scholarly journals Terrain-Trapped Airflows and Orographic Rainfall along the Coast of Northern California. Part II: Horizontal and Vertical Structures Observed by a Scanning Doppler Radar

2018 ◽  
Vol 146 (8) ◽  
pp. 2381-2402 ◽  
Author(s):  
Raul A. Valenzuela ◽  
David E. Kingsmill
Keyword(s):  
Wind Direction ◽  
Doppler Radar ◽  
Horizontal Gradient ◽  
Kinematic Structure ◽  
Northern California ◽  
Winter Storms ◽  
Air Masses ◽  
Additional Information ◽  
Orographic Rainfall ◽  
Close Proximity

Abstract This study documents the mean properties and variability of kinematic and precipitation structures associated with orographic precipitation along the coast of Northern California in the context of terrain-trapped airflows (TTAs). TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. Seven land-falling winter storms are examined with observations from a scanning X-band Doppler radar deployed on the coast at Fort Ross, California. Additional information is provided by a 915-MHz wind-profiling radar, surface meteorology, a GPS receiver, and balloon soundings. The composite kinematic structure during TTA conditions exhibits a significant horizontal gradient of wind direction from the coast to approximately 50 km offshore and a low-level jet (LLJ) that surmounts a weaker airflow offshore corresponding to the TTA, with a zone of enhanced precipitation evident between ~5 and 25 km offshore and oriented nearly parallel to the coastline. Conversely, the composite kinematic structure during NO-TTA conditions exhibits a smaller offshore horizontal gradient of wind direction and precipitation structures are generally enhanced within km of the coastline. Interstorm variability analysis reveals significant variations in kinematic structures during both TTA and NO-TTA conditions, whereas significant variations in precipitation structures are only evident during TTA conditions. The interstorm analysis also illustrates more clearly how LLJ vertical structures evident during NO-TTA conditions exhibit ascent along the coast and over the coastal mountains, which is in contrast to TTA conditions where the ascent occurs offshore and over the TTA.

scholarly journals Terrain-Trapped Airflows and Orographic Rainfall along the Coast of Northern California. Part I: Kinematic Characterization Using a Wind Profiling Radar

2017 ◽  
Vol 145 (8) ◽  
pp. 2993-3008 ◽  
Author(s):  
Raul A. Valenzuela ◽  
David E. Kingsmill
Keyword(s):  
Vertical Shear ◽  
Northern California ◽  
Wind Maximum ◽  
Easterly Wind ◽  
Air Masses ◽  
Orographic Rainfall ◽  
Close Proximity ◽  
Flow Features ◽  
Bodega Bay

This study develops an objective method of identifying terrain-trapped airflows (TTAs) along the coast of Northern California and documenting their impact on orographic rainfall. TTAs are defined as relatively narrow air masses that consistently flow in close proximity and approximately parallel to an orographic barrier. A 13-winter-seasons dataset is employed, including observations from a 915-MHz wind profiling radar along the coast at Bodega Bay (BBY, 15 m MSL) and surface meteorology stations at BBY and in the coastal mountains at Cazadero (CZD, 478 m MSL). A subset of rainy hours exhibits a profile with enhanced vertical shear and an easterly wind maximum in the lowest 500 m MSL, roughly the same depth as the nearby coastal terrain. Both flow features have a connection to TTAs along the coast of Northern California. Based on the average orientation (320°–140°) and altitude of nearby topography, mean wind direction in the lowest 500 m MSL ([Formula: see text]) between 0°–140° is used as the initial criterion to identify TTA conditions. Application of this threshold yields a CZD/BBY rainfall ratio of 1.4 (3.2) for TTA (NO TTA) conditions. More detailed analysis of the relationship between [Formula: see text] and orographic rainfall reveals that an upper threshold of 150° more precisely divides the TTA and NO-TTA regimes. A sensitivity analysis and comparison with a TTA documented in a previous case study show that the best TTA identification criteria correspond to [Formula: see text] with a duration of at least 2 h. This objective identification method is applied to seven case studies in Part II of the present study.

scholarly journals An assessment of the Ambrosia L. pollen threat at a regional scale using the example of the town of Sosnowiec (Silesian Uplands, Poland)

