Mountain wave structures occurring within a major orographic precipitation event: Part I. Analyses of airborne Doppler radar data
0
Citation
0
Reference
13
Related Paper
Keywords:
Orographic lift
Orography
Cite
Cite
Citations (1)
Observations of snowstorms were carried out around Ishikari Bay, Hokkaido, Japan, from December 1991 to February 1992. On 15 January 1992, a longitudinal-mode snowband was observed by a dual-Doppler radar system. The snowband was composed of meso-γ-scale (∼20 km) convective cloud systems. The three-dimensional kinematic structures and organization processes of the meso-γ-scale systems were studied in detail. Strong band-parallel winds appeared in the upper-north rear of the meso-γ-scale systems. Since their speed was faster than that of the cellular radar echoes, the winds formed rear-to-front currents. Increasing in volume and speed, the rear-to-front currents developed into meso-γ scale and penetrated toward the lower front and lower south of the systems. Consequently, the rear-to-front currents caused strong meso-γ-scale convergence and the enhancement of updrafts at their leading edges. The transport of the band-parallel momentum greatly contributed to the successive development of new convective cells within the systems and the organization of the meso-γ-scale systems in the snowband. Ice/snow particles were transported by the updrafts toward the rear and north of the systems in the overlying stable layer. They then evaporated outside the clouds. The downdraft caused by evaporative cooling played an important role in the transport of band-parallel momentum from the upper to the lower levels and from the outside to the inside of the snowband.
Momentum (technical analysis)
Cold front
Mode (computer interface)
Cite
Citations (33)
Airborne radar data collected within Hurricane Karl (2010) provide a high-resolution glimpse of variations in the vertical precipitation structure around complex terrain in eastern Mexico. Widespread precipitation north of Karl’s track traced the strong gradient of terrain, suggesting orographic enhancement. Although the airborne radar did not sample the period of peak precipitation, time series of surface rainfall at three locations near the inner core show greater precipitation where flow was oriented to rise over the terrain. In regions of upslope flow, radar observations reveal reflectivity enhancement within 1–2 km of the surface. The shallow nature of the enhancement points to orographically generated cloud water accreted by falling drops as a mechanism consistent with prior studies, while the heterogeneous nature of the enhancement suggests shallow convection was playing a role. In contrast, regions of downslope flow were characterized by uniform reflectivity above the ground and fallstreaks originating above the melting level. Unlike most previously studied tropical cyclones passing over topography, Karl made landfall on a mountainous continent, not an island. As Karl weakened and decayed over land, the vertical structure of the radar echo deteriorated north of the storm center, and infrared satellite imagery revealed a strong reduction in the upper-level cloud coverage; however, a small region of intense convection appeared and produced locally heavy rainfall as Karl was close to dissipation. These results indicate that orographic modification processes in a landfalling tropical cyclone are not static, and surface precipitation is highly sensitive to the changes.
Orographic lift
Orography
Precipitation types
Rainband
Cite
Citations (19)
Cite
Citations (0)
Abstract This study documents orographic precipitation forcing along the coastal mountains of Northern California during the landfall of a significant winter storm over the period 16–18 February 2004. The primary observing asset is a scanning X-band Doppler radar deployed on the coast at Fort Ross, California, which provides low-level (e.g., below 1 km MSL) horizontal and vertical scans of radial velocity and reflectivity to characterize airflow and precipitation structures. Further context is provided by a wind-profiling radar, a radio acoustic sounding system (RASS), balloon soundings, buoys, a GPS receiver, and surface meteorological sensors. The winter storm is divided into two episodes, each having pre-cold-frontal low-level jet (LLJ) structures and atmospheric river characteristics. Episode 1 has a corridor of terrain-trapped airflow (TTA) that forms an interface with the LLJ. The interface extends ~25 km offshore in a ~0.5-km vertical layer, and the western edge of this interface near the ocean surface advances toward the coast over the course of ~5 h. The TTA acts as a dynamically driven barrier, so that the incoming LLJ slopes upward offshore below 1.5 km MSL and precipitation is enhanced over the ocean and near the coast. The absence of a TTA in episode 2 allows the cross-barrier flow to slope upward and enhance precipitation directly over the coastal mountains. A theoretical analysis favors the hypothesis that a gap flow exiting the Petaluma Gap forces the TTA.
Orographic lift
Orography
Cite
Citations (11)
During the Baiu season (17 June-8 July 1991) we carried out simultaneous tropospheric observation by using the MU (Middle and Upper atmosphere) radar (VHF band, Kyoto University) and meteorological radars (C band of Osaka Meteorological Observatory, X band of Hokkaido University and C/Ku band of Kyoto University). Vertical distributions of three components of wind field and precipitation particles were observed by the MU and C/Ku-band radars, respectively. The C- and X-band radars were used to investigate horizontal distributions of precipitating clouds in the meso-α and -β scales, respectively. Several meso-β and -γ-scale cloud systems were observed around a meso-α-scale cyclone center during 4-5 July when rainfall was the strongest in the whole observational period. They were divided into two groups of convective clouds i) near a surface warm front and ii) near a surface cold front, and iii) one group of stratiform clouds on the north-western side of the surface cold front. In i), a remarkable updraft inside a precipitating cloud extending up to an altitude of 14km was produced by a convergence (inflows coming from the front and rear of the precipitating cloud) at an altitude of 4-5km and by strong southerly wind in the middle troposphere. In ii), a narrow rainband with gust front was seen at the leading edge of the surface cold front.Two meso-γ-scale rotor circulations were found in front of and inside the rainband, respectively. In iii), south-easterly (north-westerly) ascent (descent) flows were observed above (inside/under) the cold frontal surface extending up to an altitude of about 9km. Below the cold frontal surface, there was a dry region without precipitation, and a part of the descending westerly flow returned to the back of the precipitating cloud. In this study, vertical structures of meso-β and -γ-scale cloud systems with characteristic wind flows as mentioned above were revealed by the detailed three components of wind field in both clear and precipitating atmosphere. They were presented as smaller cloud systems in the hierarchical structure of cloud clusters near the meso-α-scale cyclone in the central region of the Japan Islands.
