scholarly journals Remote Trigger of Deep Convection by Cold Outflow over the Taiwan Strait in the Mei-Yu Season: A Modeling Study of the 8 June 2007 Case

2011 ◽  
Vol 139 (9) ◽  
pp. 2854-2875 ◽  
Author(s):  
Chung-Chieh Wang ◽  
George Tai-Jen Chen ◽  
Shin-Yi Huang
Keyword(s):  
Heavy Rainfall ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Taiwan Strait ◽  
Heavy Rainfall Event ◽  
Positive Pressure ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Cold Air

In this study, the heavy-rainfall event over central Taiwan during the mei-yu season on 8 June 2007 is investigated, with an emphasis on the triggering mechanism for the deep convection that produced the rain. Observations indicate that there existed two lines of forcing with convection prior to the rain: one over the northern Taiwan Strait along the mei-yu front and the other over the southern Taiwan Strait. Yet, the convection in question developed over the central strait between these two lines, in an unstable environment with strong westerly vertical wind shear. This motivated the authors to carry out the present study. The Cloud-Resolving Storm Simulation (CReSS) of Nagoya University was used and the event was reproduced at a horizontal grid size of 2 km, including the initiation of new convection over the central strait at the correct location and time. The model results suggest a crucial role played by the series of active, persistent, and propagating storms in the southern strait (along the aforementioned second forcing line). On their back (northern) side, these storms repeatedly produced pulses of cold outflow that traveled toward the north-northeast with positive pressure perturbation. With characteristics of gravity waves, the perturbation propagated faster than the cold air and the associated increase in forward-directed (horizontal) pressure gradient force led to northward acceleration of near-surface flow (by up to 4–5 m s−1 h−1). The stronger southerly flow in turn enhanced downstream convergence, and the deep convection was triggered in the central strait near the arrival of the gravity wave ahead of the cold air. When the convection moved eastward over Taiwan, heavy rainfall resulted. The mechanism presented here for remote triggering of convection over the ocean has not been documented near Taiwan during the mei-yu season. With a better understanding about the behavior of convection, these results can contribute to the improvement of quantitative precipitation forecasts and hazard prevention and reduction.

Topographic Effects on a Wintertime Cold Front in Taiwan

2006 ◽  
Vol 134 (11) ◽  
pp. 3297-3316 ◽  
Author(s):  
Fang-Ching Chien ◽  
Ying-Hwa Kuo
Keyword(s):  
Pressure Gradient ◽  
East Coast ◽  
Leading Edge ◽  
Taiwan Strait ◽  
Gradient Force ◽  
Mountain Range ◽  
Pressure Gradient Force ◽  
Cold Air ◽  
Low Level ◽  
The Taiwan Strait

Abstract This paper describes an observational and numerical study of an intense wintertime cold front that occurred in Taiwan on 8 January 1996. The front was associated with rope clouds at the leading edge, and a broad area of stratiform clouds behind. The front was blocked by the Central Mountain Range of Taiwan and divided into two sections on each side of the mountain range. As the cold air moved southward along the east coast, the increasing westward Coriolis force induced a landward acceleration. After the cold air piled up against the mountains, a coastal pressure ridge developed. The cold air damming yielded a geostrophic balance between the westward Coriolis force and the eastward component of the pressure gradient force in the x direction, and a southward acceleration in the y direction mainly caused by the southward pressure gradient force component. Over the Taiwan Strait, southward pressure gradient forces increased when the low-level stable cold air was confined over the Taiwan Strait, leading to a southward acceleration of the cold air. The formation of a windward ridge off the northwest coast of Taiwan contributed to a large southward acceleration, resulting in the development of a coastal jet. Over the Taiwan Strait, the cold air moved southward the fastest due to the channeling effect. The air parcels along the east coast of Taiwan experienced a downgradient acceleration from the cold air damming and advanced at a slower speed. Those traveling over the western plains and the nearshore coast advanced at the slowest speed. Two sensitivity runs, one without Taiwan’s topography (flat land only) and the other without Taiwan’s landmass, demonstrated the influences of Taiwan’s terrain and water–land contrast on the airflow. The run with no surface fluxes showed that the ocean modified the low-level cold air by supplying surface heat and moisture fluxes. This weakened the front, reduced low-level stability, and increased forced shallow convection (formation of rope clouds) at the leading edge.

scholarly journals Sensitivity of Hurricane Intensity Forecast to Convective Momentum Transport Parameterization

