scholarly journals Two Issues Concerning Surface Frontogenesis

2017 ◽  
Vol 74 (9) ◽  
pp. 2967-2987 ◽  
Author(s):  
Mankin Mak ◽  
Yi Lu ◽  
Yi Deng
Keyword(s):  
Diabatic Heating ◽  
Cold Front ◽  
Model Simulation ◽  
Wrf Model ◽  
Baroclinic Wave ◽  
Steering Flow ◽  
Warm Front ◽  
Surface Front ◽  
Upper Level ◽  
Ageostrophic Circulation

Abstract With the Weather Research and Forecasting (WRF) Model specifically configured to simulate the intensification and evolution of an extratropical baroclinic wave, this study first investigates why cold fronts are characteristically longer, narrower, and more intense than warm fronts in the extratropical atmosphere. It is found that the differential thermal advection by the geostrophic and ageostrophic wind components in the two frontal regions results in a greater thermal contrast across the cold front. The length of the cold front is essentially the length scale of the intensifying baroclinic wave (i.e., on the order of radius of deformation). The frontal system as a whole moves eastward under the influence of a steering flow. In addition, the cold front outpaces the warm front eastward, making the western portion of the warm front progressively occluded and the eastern portion of the warm front shorter. The dynamical processes tend to move the cold front eastward, whereas the diabatic heating processes tend to move it westward, contributing to the narrowness of the cold front. This study also investigates whether, how, and why an upper-level front (ULF) would synergistically interact with a surface front (SF). It is found that a favorable circumstance for such interaction to occur in an observed extratropical cyclone and in the WRF Model simulation is when the ULF and SF are roughly parallel to one another with the ULF aloft located a few hundred kilometers to the west of the SF. The relative importance of “forcing” for the ageostrophic circulation associated with the geostrophic circulation, diabatic heating, and friction are diagnosed in such interaction.

scholarly journals Discrete Frontal Propagation over the Sierra–Cascade Mountains and Intermountain West

2009 ◽  
Vol 137 (6) ◽  
pp. 2000-2020 ◽  
Author(s):  
W. James Steenburgh ◽  
Colby R. Neuman ◽  
Gregory L. West ◽  
Lance F. Bosart
Keyword(s):  
Sierra Nevada ◽  
Cold Front ◽  
Salt Lake ◽  
Salt Lake City ◽  
Wrf Model ◽  
Moist Convection ◽  
Intermountain West ◽  
Cascade Mountains ◽  
Frontal System ◽  
Upper Level

Abstract On 25 March 2006, a complex frontal system moved across the Sierra–Cascade Mountains and intensified rapidly over the Intermountain West where it produced one of the strongest cold-frontal passages observed in Salt Lake City, Utah, during the past 25 yr. Observational analyses and numerical simulations by the Weather Research and Forecast (WRF) Model illustrate that the frontal system propagated discretely across the Sierra–Cascade Mountains and western Nevada. This discrete propagation occurs in a synoptic environment that features a mobile upper-level cyclonic potential vorticity (PV) anomaly that is coupled initially with a landfalling Pacific cyclone and attendant occluded front. The eastward migration of the upper-level cyclonic PV anomaly ultimately encourages the development of a new surface-based cold front ahead of the landfalling occlusion as troughing, confluence, and convergence downstream of the Sierra Nevada intensify preexisting baroclinity over Nevada. Trajectories show that the new cold front represents a boundary between potentially warm air originating over the desert Southwest, some of which has been deflected around the south end of the high sierra, and potentially cool air that has traversed the sierra near and north of Lake Tahoe, some of which has been deflected around the north end of the high sierra. Although diabatic processes contribute to the frontal sharpening, they are not needed for the discrete propagation or rapid cold-frontal development. Forecasters should be vigilant for discrete frontal propagation in similar synoptic situations and recognize that moist convection or differential surface heating can contribute to but are not necessary for rapid Intermountain West frontogenesis.

scholarly journals Structure of a Narrow Cold Front in the Boundary Layer: Observations versus Model Simulation

