Slow-moving Cold Front Research Articles

Relationship between daily atmospheric circulation patterns and South Atlantic Convergence Zone (SACZ) events

This study presents the daily atmospheric circulation patterns at surface and altitude related to the South Atlantic Convergence Zone (SACZ) events that occurred between 2007 and 2017. For this analysis, Principal Pattern Sequence Analysis (PPSA) and sea level pressure and geopotential height reanalysis were used. Four typical atmospheric circulation patterns associated with SACZ episodes were identified. Three of them are related to the propagation of a cold front toward southeastern Brazil with a post-frontal anticyclone moving over the Atlantic Ocean. The fourth pattern is associated with the presence of a cut-off low centered over the central-south region of Brazil that induces moisture transport from the Atlantic Ocean and from the Amazon basin to most of southeastern Brazil. Considering the first three patterns, they represent, respectively, a frontal wave, a classical cold front, and a slow-moving cold front. All of these patterns provide permanent northwesterly winds in the lower troposphere over southeastern Brazil from two days before the SACZ occurrence.

Ensemble simulations of the urban effect on a summer rainfall event in the Great Beijing Metropolitan Area

The Great Beijing Metropolitan Area (GBMA), located in North China, is one of the most rapidly developing regions in the world. In this study, ensemble simulations are conducted to investigate the urban effects on a summertime heavy rainfall event in the GBMA. The Weather Research and Forecasting (WRF) model that couples with a single-layer Urban Canopy Model (UCM) is used for the ensemble simulation. Results show that the ensemble simulation with a realistic land-use representation of urban areas (i.e. control run) can well reproduce the spatial distribution and temporal variation of the rainfall event. The simulated total precipitation agrees well with observation. Compared with the sensitivity ensemble simulation, in which the urban area is replaced by cropland, the control run generates more precipitation over the southwest of Beijing, while less rainfall is found in the area to the northeast of Beijing. This result suggests that the underlying urban surface and urban canopy physics in the surface layer have remarkable impacts on precipitation. The stronger upward motion along with larger convergence and more moisture transportation caused by the urban dynamic and thermodynamic effects directly contribute to the differences in rainfall distribution between the control run and the sensitivity run. In addition, the urban effects are found to slow the cold front movement due to the intense warm air over the urban area, leading to a delayed occurrence of the peak rainfall. However, the slow-moving cold front over the urban area enhances the maximum precipitation intensity. The evolution of the rainfall pattern during the intensification period of the precipitation event is dependent on the movement of the cold front in both the control and sensitivity experiments, indicating that urban effects tend to modify the precipitation distribution and influence the temporal variation of the rainfall process.

Two major heavy rain/flood events in the mid-Atlantic: June 2006 and September 2011

In June 2006, significant flooding and flash flooding impacted much of the mid-Atlantic region as a continuous supply of deep tropical moisture moved north from the subtropical Atlantic ahead of a slowmoving cold front. A 3-day period of heavy rain resulted in nearly 38.1 cm (15 in) of rain across portions of the northern mid-Atlantic with record flooding along the mainstem Susquehanna and Delaware Rivers. In September 2011, moisture associated with the remnants of Tropical Storm Lee resulted in a 24-h period of heavy rain over which rainfall totals approached 30.3 cm (12 in) across portions of central New York and northern Pennsylvania. Numerous river-stage records that were set in the June 2006 event were shattered along the mainstem Susquehanna River during the September 2011 flood. Damage estimates resulting from the flooding in both events were >2 billion dollars, and 22 lives were lost. Multiple counties across the northern mid-Atlantic were declared disaster areas. Both flood events were investigated to identify the similar meteorological features and patterns responsible for extreme rainfall. Several crucial similarities were identified that likely combined to produce historic socioeconomic and environmental impacts. One of the similarities was that each event had a well-established atmospheric river in place that provided the uninterrupted supply of deep tropical moisture. Additionally, although these events displayed many of the large-scale characteristics identified in previous flash flood classification schemes, both events were associated with the presence of coastal fronts that appeared to make these cases different from many otherwise similar and previously documented flood cases.