2012 ◽  
Vol 64 (2) ◽  
pp. 51-62 ◽  
Author(s):  
Kazimiera Chłopek ◽  
Katarzyna Dąbrowsa-Zapart ◽  
Barbara Tokarska-Guzik
Keyword(s):  
Wind Direction ◽  
Regional Scale ◽  
Pollen Grains ◽  
Maximum Temperature ◽  
Sampling Point ◽  
Air Masses ◽  
Daily Count ◽  
Annual Total ◽  
The Town ◽  
Maximum Air Temperature

The investigation of <i>Ambrosia</i> pollen counts in the air of Sosnowiec was carried out from 1998 to 2010 by means of the volumetric method. The pollen season which was determined by means of the 98% method started at the end of July and the beginning of August and lasted until the end of October. The highest <i>Ambrosia</i> pollen count was recorded in 1999 (222 grains x m<sup>-3</sup>) and the lowest in 2001 (18 grains x m<sup>-3</sup>). It was stated that the daily count of pollen grains depended on the wind direction and maximum air temperature. The strongest correlations were found with maximum temperature and with a wind direction from the south east. A high negative correlation coefficient was found between the frequency of inflows of air masses from the west and the annual total of pollen grains and the value of the maximum daily count. The closest sites of <i>Ambrosia</i> L. are at a distance of 25-40 kilometres from the sampling point. Significant correlations with the frequency of inflow of air masses can support the conclusion that <i>Ambrosia</i> pollen grains recorded in Sosnowiec were most probably carried not only from local sources but also, at least in part, from distant places.

Insights into the Evolving Microphysical and Kinematic Structure of Northeastern U.S. Winter Storms from Dual-Polarization Doppler Radar

2017 ◽  
Vol 145 (3) ◽  
pp. 1033-1061 ◽  
Author(s):  
Matthew R. Kumjian ◽  
Kelly A. Lombardo
Keyword(s):  
Doppler Radar ◽  
Elevation Angle ◽  
Model Performance ◽  
Radar Data ◽  
Added Value ◽  
Dual Polarization ◽  
Differential Phase Shift ◽  
Winter Storms ◽  
Microphysical Processes ◽  
Ice Production

The recent Weather Surveillance Radar-1988 Doppler (WSR-88D) network upgrade to dual-polarization capabilities allows for bulk characterization of microphysical processes in northeastern U.S. winter storms for the first time. In this study, the quasi-vertical profile (QVP) technique (wherein data from a given elevation angle scan are azimuthally averaged and the range coordinate is converted to height) is extended and applied to polarimetric WSR-88D observations of six Northeast winter storms to survey their evolving, bulk vertical microphysical and kinematic structures. These analyses are supplemented using hourly analyses from the Rapid Refresh (RAP) model. Regions of ascent inferred from QVPs were consistently associated with notable polarimetric signatures, implying planar crystal growth when near −15°C, and riming and secondary ice production at higher temperatures. The heaviest snowfall occurred most often when ascent and enhanced propagation differential phase shift ([Formula: see text]) occurred near −15°C. When available, limited surface observations confirmed heavy snowfall rates and revealed large snow-to-liquid ratios at these times. Other cases revealed sudden, large melting-layer excursions associated with precipitation-type transitions near the surface. RAP analyses failed to capture such complex evolution, demonstrating the added value of dual-polarization radar observations in these scenarios and the potential use of radar data for assessing model performance in real time. These insights are a preliminary step toward better understanding the complex processes in northeastern U.S. winter storms.

scholarly journals Analysis of Convergence Boundaries Observed during IHOP_2002

2010 ◽  
Vol 138 (7) ◽  
pp. 2737-2760 ◽  
Author(s):  
Roger M. Wakimoto ◽  
Hanne V. Murphey
Keyword(s):  
Doppler Radar ◽  
The Other ◽  
Horizontal Gradient ◽  
Mobile Platforms ◽  
Thermodynamic Structure ◽  
Low Level ◽  
Axis Parallel ◽  
Using Data ◽  
Outflow Boundary ◽  
Convection Initiation