2006 ◽  
Vol 134 (2) ◽  
pp. 664-674 ◽  
Author(s):  
Jongil Han ◽  
Hua-Lu Pan
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Momentum Transport ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Momentum Exchange ◽  
Forecast System ◽  
Hurricane Intensity ◽  
Convective Momentum Transport ◽  
The Many

Abstract A parameterization of the convection-induced pressure gradient force (PGF) in convective momentum transport (CMT) is tested for hurricane intensity forecasting using NCEP's operational Global Forecast System (GFS) and its nested Regional Spectral Model (RSM). In the parameterization the PGF is assumed to be proportional to the product of the cloud mass flux and vertical wind shear. Compared to control forecasts using the present operational GFS and RSM where the PGF effect in CMT is taken into account empirically, the new PGF parameterization helps increase hurricane intensity by reducing the vertical momentum exchange, giving rise to a closer comparison to the observations. In addition, the new PGF parameterization forecasts not only show more realistically organized precipitation patterns with enhanced hurricane intensity but also reduce the forecast track error. Nevertheless, the model forecasts with the new PGF parameterization still largely underpredict the observed intensity. One of the many possible reasons for the large underprediction may be the absence of hurricane initialization in the models.

scholarly journals On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

2019 ◽  
Author(s):  
Étienne Vignon ◽  
Olivier Traullé ◽  
Alexis Berne
Keyword(s):  
Pressure Gradient ◽  
Vertical Structure ◽  
East Antarctica ◽  
Radiosonde Data ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Ice Shelves ◽  
Low Level ◽  
Humidity Profiles

Abstract. Eight years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large scale characterization of the fine scale vertical structure of the low troposphere up to 3 km of altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., Mc Murdo). At Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent- ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice-shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performances. A free simulation with the regional model Polar WRF (at a 35-km resolution) over the entire continent shows too strong and too shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an understimated coastal cold air bump and associated sea-continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East-Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth and it raises the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art climate model.

scholarly journals Large Sensitivity of Near-Surface Vertical Vorticity Development to Heat Sink Location in Idealized Simulations of Supercell-Like Storms

2017 ◽  
Vol 74 (4) ◽  
pp. 1095-1104 ◽  
Author(s):  
Paul M. Markowski ◽  
Yvette P. Richardson
Keyword(s):  
Heat Sink ◽  
Vertical Axis ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Vertical Vorticity ◽  
Supercell Storm ◽  
History Of ◽  
Perturbation Pressure ◽  
Negative Buoyancy

Abstract In idealized numerical simulations of supercell-like “pseudostorms” generated by a heat source and sink in a vertically sheared environment, a tornado-like vortex develops if air possessing large circulation about a vertical axis at the lowest model levels can be converged. This is most likely to happen if the circulation-rich air possesses only weak negative buoyancy (the circulation-rich air has a history of descent, so typically possesses at least some negative buoyancy) and is subjected to an upward-directed vertical perturbation pressure gradient force. This paper further explores the sensitivity of the development of near-surface vertical vorticity to the horizontal position of the heat sink. Shifting the position of the heat sink by only 2–3 km can significantly influence vortex intensity by altering both the baroclinic generation of circulation and the buoyancy of circulation-rich air. Many of the changes in the pseudostorms that arise from shifting the position of the heat sink would be difficult to anticipate. The sensitivity of the pseudostorms to heat sink position probably at least partly explains the well-known sensitivity of near-surface vertical vorticity development to the microphysics parameterizations in more realistic supercell storm simulations, as well as some of the failures of actual supercells to produce tornadoes in seemingly favorable environments.

scholarly journals A numerical study of back-building process in a quasistationary rainband with extreme rainfall over northern Taiwan during 11–12 June 2012

2016 ◽  
Vol 16 (18) ◽  
pp. 12359-12382 ◽  
Author(s):  
Chung-Chieh Wang ◽  
Bing-Kui Chiou ◽  
George Tai-Jen Chen ◽  
Hung-Chi Kuo ◽  
Ching-Hwang Liu
Keyword(s):  
Numerical Study ◽  
Leading Edge ◽  
Extreme Rainfall ◽  
Cold Pool ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Northern Taiwan

