scholarly journals Semi-Permanent Zones of Radar Radial Shear within the Planetary Boundary Layer: Observations and Effects on High Intensity Precipitation in the Wider Auckland region, New Zealand

2021 ◽  
Author(s):  
◽  
Frances Russell
Keyword(s):  
Boundary Layer ◽  
New Zealand ◽  
High Intensity ◽  
Heavy Precipitation ◽  
Convective Precipitation ◽  
Windward Side ◽  
Doppler Velocity ◽  
Low Level ◽  
Auckland Region ◽  
Mechanical Forcing

<p>This study investigates the role of mechanical forcing within the boundary layer in enhancing low-level precipitation and initiating/intensifying convective precipitation during cases of high intensity precipitation in the wider Auckland region, New Zealand. Eight cases, that occurred between 2001 and 2008 have been investigated. All cases were observed to be strongly dynamically forced, resulting from the passage of mid-latitude cyclones. These features were observed to be centred mainly to the north and west of the study area, with surface winds from the northeast quadrant over the wider Auckland region. Radar imagery is characterised by regions of both convective and stratiform precipitation for all the cases investigated; areas of convection are often observed to be embedded in areas of larger scale precipitation. These cases were subdivided into eleven heavy precipitation events. Nine of these events were subject to further investigation. Environmental conditions during these events were characterised by steady low-level winds from the northeast quadrant, weak to moderate convective instability, with 0-3km wind shear indicating a high level of directional shear in the lower atmosphere. To investigate mechanical forcing in the boundary layer, low-level Doppler velocity and reflectivity fields measured by the Mt Tamahunga radar, were examined. These data revealed mesoscale structures of the Doppler velocity field not previously documented in this region. Mechanical forcing was identified by the presence of mesoscale zones of radar radial shear, resulting from horizonal convergence and/or zones of horizontal shear. These features were observed to be semi-permanent on the windward side of Little Barrier and Great Barrier islands, the windward side of the Coromandel ranges, and along the west coast of the Auckland region. Further, zones of semi-permanent radar radial shear were observed to extend downstream (lee side) of Mt Moehau and Great Barrier, Little Barrier and Taranga islands in the Hauraki Gulf. These features have not been documented previously for this study area. The features, observed downstream of each obstacle, were characterised by a long thin low velocity zone present in PPI images of radar radial velocity and were bounded by the above mentioned shear zones. Further, these features were aligned parallel to the surface wind direction, with widths approximately equal to the diameter of the obstacle and extended up to 57km downstream of each obstacle. These features are consistent with characteristics of mountain wakes described in the literature. A partitioning algorithm was calibrated to identify the convective and stratiform components of the radar reflectivity field. This algorithm was applied to reflectivity data for each heavy precipitation event. Local maxima in the frequency of low-level enhanced precipitation were observed in the vicinity of topographic features such as the Coromandel Peninsula and Mt Tamahunga, in addition to the observed location of wakes in the lee of Great Barrier and Little Barrier Island. Finally, the relationship between mountain wakes observed in the Hauraki Gulf and low-level precipitation enhancement was examined. Investigations showed that when large scale areas of precipitation interacted with these wakes, in some cases convective precipitation was observed to be initiated or intensified. However, the observed areas of enhancement were observed to be short lived and shallow, reaching heights below the radar bright band at [approximately ]3.5 km.</p>

Semi-Permanent Zones of Radar Radial Shear within the Planetary Boundary Layer: Observations and Effects on High Intensity Precipitation in the Wider Auckland region, New Zealand

2021 ◽  
Author(s):  
◽  
Frances Russell
Keyword(s):  
Boundary Layer ◽  
New Zealand ◽  
High Intensity ◽  
Heavy Precipitation ◽  
Convective Precipitation ◽  
Windward Side ◽  
Doppler Velocity ◽  
Low Level ◽  
Auckland Region ◽  
Mechanical Forcing