Rainband
Cold front
Cite
Citations (24)
We carried out a three-week observation campaign using the MU radar and meteorological (C-, X-, Ku-band) radars during the Baiu season in 1991 (17 June-8 July). A complete data set of three-dimensional atmospheric motions with high reliability and resolution was produced after removing raindrop echoes. The Baiu front was located to the south of the radar site during 17-24 June, and to the north during 25-28 June. After 29 June, surface medium-scale (meso-α-scale) cyclone centers passed near the MU observatory, and the tropopause jet stream became strong (one or two days) after the low-level jet stream appeared near the surface cyclone center. We have investigated meso-β-scale vertical velocity fluctuations and precipitating cloud clusters with temporal scales of several hours for various locations relative to the surface cyclone centers: (i) one case in the northern side of a cyclone center on the Baiu front, (ii) four cases near developing surface cyclone centers and (iii) one case in the southern side of the Baiu front far from a cyclone center. For (i) and (ii), the vertical distributions of upward-velocity regions were strongly dependent on the level of lowest stratiform turbulence near the tropopause level (LSTT) and the frontal surface. The upward velocities in the cases (ii) were associated with the developing cloud cluster on the northern side of the surface warm front, while they were dominant over the region of meso-β scale but did not always have precipitation on the northern side of the surface cold front. For (iii) some upward-velocity regions without precipitation penetrated LSTT. Each meso-β-scale fluctuation for all the cases (i)-(iii) included a number of upward-velocity peaks corresponding to meso-γ-scale disturbances. Some of them were coincident with surface rainfall (and lower-tropospheric precipitation echo) peaks. Based on all the observational evidence mentioned above, we propose a schematic picture of the 'hierarchical structure' of the vertical velocity fluctuations near the Baiu front. This is partly the same as the well-known multi-scale structure composed of meso-α-scale cyclones, meso-β-scale cloud clusters and meso-γ-scale precipitating clouds, but covers more broad clear regions which cannot be observed by foregoing studies based on only meteorological radars and satellites.
Tropopause
Cold front
Jet stream
Rainband
Warm front
Cite
Citations (29)
A classification of snow clouds, called the "snowfall mode," is proposed based on Doppler radar observations at 10-minute intervals at Nagaoka in 1999/2000 winter season. Using 795 hours of data at an altitude of 1.6 km, six snowfall modes were defined: longitudinal line (L-mode), transversal line (T-mode), spreading precipitation (S-mode), meso-β scale vortex (V-mode), mountain-slope precipitation (M-mode), and local-frontal (discontinuity) band (D-mode). In migrating snow clouds, a sub-class, referred to as snowfall with coastal intensification (xI-mode, where x is L, T, S and V) was defined. A sample snapshot and the mean Ze are shown for each snowfall mode. The frequency of occurrence of the snowfall modes indicated that both of the longitudinal cloud streets and the meso-α scale disturbances occupied about 1/3 of the analysis period. About 18% of the precipitation in the analysis period was considered to be under orographic effects. The prevailing wind direction differed between the snowfall modes although a west-northwesterly wind dominated.
Orographic lift
Mode (computer interface)
Cite
Citations (23)
Abstract Airborne vertically pointing Doppler radar data collected in 10 winter storms over the Medicine Bow Range in Wyoming are used to examine the importance of boundary layer (BL) turbulence for orographic precipitation growth. In all 10 cases, the cloud-base temperature was below 0°C and the bulk Froude number was more than 1.0, implying little or no blocking of the flow by the mountain barrier. Seven of the 10 storms sampled were postfrontal, with weak static stability and relatively shallow cloud tops. Doppler vertical velocity transects depict an approximately 1-km-deep turbulent layer draped over the terrain, sometimes clearly distinct from the stratified flow in the free troposphere aloft, where vertical motion is largely controlled by gravity wave dynamics. Spectral analysis of near-surface Doppler vertical velocity data in terrain-following coordinates reveals an inertial subrange with decreasing power with height toward the BL top. The composite of radar data profiles from the 10 flights is analyzed in frequency-by-altitude diagrams, with altitude expressed above ground level. These diagrams indicate a wide range of vertical velocities in the BL, and rapid snow growth within the BL as air rises through the cloud base, especially when BL turbulence is more intense. This snow growth is concentrated on the windward side of mountains, above the terrain–cloud base intersection. The dominant snow growth mechanism in the BL (i.e., by accretion or vapor deposition) cannot be established because of restrictions in aircraft flight level over complex terrain. Snow aggregation may have contributed to the observed rapid increase in reflectivity in the BL along the windward slope.
Orographic lift
Cloud top
Cite
Citations (48)