Abstract. During 11–12 June 2012, quasistationary linear mesoscale convective systems (MCSs) developed near northern Taiwan and produced extreme rainfall up to 510 mm and severe flooding in Taipei. In the midst of background forcing of low-level convergence, the back-building (BB) process in these MCSs contributed to the extreme rainfall and thus is investigated using a cloud-resolving model in the case study here. Specifically, as the cold pool mechanism is not responsible for the triggering of new BB cells in this subtropical event during the meiyu season, we seek answers to the question why the location about 15–30 km upstream from the old cell is still often more favorable for new cell initiation than other places in the MCS. With a horizontal grid size of 1.5 km, the linear MCS and the BB process in this case are successfully reproduced, and the latter is found to be influenced more by the thermodynamic and less by dynamic effects based on a detailed analysis of convective-scale pressure perturbations. During initiation in a background with convective instability and near-surface convergence, new cells are associated with positive (negative) buoyancy below (above) due to latent heating (adiabatic cooling), which represents a gradual destabilization. At the beginning, the new development is close to the old convection, which provides stronger warming below and additional cooling at mid-levels from evaporation of condensates in the downdraft at the rear flank, thus yielding a more rapid destabilization. This enhanced upward decrease in buoyancy at low levels eventually creates an upward perturbation pressure gradient force to drive further development along with the positive buoyancy itself. After the new cell has gained sufficient strength, the old cell's rear-flank downdraft also acts to separate the new cell to about 20 km upstream. Therefore, the advantages of the location in the BB process can be explained even without the lifting at the leading edge of the cold outflow.

scholarly journals On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020

2021 ◽  
Vol 38 (12) ◽  
pp. 2183-2200
Author(s):  
Rudi Xia ◽  
Yali Luo ◽  
Da-Lin Zhang ◽  
Mingxin Li ◽  
Xinghua Bao ◽  
...  
Keyword(s):  
Diurnal Cycle ◽  
Sichuan Basin ◽  
Heavy Rainfall ◽  
Convective Available Potential Energy ◽  
Water Vapor Transport ◽  
Inertial Oscillation ◽  
Moist Convection ◽  
Near Surface ◽  
Cold Air ◽  
The Sichuan Basin

AbstractA sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.

scholarly journals Origin of Strong Winds in an Explosive Mediterranean Extratropical Cyclone

2019 ◽  
Vol 147 (10) ◽  
pp. 3649-3671 ◽  
Author(s):  
Mihaela Brâncuş ◽  
David M. Schultz ◽  
Bogdan Antonescu ◽  
Christopher Dearden ◽  
Sabina Ştefan
Keyword(s):  
Boundary Layer ◽  
Conveyor Belt ◽  
Heat Fluxes ◽  
The Black Sea ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Strong Wind ◽  
Near Surface ◽  
Cyclone Center ◽  
Strong Winds

Abstract During 2–3 December 2012, the Black Sea and east coast of Romania were affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale model simulation was used to analyze the evolution of the wind field, to investigate the physical processes that were responsible for the strong winds and their acceleration, and to investigate the relative importance of the stability of the boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated with a sting jet, a descending airstream from the midlevels of the cloud head and the lower part of the cyclonic branch of the warm conveyor belt. The sting jet started to descend west of the cyclone center, ending at the frontolytic tip of the bent-back front. The second was a low-level airstream associated with the cold conveyor belt that originated northeast of the cyclone center and traveled below 900 hPa along the cold side of the bent-back front, ending behind the cold front. Both airstreams were accelerated by the along-flow pressure gradient force, with the largest accelerations acting on the sting-jet air before entering into the near-surface strong-wind area. The sensible heat fluxes destabilized the boundary layer to near-neutral conditions south of the cyclone center, facilitating downward mixing and allowing the descending air to reach the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet formation.

Nocturnal Destabilization Associated with the Summertime Great Plains Low-Level Jet

2020 ◽  
Vol 148 (11) ◽  
pp. 4641-4656
Author(s):  
Thomas R. Parish ◽  
Richard D. Clark ◽  
Todd D. Sikora
Keyword(s):  
Great Plains ◽  
The United States ◽  
Temperature Gradients ◽  
Gradient Force ◽  
Wind Component ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Low Level ◽  
Temperature Advection ◽  
Low Level Jet

