecasts at 0.25° × 0.25° latitude–longitude gridded analyses. At
was discovered during the third author’s master’s thesis research ( Chiariello 2006 ). Therefore, the purpose of this article is to present this cold-type occluded front and to demonstrate its consistency with the static-stability rule. 2. Case study of a cold-type occluded front This case was found from looking at occluded fronts over the North Atlantic Ocean using model output from the European Centre for Medium-Range Weather Forecasts at 0.25° × 0.25° latitude–longitude gridded analyses. At
, just above the minimum in the analysis at 1200 UTC. Figure 4a shows the structure of wind speed, θ e , and RH ice obtained from the sondes. The southern arm of the bent-back front was crossed between 57° and 57.3°N and divides two distinct air masses: the cyclone’s warm seclusion to the north and the frontal fracture zone to the south. The strongest winds are confined below 720 hPa near the bent-back front with a maximum at 866 hPa, just above the boundary layer (51 m s −1 ). At this level, the
, just above the minimum in the analysis at 1200 UTC. Figure 4a shows the structure of wind speed, θ e , and RH ice obtained from the sondes. The southern arm of the bent-back front was crossed between 57° and 57.3°N and divides two distinct air masses: the cyclone’s warm seclusion to the north and the frontal fracture zone to the south. The strongest winds are confined below 720 hPa near the bent-back front with a maximum at 866 hPa, just above the boundary layer (51 m s −1 ). At this level, the
-and-gap structures ( Hobbs and Biswas 1979 ; James and Browning 1979 ; Hobbs and Persson 1982 ). Often, the cores are rotated anticyclonically from the orientation of the surface cold front. These features are clearly observed over land by precipitation radar (e.g., over the British Isles as in Fig. 1 ) and have also been observed over open ocean (e.g., Hobbs and Biswas 1979 ; Hobbs and Persson 1982 ; Wakimoto and Bosart 2000 ; Jorgensen et al. 2003 ). The precipitation associated with these fronts is
-and-gap structures ( Hobbs and Biswas 1979 ; James and Browning 1979 ; Hobbs and Persson 1982 ). Often, the cores are rotated anticyclonically from the orientation of the surface cold front. These features are clearly observed over land by precipitation radar (e.g., over the British Isles as in Fig. 1 ) and have also been observed over open ocean (e.g., Hobbs and Biswas 1979 ; Hobbs and Persson 1982 ; Wakimoto and Bosart 2000 ; Jorgensen et al. 2003 ). The precipitation associated with these fronts is
( Roebber et al. 2002 ) have produced counterintuitive results. In both cases, the interaction of a northern and a southern PV anomaly was hypothesized to have caused a more extreme event. In the case of the March 1993 Superstorm, the interaction of the two PV anomalies was hypothesized to reduce static stability and enhance baroclinicity. However, when the southern anomaly was removed using PV inversion, a stronger cyclone developed because the stronger northern anomaly was able to move eastward more
( Roebber et al. 2002 ) have produced counterintuitive results. In both cases, the interaction of a northern and a southern PV anomaly was hypothesized to have caused a more extreme event. In the case of the March 1993 Superstorm, the interaction of the two PV anomalies was hypothesized to reduce static stability and enhance baroclinicity. However, when the southern anomaly was removed using PV inversion, a stronger cyclone developed because the stronger northern anomaly was able to move eastward more
Chilbolton Facility for Atmospheric and Radio Research (CFARR) in Hampshire, southern England (51.14°N, 1.44°W). This is a fully steerable 3-GHz dual-polarization Doppler radar. During this flight the BAe-146 aircraft flew along a radial of 240° while CAMRa performed range–height indicator (RHI) scans along the same radial. Other data collected at CFARR to monitor the passage of the cold front included atmospheric pressure, temperature, wind speed, wind direction, and rainfall rate as measured by a rapid
Chilbolton Facility for Atmospheric and Radio Research (CFARR) in Hampshire, southern England (51.14°N, 1.44°W). This is a fully steerable 3-GHz dual-polarization Doppler radar. During this flight the BAe-146 aircraft flew along a radial of 240° while CAMRa performed range–height indicator (RHI) scans along the same radial. Other data collected at CFARR to monitor the passage of the cold front included atmospheric pressure, temperature, wind speed, wind direction, and rainfall rate as measured by a rapid
resolved on the small mesoscale, including the eastward-bulging configuration of the front near the center of the domain, the very sharp gradients in temperature and wind direction near the apex of the bulge, and the comparatively weak gradients over the southern parts of the domain, where the frontal wind veer and temperature decrease are spread over a zone ~40 km wide in the cross-front direction. A smaller region of weaker cross-front gradients is also apparent to the north of the line bulge, in the
resolved on the small mesoscale, including the eastward-bulging configuration of the front near the center of the domain, the very sharp gradients in temperature and wind direction near the apex of the bulge, and the comparatively weak gradients over the southern parts of the domain, where the frontal wind veer and temperature decrease are spread over a zone ~40 km wide in the cross-front direction. A smaller region of weaker cross-front gradients is also apparent to the north of the line bulge, in the
test profile The test ensemble comprises a 24-member ensemble of 3-h forecasts from a high-resolution weather forecasting model [the Met Office’s 1.5-km nowcasting model with a domain over the southern United Kingdom ( Golding et al. 2014 )]. The ensemble’s analysis perturbations are produced by the Met Office Global and Regional Ensemble Prediction System (MOGREPS) ( Bowler et al. 2008 ) and adapted for this domain ( Migliorini et al. 2011 ; Caron 2013 ; Baker et al. 2014 ). The MOGREPS
test profile The test ensemble comprises a 24-member ensemble of 3-h forecasts from a high-resolution weather forecasting model [the Met Office’s 1.5-km nowcasting model with a domain over the southern United Kingdom ( Golding et al. 2014 )]. The ensemble’s analysis perturbations are produced by the Met Office Global and Regional Ensemble Prediction System (MOGREPS) ( Bowler et al. 2008 ) and adapted for this domain ( Migliorini et al. 2011 ; Caron 2013 ; Baker et al. 2014 ). The MOGREPS
the 6-h period prior to this, with only a small reduction in the cyclone’s central low pressure during this time (1004–1002 hPa). Rainfall radar data (not shown) reveal that the frontal rainband had established itself over central and southern Scotland by 0600 UTC. Between 1200 and 1800 UTC, the system moved slowly eastward over the North Sea as the cyclone continued to deepen gradually to 998 hPa, although the orientation of the occluded front meant that rain continued to affect Scotland for much
the 6-h period prior to this, with only a small reduction in the cyclone’s central low pressure during this time (1004–1002 hPa). Rainfall radar data (not shown) reveal that the frontal rainband had established itself over central and southern Scotland by 0600 UTC. Between 1200 and 1800 UTC, the system moved slowly eastward over the North Sea as the cyclone continued to deepen gradually to 998 hPa, although the orientation of the occluded front meant that rain continued to affect Scotland for much