Warm Winter Storms in Central Chile

Abstract Central Chile is a densely populated region along the west coast of subtropical South America (30°–36°S), limited to the east by the Andes. Precipitation is concentrated in austral winter, mostly associated with the passage of cold fronts. The freezing level over central Chile is typically between 1500 and 2500 m when precipitation is present. In about a third of the cases, however, precipitation occurs accompanied by warm temperatures and freezing levels above 3000 m, leading to a sizeable increment in the pluvial area of Andean basins and setting the stage for hydrometeorological hazards. Here, warm winter storms in central Chile are studied, including a statistical description of their occurrence and an estimate of their hydrological impacts. Remote-sensed data and high-resolution reanalysis are used to explore the synoptic-scale environment of a typical case, generalized later by a compositing analysis. The structure of warm storms is also contrasted with that of the more recurrent cold cases. Precipitation during warm events occurs in the warm sector of a slow-moving cold front because of the intense moisture flux against the mountains in connection with a land-falling atmospheric river. This is in turn driven by a strong zonal jet aloft and reduced mechanical blocking upstream of the Andes. On a broader scale, a key element is the presence of a slowly moving anticyclone over the south Pacific, fostering advection of cold air into midlatitudes. The intense and persistent zonal jet stretches a moist-air corridor from the central Pacific to the west coast of South America.

Interpolation of nonstationary high frequency spatial–temporal temperature data

The Atmospheric Radiation Measurement program is a U.S. Department of Energy project that collects meteorological observations at several locations around the world in order to study how weather processes affect global climate change. As one of its initiatives, it operates a set of fixed but irregularly-spaced monitoring facilities in the Southern Great Plains region of the U.S. We describe methods for interpolating temperature records from these fixed facilities to locations at which no observations were made, which can be useful when values are required on a spatial grid. We interpolate by conditionally simulating from a fitted nonstationary Gaussian process model that accounts for the time-varying statistical characteristics of the temperatures, as well as the dependence on solar radiation. The model is fit by maximizing an approximate likelihood, and the conditional simulations result in well-calibrated confidence intervals for the predicted temperatures. We also describe methods for handling spatial-temporal jumps in the data to interpolate a slow-moving cold front.

Physical Processes Associated with Heavy Flooding Rainfall in Nashville, Tennessee, and Vicinity during 1–2 May 2010: The Role of an Atmospheric River and Mesoscale Convective Systems

A multiscale analysis is conducted in order to examine the physical processes that resulted in prolonged heavy rainfall and devastating flash flooding across western and central Tennessee and Kentucky on 1–2 May 2010, during which Nashville, Tennessee, received 344.7 mm of rainfall and incurred 11 flood-related fatalities. On the synoptic scale, heavy rainfall was supported by a persistent corridor of strong water vapor transport rooted in the tropics that was manifested as an atmospheric river (AR). This AR developed as water vapor was extracted from the eastern tropical Pacific and the Caribbean Sea and transported into the central Mississippi Valley by a strong southerly low-level jet (LLJ) positioned between a stationary lee trough along the eastern Mexico coast and a broad, stationary subtropical ridge positioned over the southeastern United States and the subtropical Atlantic. The AR, associated with substantial water vapor content and moderate convective available potential energy, supported the successive development of two quasi-stationary mesoscale convective systems (MCSs) on 1 and 2 May, respectively. These MCSs were both linearly organized and exhibited back-building and echo-training, processes that afforded the repeated movement of convective cells over the same area of western and central Tennessee and Kentucky, resulting in a narrow band of rainfall totals of 200–400 mm. Mesoscale analyses reveal that the MCSs developed on the warm side of a slow-moving cold front and that the interaction between the southerly LLJ and convectively generated outflow boundaries was fundamental for generating convection.

The outbreak of severe storms along convergence lines northeast of the Alps. Case study of the 3 August 2001 mesoscale convective system with a pronounced bow echo