AbstractThe Great Plains low-level jet (LLJ) has long been associated with summertime nocturnal convection over the central Great Plains of the United States. Destabilization effects of the LLJ are examined using composite fields assembled from the North American Mesoscale Forecast System for June and July 2008–12. Of critical importance are the large isobaric temperature gradients that become established throughout the lowest 3 km of the atmosphere in response to the seasonal heating of the sloping Great Plains. Such temperature gradients provide thermal wind forcing throughout the lower atmosphere, resulting in the establishment of a background horizontal pressure gradient force at the level of the LLJ. The attendant background geostrophic wind is an essential ingredient for the development of a pronounced summertime LLJ. Inertial turning of the ageostrophic wind associated with LLJ provides a westerly wind component directed normal to the terrain-induced orientation of the isotherms. Hence, significant nocturnal low-level warm-air advection occurs, which promotes differential temperature advection within a vertical column of atmosphere between the level just above the LLJ and 500 hPa. Such differential temperature advection destabilizes the nighttime troposphere above the radiatively cooled near-surface layer on a recurring basis during warm weather months over much of the Great Plains and adjacent states to the east. This destabilization process reduces the convective inhibition of air parcels near the level of the LLJ and may be of significance in the development of elevated nocturnal convection. The 5 July 2015 case from the Plains Elevated Convection at Night field program is used to demonstrate this destabilization process.

scholarly journals Strong Down-Valley Low-Level Jets over the Atacama Desert: Observational Characterization

2013 ◽  
Vol 52 (12) ◽  
pp. 2735-2752 ◽  
Author(s):  
Ricardo C. Muñoz ◽  
Mark J. Falvey ◽  
Marcelo Araya ◽  
Martin Jacques-Coper
Keyword(s):  
Surface Wind ◽  
Atacama Desert ◽  
Surface Friction ◽  
Gradient Force ◽  
Turbulence Structure ◽  
Pressure Gradient Force ◽  
Surface Cooling ◽  
Environmental Implications ◽  
Near Surface ◽  
Drainage Flows

AbstractThe near-surface wind and temperature regime at three points in the Atacama Desert of northern Chile is described using two years of multilevel measurements from 80-m towers located in an altitude range between 2100 and 2700 m MSL. The data reveal the frequent development of strong nocturnal drainage flows at all sites. Down-valley, nose-shaped wind speed profiles are observed, with maximum values occurring at heights between 20 and 60 m AGL. The flow intensity shows considerable interdaily variability and a seasonal modulation of maximum speeds, which in the cold season can attain hourly average values of more than 20 m s−1. Turbulent mixing appears to be important over the full tower layer, affecting the curvature of the nighttime temperature profile and possibly explaining the observed increase of surface temperatures in the down-valley direction. Nocturnal valley winds and temperatures are weakly controlled by upper-air conditions observed at the nearest aerological station. Estimates of terms in the momentum budget for the development and quasi-stationary phases of the down-valley flows suggest that the pressure gradient force due to the near-surface cooling along the sloping valley axes plays an important role in these drainage flows. A scale for the jet nose height of equilibrium turbulent down-slope jets is proposed that is based on surface friction velocity and surface inversion intensity. At one of the sites, this scale explains about 70% of the case-to-case observed variance of jet nose heights. Further modeling and observations are needed, however, to define better the dynamics, extent, and turbulence structure of this flow system, which has significant wind-energy, climatic, and environmental implications.

scholarly journals Multiple Regimes of Wind, Stratification, and Turbulence in the Stable Boundary Layer

2015 ◽  
Vol 72 (8) ◽  
pp. 3178-3198 ◽  
Author(s):  
Adam H. Monahan ◽  
Tim Rees ◽  
Yanping He ◽  
Norman McFarlane
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Cloud Cover ◽  
Large Scale ◽  
Potential Temperature ◽  
Surface Wind ◽  
Geostrophic Wind ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface

Abstract A long time series of temporally high-resolution wind and potential temperature data from the 213-m tower at Cabauw in the Netherlands demonstrates the existence of two distinct regimes of the stably stratified nocturnal boundary layer at this location. Hidden Markov model (HMM) analysis is used to objectively characterize these regimes and classify individual observed states. The first regime is characterized by strongly stable stratification, large wind speed differences between 10 and 200 m, and relatively weak turbulence. The second is associated with near-neutral stratification, weaker wind speed differences between 10 and 200 m, and relatively strong turbulence. In this second regime, the state of the boundary layer is similar to that during the day. The occupation statistics of these regimes are shown to covary with the large-scale pressure gradient force and cloud cover such that the first regime predominates under clear skies with weak geostrophic wind speed and the second regime predominates under conditions of extensive cloud cover or large geostrophic wind speed. These regimes are not distinguished by standard measures of stability, such as the Obukhov length or the bulk Richardson number. Evidence is presented that the mechanism generating these distinct regimes is associated with a previously documented feedback resulting from the existence of an upper limit on the maximum downward heat flux that can be sustained for a given near-surface wind speed.

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