2012 ◽  
Vol 140 (8) ◽  
pp. 2497-2519 ◽  
Author(s):  
Victoria A. Sinclair ◽  
Sami Niemelä ◽  
Matti Leskinen
Keyword(s):  
Boundary Layer ◽  
Frontal Zone ◽  
Cold Front ◽  
Model Simulation ◽  
Complex Structure ◽  
Frontal Surface ◽  
Near Surface ◽  
Thermal Inversion ◽  
Upper Level

Abstract A narrow and shallow cold front that passed over Finland during the night 30–31 October 2007 is analyzed using model output and observations primarily from the Helsinki Testbed. The aim is to describe the structure of the front, especially within the planetary boundary layer, identify how this structure evolved, and determine the ability of a numerical model to correctly predict this structure. The front was shallow with a small (2.5–3 K) temperature decrease associated with it, which is attributed to the synoptic evolution of the cold front from a frontal wave on a mature, trailing cold front in a region of weak upper-level forcing and where the midtroposphere was strongly stratified. Within the boundary layer, the frontal surface was vertical and the frontal zone was narrow (<8 km). The small cross-front scale was probably a consequence of the weak frontolytical turbulent mixing occurring at night, at high latitudes, combined with strong, localized frontogenetic forcing driven by convergence. The model simulated the mesoscale evolution of the front well, but overestimated the width of the frontal zone. Within the boundary layer, the model adequately predicted the stratification and near-surface temperatures ahead of, and within, the frontal zone, but failed to correctly predict the thermal inversion that developed in the stably stratified postfrontal air mass. This case study highlights the complex structure of fronts both within the nocturnal boundary layer, and in a location far from regions of cyclogenesis, and hence the challenges that both forecasters and operational models face.

scholarly journals Dynamics of Upper-Level Frontogenesis in Baroclinic Waves

2016 ◽  
Vol 73 (7) ◽  
pp. 2699-2714 ◽  
Author(s):  
Mankin Mak ◽  
Yi Lu ◽  
Yi Deng
Keyword(s):  
Diabatic Heating ◽  
Weather Research And Forecasting ◽  
Baroclinic Wave ◽  
Baroclinic Waves ◽  
Structure And Dynamics ◽  
Upper Level ◽  
Vertically Propagating ◽  
Necessary And Sufficient ◽  
Stretching Process ◽  
High Potential Vorticity

Abstract This paper reports a diagnosis of the structure and dynamics of upper-level fronts (ULFs) simulated with a high-resolution Weather Research and Forecasting Model with diabatic heating versus one without diabatic heating. The ULFs of both simulations develop in about 6 days as integral parts of intensifying baroclinic waves. Each has a curvilinear structure along the southern edge of a relatively narrow long tongue of high potential vorticity in which stratospheric air is subducted to different tropospheric levels by synoptic-scale subsidence. It resembles a veil in the sky of varying thickness across the midsection upstream of the trough of the baroclinic wave. The 3D frontogenetical function is shown to be a necessary and sufficient metric for quantifying the rate of development of ULFs. Its value is mostly associated with the contribution of the 3D ageostrophic velocity component. Upper-level frontogenesis is attributable to the joint direct influence of the vortex-stretching process and the deformation property of the 3D ageostrophic flow component. The model also generates a spectrum of vertically propagating mesoscale gravity waves. The ULFs simulated with and without diabatic heating processes are qualitatively similar. The ULF is considerably more intense when there is heating. The heating, however, does not make a significant direct contribution to but indirectly does so through its impacts on the subsidence field of the baroclinic wave.

scholarly journals On the Quantification of Imbalance and Inertia–Gravity Waves Generated in Numerical Simulations of Moist Baroclinic Waves Using the WRF Model

2017 ◽  
Vol 74 (12) ◽  
pp. 4241-4263 ◽  
Author(s):  
Mohammad Mirzaei ◽  
Ali R. Mohebalhojeh ◽  
Christoph Zülicke ◽  
Riwal Plougonven
Keyword(s):  
Numerical Simulations ◽  
Gravity Waves ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Baroclinic Waves ◽  
Surface Front ◽  
The Difference ◽  
Upper Level ◽  
Upper Level Jet ◽  
Inertia Gravity Waves