<p>This study investigates the role of mechanical forcing within the boundary layer in enhancing low-level precipitation and initiating/intensifying convective precipitation during cases of high intensity precipitation in the wider Auckland region, New Zealand. Eight cases, that occurred between 2001 and 2008 have been investigated. All cases were observed to be strongly dynamically forced, resulting from the passage of mid-latitude cyclones. These features were observed to be centred mainly to the north and west of the study area, with surface winds from the northeast quadrant over the wider Auckland region. Radar imagery is characterised by regions of both convective and stratiform precipitation for all the cases investigated; areas of convection are often observed to be embedded in areas of larger scale precipitation. These cases were subdivided into eleven heavy precipitation events. Nine of these events were subject to further investigation. Environmental conditions during these events were characterised by steady low-level winds from the northeast quadrant, weak to moderate convective instability, with 0-3km wind shear indicating a high level of directional shear in the lower atmosphere. To investigate mechanical forcing in the boundary layer, low-level Doppler velocity and reflectivity fields measured by the Mt Tamahunga radar, were examined. These data revealed mesoscale structures of the Doppler velocity field not previously documented in this region. Mechanical forcing was identified by the presence of mesoscale zones of radar radial shear, resulting from horizonal convergence and/or zones of horizontal shear. These features were observed to be semi-permanent on the windward side of Little Barrier and Great Barrier islands, the windward side of the Coromandel ranges, and along the west coast of the Auckland region. Further, zones of semi-permanent radar radial shear were observed to extend downstream (lee side) of Mt Moehau and Great Barrier, Little Barrier and Taranga islands in the Hauraki Gulf. These features have not been documented previously for this study area. The features, observed downstream of each obstacle, were characterised by a long thin low velocity zone present in PPI images of radar radial velocity and were bounded by the above mentioned shear zones. Further, these features were aligned parallel to the surface wind direction, with widths approximately equal to the diameter of the obstacle and extended up to 57km downstream of each obstacle. These features are consistent with characteristics of mountain wakes described in the literature. A partitioning algorithm was calibrated to identify the convective and stratiform components of the radar reflectivity field. This algorithm was applied to reflectivity data for each heavy precipitation event. Local maxima in the frequency of low-level enhanced precipitation were observed in the vicinity of topographic features such as the Coromandel Peninsula and Mt Tamahunga, in addition to the observed location of wakes in the lee of Great Barrier and Little Barrier Island. Finally, the relationship between mountain wakes observed in the Hauraki Gulf and low-level precipitation enhancement was examined. Investigations showed that when large scale areas of precipitation interacted with these wakes, in some cases convective precipitation was observed to be initiated or intensified. However, the observed areas of enhancement were observed to be short lived and shallow, reaching heights below the radar bright band at [approximately ]3.5 km.</p>

Strong Cross-Barrier Flow under Stable Conditions Producing Intense Winter Orographic Precipitation: A Case Study over the Subtropical Central Andes

2009 ◽  
Vol 24 (4) ◽  
pp. 1009-1031 ◽  
Author(s):  
Maximiliano Viale ◽  
Federico A. Norte
Keyword(s):  
Heavy Precipitation ◽  
Stable Stratification ◽  
Central Andes ◽  
Precipitation Event ◽  
Windward Side ◽  
Orographic Precipitation ◽  
Near Surface ◽  
Low Level ◽  
Precipitation Enhancement ◽  
Subtropical Central Andes

Abstract The most intense orographic precipitation event over the subtropical central Andes (36°–30°S) during winter 2005 was examined using observational data and a regional model simulation. The Eta-Programa Regional de Meteorología (PRM) model forecast was evaluated and used to explore the airflow structure that generated this heavy precipitation event, with a focus on orographic influences. Even though the model did not realistically reproduce any near-surface variables, nor the precipitation shadow in the leeside lowlands, its reliable forecast of heavy precipitation over the windward side and the wind fields suggests that it can be used as a valuable forecasting tool for such events in the region. The synoptic flow of the 26–29 August 2005 storm responded to a well-defined dipole from low to upper levels with anomalous low (high) geopotential heights at midlatitudes (subtropical) latitudes located off the southeast Pacific coast, resulting in a large meridional geopotential height gradient that drove a strong anomalous cross-barrier flow. Precipitation enhancement in the Andes was observed during the entire event; however, the highest rates were in the prefrontal sector under the low-level stable stratification and cross-barrier winds exceeding 2.5 standard deviations (σ) from the climatological monthly mean. The combination of strong cross-mountain winds with the stable stratification in the air mass of a frontal system, impinging on the high Andes range, appears to be the major factor in determining the flow structure that produced the pattern of precipitation enhancement, with uplift maximized near mountaintops and low-level blocking upwindleading to the formation of a low-level along-barrier jet. Additionally, only the upstream wind anomalies for the 15 heaviest events over a 10-yr (1967–76) period were investigated. They exhibited strong anomalous northwesterly winds for 14 of the 15 events, whereas for the remaining event there were no available observations to evaluate. Thus, these anomalies may also be exploited for forecasting capabilities.