Abstract An analysis of six convergence boundaries observed during the International H2O Project (IHOP_2002) is presented. The detailed kinematic and thermodynamic structure of these boundaries was examined using data collected by an airborne Doppler radar and a series of dropsondes released by a jet flying at ∼500 mb. The former and latter platforms were able to resolve the meso-γ- and meso-β-scale circulations, respectively. Convection initiated on three of the days while no storms developed in the regions targeted by the mobile platforms on the other days (referred to as null cases). The airborne radar resolved the finescale structure of four drylines, a cold front, and an outflow boundary on the six days. Horizontal profiles through radar-detected thin lines revealed “bell-shaped distributions” and there appeared to be a seasonal dependence of the peak values of radar reflectivity. The echo profiles through the fine line in May were, in general, greater than those plotted for the June cases. There was no apparent relationship between the intensity of the low-level updraft and convection initiation. The strongest updraft resolved in the dual-Doppler wind synthesis was associated with a null case. There was also no relationship between the strength of the moisture discontinuity across the boundaries and convection initiation. The three days during which the storms developed were all associated with two convergence boundaries that were adjacent to each other. The two boundaries collided on one of the days; however, the boundaries on the other two days were approximately parallel and remained separated by a distance of 5–15 km. The total derivative of the horizontal vorticity rotating along an axis parallel to the boundary was calculated using dropsonde data. The horizontal gradient of buoyancy was the largest contributor to the change in vorticity and revealed maximum and minimum values that would support the generation of counterrotating circulations, thus promoting vertically rising air parcels. These updrafts would be more conducive to convection initiation. The null cases were characterized by a low-level vorticity generation of only one sign. This pattern would support tilted updrafts. The results presented in this study suggest that it is not necessary for two boundaries to collide in order for thunderstorms to develop. Solenoidally generated horizontal circulations can produce conditions favorable for convection initiation even if the boundaries remain separate.

Convective Cells in Altocumulus Observed with a High-Resolution Radar

2014 ◽  
Vol 71 (6) ◽  
pp. 2130-2154 ◽  
Author(s):  
Jerome M. Schmidt ◽  
Piotr J. Flatau ◽  
Robert D. Yates
Keyword(s):  
High Resolution ◽  
Radiative Forcing ◽  
Doppler Radar ◽  
Cloud Layer ◽  
Middle Level ◽  
Horizontal Gradient ◽  
Finite Width ◽  
Layer Width ◽  
Thermodynamic Structure ◽  
Rayleigh Benard

Abstract Very-high-resolution Doppler radar observations are used together with aircraft measurements to document the dynamic and thermodynamic structure of a dissipating altocumulus cloud system associated with a deep virga layer. The cloud layer circulation is shown to consist of shallow vertical velocity couplets near cloud top and a series of subkilometer-scale Rayleigh–Bénard-like cells that extend vertically through the depth of the cloud layer. The subcloud layer was observed to contain a number of narrow virga fall streaks that developed below the more dominant Rayleigh–Bénard updraft circulations in the cloud layer. These features were discovered to be associated with kilometer-scale horizontally orientated rotor circulations that formed along the lateral flanks of the streaks collocated downdraft circulation. The Doppler analysis further reveals that a layer mean descent was present throughout both the cloud and subcloud layers. This characteristic of the circulation is analyzed with regard to the diabatic and radiative forcing on horizontal length scales ranging from the Rayleigh–Bénard circulations to the overall cloud layer width. In particular, linear analytical results indicate that a deep and broad mesoscale region of subsidence is quickly established in middle-level cloud layers of finite width when a layer-wide horizontal gradient in the cloud-top radiative cooling rate is present. A conceptual model summarizing the primary observed and inferred circulation features of the altocumulus layer is presented.

scholarly journals Improving the Model Convective Storm Quantitative Precipitation Nowcasting by Assimilating State Variables Retrieved from Multiple-Doppler Radar Observations

2014 ◽  
Vol 142 (11) ◽  
pp. 4017-4035 ◽  
Author(s):  
Yu-Chieng Liou ◽  
Jian-Luen Chiou ◽  
Wei-Hao Chen ◽  
Hsin-Yu Yu
Keyword(s):  
Data Assimilation ◽  
Doppler Radar ◽  
Scale Up ◽  
Three Dimensional ◽  
Radar Data ◽  
State Variables ◽  
Weather System ◽  
Additional Information ◽  
Convective Scale ◽  
Fine Resolution