Abstract Quantification of inertia–gravity waves (IGWs) generated by upper-level jet–surface front systems and their parameterization in global models of the atmosphere relies on suitable methods to estimate the strength of IGWs. A harmonic divergence analysis (HDA) that has been previously employed for quantification of IGWs combines wave properties from linear dynamics with a sophisticated statistical analysis to provide such estimates. A question of fundamental importance that arises is how the measures of IGW activity provided by the HDA are related to the measures coming from the wave–vortex decomposition (WVD) methods. The question is addressed by employing the nonlinear balance relations of the first-order δ–γ, the Bolin–Charney, and the first- to third-order Rossby number expansion to carry out WVD. The global kinetic energy of IGWs given by the HDA and WVD are compared in numerical simulations of moist baroclinic waves by the Weather Research and Forecasting (WRF) Model in a channel on the f plane. The estimates of the HDA are found to be 2–3 times smaller than those of the optimal WVD. This is in part due to the absence of a well-defined scale separation between the waves and vortical flows, the IGW estimates by the HDA capturing only the dominant wave packets and with limited scales. It is also shown that the difference between the HDA and WVD estimates is related to the width of the IGW spectrum.

The Life Cycle of an Undular Bore and Its Interaction with a Shallow, Intense Cold Front

2010 ◽  
Vol 138 (3) ◽  
pp. 886-908 ◽  
Author(s):  
Daniel C. Hartung ◽  
Jason A. Otkin ◽  
Jonathan E. Martin ◽  
David D. Turner
Keyword(s):  
Thermodynamic Properties ◽  
Great Plains ◽  
Cold Front ◽  
Model Simulation ◽  
Wrf Model ◽  
The United States ◽  
Undular Bore ◽  
Frontal Passage ◽  
South Central ◽  
Short Period

Abstract The evolution of an undular bore and its associated wind shift, spawned by the passage of a shallow surface cold front over the Southern Great Plains of the United States, is examined using surface and remote sensing observations along with output from a high-resolution numerical model simulation. Observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south-central Kansas around 0200 UTC 29 November 2006. By the time the front–bore complex passed through Lamont, Oklahoma, approximately 4 h later, the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min. Within several hours the bore decayed and a cold frontal passage, characterized by a wind shift coincident with thermodynamic properties was observed at Okmulgee, Oklahoma. Thus, a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relationship to the parent surface front occurred during a short period of time. The details of this evolution are examined using output from a finescale numerical simulation, performed using the Weather Research and Forecasting (WRF) model. Analysis of the output reveals that as the bore advanced southeastward it moved into a region with a weaker surface stable layer. Consequently, the wave duct that had supported its maintenance steadily weakened resulting in dissipation of the bore. This circumstance led to a merger of the surface temperature and moisture boundaries with the orphaned wind shift, resulting in the cold frontal passage observed at Okmulgee.

scholarly journals Airborne Doppler Radar and Sounding Analysis of an Oceanic Cold Front

2008 ◽  
Vol 136 (4) ◽  
pp. 1475-1491 ◽  
Author(s):  
Roger M. Wakimoto ◽  
Hanne V. Murphey
Keyword(s):  
Diabatic Heating ◽  
Doppler Radar ◽  
Frontal Zone ◽  
Cold Front ◽  
Deep Convection ◽  
Dominant Term ◽  
Low Level ◽  
Frontal Zones ◽  
Low Level Jet ◽  
Upper Level