scholarly journals Diurnal Spatial Variability of Great Plains Summer Precipitation Related to the Dynamics of the Low-Level Jet

2014 ◽  
Vol 71 (5) ◽  
pp. 1807-1817 ◽  
Author(s):  
Bing Pu ◽  
Robert E. Dickinson
Keyword(s):  
Boundary Layer ◽  
Great Plains ◽  
Summer Precipitation ◽  
Meridional Wind ◽  
Convective Precipitation ◽  
Diurnal Variability ◽  
Near Surface ◽  
Low Level ◽  
Diurnal Oscillation ◽  
Low Level Jet

Abstract Diurnal variations of the Great Plains low-level jet (GPLLJ) and vertical motions have been related to the development of summer precipitation individually, but their underlying connection and consequences for the nocturnal and afternoon precipitation peaks are less discussed. This paper examines how together they help explain the spatial pattern of the frequency of summer convective precipitation over the Great Plains. A one-layer linearized boundary layer model is used to reproduce the diurnal cycle of the GPLLJ. Its periodic rising and sinking motions compare favorably with those of the North American Regional Reanalysis (NARR) climatology. Its development of rising motion is also consistent with the enhanced occurrence of nocturnal convective precipitation over the central and eastern Great Plains (90°–100°W) and afternoon maximum over the western Great Plains (100°–105°W). The diurnal phasing of the vertical motions can be captured by the model only if the diurnal oscillation of the jet is forced by both near surface geopotential gradients and friction with observed diurnal variability. The diurnal variation of the vertical velocity (or boundary layer convergence and divergence) is explained by local vorticity balance; that is, following the diurnal oscillation of the jet, the zonal gradient of the meridional wind oscillates and, thus, relative vorticity and its tendency. The slowing down of the jet after midnight decreases the anticyclonic (cyclonic) vorticity and consequently gives a positive (negative) vorticity tendency to the east (west) of the jet core; anomalous rising (sinking) motions occur to balance these positive (negative) vorticity tendencies. The pattern reverses when the jet is relatively weak.

scholarly journals Climatology of Low-Level Jets and Their Impact on Rainfall over Southern China during the Early-Summer Rainy Season

2019 ◽  
Vol 32 (24) ◽  
pp. 8813-8833 ◽  
Author(s):  
Yu Du ◽  
Guixing Chen
Keyword(s):  
Boundary Layer ◽  
Southern China ◽  
Early Summer ◽  
Windward Side ◽  
The South ◽  
Rainfall Distribution ◽  
Wind Maximum ◽  
Low Level ◽  
The North ◽  
Low Level Jets

Abstract Low-level jets (LLJs) are a key factor regulating the early-summer rainfall over southern China. Their detailed activities and impact are examined using 21-yr ERA5 and TRMM rainfall data. The LLJs typically consist of boundary layer jets (BLJs) and synoptic-system-related LLJs (SLLJs). The BLJ is usually characterized by a southerly wind maximum at 950 hPa over the northern area of South China Sea, whereas the SLLJ features a southwesterly wind maximum at 850–700 hPa located more north on land. Meanwhile, the BLJ (SLLJ) has a maximum occurrence in April–June (May–July) and at late night (in the early morning), indicating the differences in seasonal and diurnal variations. The two types of LLJs are found to influence the rainfall distribution via terrain effects, synoptic disturbances, and moisture transport. During the BLJ events, rainfall is mainly confined to the south side of the Nanling and Wuyi Mountains and Yun-Gui Plateau (south region), whereas during the SLLJ events rainfall occurs both in the coastal region and to the north of the mountains (north region). The difference is caused by the southerly BLJ that induces strong orographic lifting on the windward side of the mountains, while the elevated SLLJ can pass over the mountains driving an additional upward motion more north. Active synoptic disturbances accompanied by SLLJs are also favorable for the rainfall in the north region. The moisture transportation by LLJs is another important factor regulating rainfall distribution. Rainfall in the south (north) region is mainly attributed to the net moisture flux in the boundary layer (more elevated layers) due to the BLJ (SLLJ).