Abstract This research combines an advanced multiple-Doppler radar synthesis technique with the thermodynamic retrieval method, originally proposed by Gal-Chen, and a moisture/temperature adjustment scheme, and formulates a sequential procedure. The focus is on applying this procedure to improve the model quantitative precipitation nowcasting (QPN) skill in the convective scale up to 3 hours. A series of (observing system simulation experiment) OSSE-type tests and a real case study are conducted to investigate the performance of this algorithm under different conditions. It is shown that by using the retrieved three-dimensional wind, thermodynamic, and microphysical parameters to reinitialize a fine-resolution numerical model, its QPN skill can be significantly improved. Since the Gal-Chen method requires the horizontal average properties of the weather system at each altitude, utilization of in situ radiosonde(s) to obtain this additional information for the retrieval is tested. When sounding data are not available, it is demonstrated that using the model results to replace the role played by observing devices is also a feasible choice. The moisture field is obtained through a simple, but effective, adjusting scheme and is found to be beneficial to the rainfall forecast within the first hour after the reinitialization of the model. Since this algorithm retrieves the unobserved state variables instantaneously from the wind measurements and directly uses them to reinitialize the model, fewer radar data and a shorter model spinup time are needed to correct the rainfall forecasts, in comparison with other data assimilation techniques such as four-dimensional variational data assimilation (4DVAR) or ensemble Kalman filter (EnKF) methods.

scholarly journals Precipitation characteristics and associated weather conditions on the eastern slopes of the Canadian Rockies during March–April 2015

2018 ◽  
Vol 22 (8) ◽  
pp. 4491-4512 ◽  
Author(s):  
Julie M. Thériault ◽  
Ida Hung ◽  
Paul Vaquer ◽  
Ronald E. Stewart ◽  
John W. Pomeroy
Keyword(s):  
Doppler Radar ◽  
Water Cycle ◽  
Weather Conditions ◽  
Doppler Velocity ◽  
Canadian History ◽  
Critical Aspect ◽  
Air Masses ◽  
Precipitation Characteristics ◽  
Flooding Events ◽  
Precipitation Events

Abstract. Precipitation events that bring rain and snow to the Banff–Calgary area of Alberta are a critical aspect of the region's water cycle and can lead to major flooding events such as the June 2013 event that was the second most costly natural disaster in Canadian history. Because no special atmospheric-oriented observations of these events have been made, a field experiment was conducted in March and April 2015 in Kananaskis, Alberta, to begin to fill this gap. The goal was to characterize and better understand the formation of the precipitation at the surface during spring 2015 at a specific location in the Kananaskis Valley. Within the experiment, detailed measurements of precipitation and weather conditions were obtained, a vertically pointing Doppler radar was deployed and weather balloons were released. Although 17 precipitation events occurred, this period was associated with much less precipitation than normal (−35 %) and above-normal temperatures (2.5 ∘C). Of the 133 h of observed precipitation, solid precipitation occurred 71 % of the time, mixed precipitation occurred 9 % and rain occurred 20 %. An analysis of 17 504 precipitation particles from 1181 images showed that a wide variety of crystals and aggregates occurred and approximately 63 % showed signs of riming. This was largely independent of whether flows aloft were upslope (easterly) or downslope (westerly). In the often sub-saturated surface conditions, hydrometeors containing ice occurred at temperatures as high as 9 ∘C. Radar structures aloft were highly variable with reflectivity sometimes >30 dBZe and Doppler velocity up to −1 m s−1, which indicates upward motion of particles within ascending air masses. Precipitation was formed in this region within cloud fields sometimes having variable structures and within which supercooled water at least sometimes existed to produce accreted particles massive enough to reach the surface through the relatively dry sub-cloud region.

scholarly journals Orographic Precipitation Forcing along the Coast of Northern California during a Landfalling Winter Storm

2015 ◽  
Vol 143 (9) ◽  
pp. 3570-3590 ◽  
Author(s):  
Raul A. Valenzuela ◽  
David E. Kingsmill
Keyword(s):  
Doppler Radar ◽  
Orographic Precipitation ◽  
Northern California ◽  
Gps Receiver ◽  
Winter Storm ◽  
Acoustic Sounding ◽  
Low Level ◽  
Gap Flow ◽  
X Band ◽  
Low Level Jet

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.

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