Thunderstorm development in Austria is highly influenced by orographical forcing especially in summer. However, the most severe thunderstorms occur in the lower areas north and southeast of the Alps where mountains are not high. The reason is the undisturbed initiation and intensification of thunderstorms due to the low-level wind field which is modified near the edge of the Alps. Typically, severe convective weather events in Austria develop when a slow-moving cold front approaches from west (e.g., Switzerland) and when unstable air is advected from the Mediterranean by southwesterly flow. During such conditions, a convergence zone is frequently formed north of the Alps by a low-level airflow as a result of diabatic heating and evaporative cooling along the cloud edge of the cold front. This convergence line moves faster to the east than the cold front itself and triggers new cells or intensifies existing cells. The case study presented in this work is based on analyses of data from the Austrocontrol Weather Radar Network and objective surface pressure analyses. Two supercells initiated along a convergence line due to enhanced low level wind shear on the lee side of the Alps. Both severe storms, linked to the northern slopes of the Alps, approached Austria from Bavaria which evolved into a Mesoscale Convective System with a pronounced bow echo within the orographic gap between the mountain range of Bohemian Forest and the Alps. The case study shows a typical storm configuration and how to improve nowcasting of mesoscale modified thunderstorms north of the Alps.

Sensitivity of Forecast Rainfall in a Texas Convective System to Soil Moisture and Convective Parameterization

The impact of soil moisture on the forecast of a small-scale convective system, and sensitivity of results to the convective parameterization used, are investigated through Eta Model simulations (run in an operationallike setting) of a convective system occurring on 27 May 1997 in Texas. The event was influenced by a southwestward-propagating gravity wave from early morning convection in Arkansas that intersected a slowmoving cold front, releasing extreme conditional static instability. Isolated heavy rainfall, over 100 mm, occurred in some regions. A control simulation with 22-km horizontal resolution reasonably simulated the event, even though mesoscale influences such as the gravity wave important to this event are often poorly captured by numerical models. A series of sensitivity tests were performed to examine the impact of soil moisture on the simulations. Two different convective parameterizations were used for the tests. Although domain average precipitation is found to generally vary in a straightforward way with soil moisture, peak precipitation in the regions of intense convection shows more complex behavior. Sensitivity of precipitation amounts to soil moisture differs significantly among runs having different convective parameterizations. For instance, with the Kain‐Fritsch convective scheme, relatively dry soil is found to result in stronger convective outflows that converge with stronger ambient flow to greatly enhance the precipitation in the region where heaviest rainfall occurs. With the Betts‐Miller‐Janjic scheme, drier soil generally results in less precipitation than in the control run, although some enhancement in peak amount does occur within a narrow range of drying. The differences between the peak quantitative precipitation forecasts in the runs is primarily due to the inclusion of a convective downdraft in the Kain‐Fritsch parameterization, and its impact on secondary convective development. Additional sensitivity tests find limited impact from prescribed vegetation coverage. A final sensitivity test shows that precipitation amounts are even more strongly affected by the vertical resolution of the data used to initialize the shallow but moist boundary layer than by variations in the soil moisture or vegetation fraction.

The Vaison-La-Romaine Flash Flood: Mesoscale Analysis and Predictability Issues

Abstract On the morning and early afternoon of 22 September 1992, a flash flood (220 mm of rain in 3 h) occurred in the city of Vaison-La-Romaine, located in southeastern France, causing numerous casualties and considerable property damage. It was generated by a combination of several mesoscale convective systems ahead of a slow-moving cold front associated with a cutoff low. The large-scale setting and a mesoscale analysis of the case, together with estimates of radar-derived rain accumulations, are presented. The mesoscale analysis demonstrates the complexity of the case, which involved five precipitation systems. Orographic influences generated a cold pool and focused convective cell development in a confined area, enabling two precipitation systems to be quasi-stationary. Precipitation forecasts by different versions of two models (a fine mesh version of an operational limited area model, and an operational stretched global model) are summarized. They demonstrate the extent to which realistic rainfall...

The chemistry and microphysics of intrastorm sequential precipitation samples

A slow moving cold front, which deposited over 40 mm of rain, was investigated for relationshipsbetween pH, ion concentrations and precipitation microphysics. For the first time. precipitationdroplet size spectra were available using a Particle Measuring System’s ground-basedprecipitation probe. The precipitation spectra allowed a preliminary analysis of below cloud rainscavenging: adsorption and impaction.The cumulative rate of chemical precipitation versus cumulative water indicated two differentslopes for the major ions contributing to the acidity of the precipitation. The slopes wereconsistent with a convective stage and a non-convective storm stage and this was consistent withrate of precipitation. DOI: 10.1111/j.1600-0889.1985.tb00063.x