scholarly journals A “Cold Path” for the Gulf Stream–Troposphere Connection

2017 ◽  
Vol 30 (4) ◽  
pp. 1363-1379 ◽  
Author(s):  
Benoît Vannière ◽  
Arnaud Czaja ◽  
Helen Dacre ◽  
Tim Woollings
Keyword(s):  
Boundary Layer ◽  
Gulf Stream ◽  
Surface Wind ◽  
Extratropical Cyclone ◽  
Convective Precipitation ◽  
Dynamical Response ◽  
Sst Front ◽  
Simple Boundary ◽  
Pressure Anomaly ◽  
Sst Gradient

Abstract The mechanism by which the Gulf Stream sea surface temperature (SST) front anchors a band of precipitation on its warm edge is still a matter of debate, and little is known about how synoptic activity contributes to the mean state. In the present study, the influence of the SST front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the Met Office Unified Model. The comparison of a control run with a simulation in which SST gradients were smoothed brought the following conclusions: a band of precipitation is reproduced for a single extratropical cyclone, and the response to the SST gradient is dominated by a change of convective precipitation in the cold sector of the storm. Several climatological features described by previous studies, such as surface wind convergence on the warm edge or a meridional circulation cell across the SST front, are also reproduced at synoptic time scales in the cold sector. Based on these results, a simple boundary layer model is proposed to explain the convective and dynamical response to the SST gradient in the cold sector. In this model, cold and dry air parcels acquire more buoyancy over a sharp SST gradient and become more convectively unstable. The convection sets a pressure anomaly over the entire depth of the boundary layer that drives wind convergence. This case study offers a new pathway by which the SST gradient can anchor a climatological band of precipitation.

Role of low-level jets and boundary-layer properties on the NBL budget technique

2005 ◽  
Vol 135 (1-4) ◽  
pp. 35-43 ◽  
Author(s):  
N. Mathieu ◽  
I.B. Strachan ◽  
M.Y. Leclerc ◽  
A. Karipot ◽  
E. Pattey
Keyword(s):  
Boundary Layer ◽  
Low Level ◽  
Low Level Jets

The Relationship Between Atmospheric Boundary Layer Structure, Brown Haze, and Air Pollution in Auckland, New Zealand

2021 ◽  
Author(s):  
Hannah Marley ◽  
Kim Dirks ◽  
Andrew Neverman ◽  
Ian McKendry ◽  
Jennifer Salmond
Keyword(s):  
Boundary Layer ◽  
Air Pollution ◽  
New Zealand ◽  
Atmospheric Boundary Layer ◽  
Boundary Layers ◽  
Layer Structure ◽  
High Surface ◽  
Residual Layer ◽  
Pollution Levels ◽  
Haze Formation

&lt;p&gt;&lt;span&gt;&lt;span&gt;A brown air pollution haze that forms over some international cities during the winter has been found to be associated with negative health outcomes and high surface air pollution levels. Previous research has demonstrated a well-established link between the structure of the atmospheric boundary layer (ABL) and surface air quality; however, the degree to which the structure of the ABL influences for formation of local-&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;scale&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; brown haze is unknown. Using continuous ceilometer data covering seven consecutive winters, we investigate the influence of the structure of the ABL in relation to surface air pollution and brown haze formation over an urban area of complex coastal terrain in the Southern Hemisphere city of Auckland, New Zealand. Our results suggest the depth and evolution of the ABL has a strong influence on severe brown haze formation. When days with severe brown haze are compared with those when brown haze is expected but not observed (based on favorable meteorology and high surface air pollution levels), days with severe brown haze are found to coincide with significantly shallower daytime convective boundary layers (~ 48% lower), and the nights preceding brown haze formation are found to have significantly shallower nocturnal boundary layers (~ 28% lower). On severe brown haze days the growth rate during the morning transition phase from a nocturnal boundary layer to a convective daytime boundary layer is found to be significantly reduced (70 m h&lt;/span&gt;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;&lt;span&gt;) compared to days on which brown haze is expected but not observed (170 m h&lt;/span&gt;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;&lt;span&gt;). Compared with moderate brown haze, severe brown haze conditions are found to be associated with a significantly higher proportion of days with a distinct residual layer present in the ceilometer profiles, suggesting the entrainment of residual layer pollutants may contribute to the severity of the haze. This study illustrates the complex interaction between the ABL structure, air pollution, and the presence of brown haze, and demonstrates the utility of a ceilometer instrument in understanding and predicting the occurrence of brown haze events. &lt;/span&gt;&lt;/span&gt;&lt;/p&gt;

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