Abstract An analysis of a cold front over the eastern Atlantic Ocean based on airborne Doppler wind syntheses and dropsonde data is presented. The focus and unique aspect of this study is a segment of the front that was near the center of the cyclone. The dual-Doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of ∼30 km covers a total distance of >450 km in the cross-frontal direction. The finescale resolution and areal coverage of the dataset are believed to be unprecedented. The cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient. The latter was most intense aloft and not at the surface, in contrast to the classical paradigm of surface cold fronts. The shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone. This observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface. The surface convergence within the frontal zone was weak. This may have been related to the closeness of the analysis region to the surface low pressure. The prefrontal low-level jet and the upper-level polar jet were both shown to be supergeostrophic based on the analysis of the geopotential height field. It is believed that a major contributing factor to the former was the isallobaric wind from the large pressure tendencies associated with the moving cyclone. A dry pocket accompanied by descending air was noted out ahead of the low-level jet. This pocket produced a region of potential instability that could have supported deep convection, although none was observed on this day. The vertical structure of the front revealed couplets of potential vorticity that appeared to be the result of diabatic heat sources from condensation. The diabatic effect in the frontogenesis equation was the dominant term, exceeding the combined effects of the confluence and tilting terms. As a result, an alternating pattern of frontogenesis–frontolysis developed along the flanks of the maxima of diabatic heating. This study highlights the importance of taking diabatic heating into account even in the absence of deep convection.

scholarly journals High-Resolution Observations of the Trowal–Warm-Frontal Region of Two Continental Winter Cyclones

2007 ◽  
Vol 135 (5) ◽  
pp. 1629-1646 ◽  
Author(s):  
Joseph A. Grim ◽  
Robert M. Rauber ◽  
Mohan K. Ramamurthy ◽  
Brian F. Jewett ◽  
Mei Han
Keyword(s):  
High Resolution ◽  
Thermal Gradient ◽  
Cold Front ◽  
Leading Edge ◽  
The Other ◽  
Frontal Region ◽  
Dry Air ◽  
Warm Front ◽  
Upper Level ◽  
Different Origins

Abstract This paper compares the structure of the trough of warm air aloft (trowal)–warm-frontal region of two continental wintertime cyclones. The cyclones were observed over the central Great Lakes region during the Lake-Induced Convection Experiment/Snowband Dynamics Project field campaign. The cyclones had different origins, with the first forming east of the Colorado Rockies and the second forming over the Gulf of Mexico. They were associated with different upper-level flow regimes, one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet. Both storms produced heavy swaths of snow across the states of Illinois, Wisconsin, and Michigan. High-resolution observations of frontal structure were made during flights of the National Center for Atmospheric Research Electra aircraft using dropsondes and the Electra Doppler Radar tail radar system. The high-resolution observations suggest a different arrangement of air masses in the trowal region compared with the classical occlusion model, where the trowal axis forms at the intersection of a warm front and a cold front that has overtaken and subsequently ascended the warm front. In both cyclones dry air intruded over the warm front, isolating the warm, moist airflow within the trowal airstream. Very sharp moisture gradients were present at the leading edge of the dry air in both cyclones. In each case, relative humidity differences of over 50% were observed over distances of 10–20 km. The thermal gradient near the leading edge of the dry air in one cyclone was diffuse, so that the moist–dry boundary could best be characterized as an upper-level humidity front. In the other cyclone, the thermal gradient was sharper and aligned with the moisture boundary and was best characterized as a cold front aloft. The analyses suggest that the classical conceptual model of the trowal, at least in some cyclones such as the two illustrated here, needs to be revised to include the possibility that the warm moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front. Since the two cyclones discussed here had different origins, tracks, and flow regimes, the similarity of their structure suggests that these features may be common.

scholarly journals A CloudSat–CALIPSO View of Cloud and Precipitation Properties across Cold Fronts over the Global Oceans

2015 ◽  
Vol 28 (17) ◽  
pp. 6743-6762 ◽  
Author(s):  
Catherine M. Naud ◽  
Derek J. Posselt ◽  
Susan C. van den Heever
Keyword(s):  
Large Scale ◽  
Cold Front ◽  
Conveyor Belt ◽  
Low Level ◽  
Scale Parameters ◽  
Convective Clouds ◽  
Cold Fronts ◽  
Surface Front ◽  
High Level ◽  
The Impact

Abstract The distribution of cloud and precipitation properties across oceanic extratropical cyclone cold fronts is examined using four years of combined CloudSat radar and CALIPSO lidar retrievals. The global annual mean cloud and precipitation distributions show that low-level clouds are ubiquitous in the postfrontal zone while higher-level cloud frequency and precipitation peak in the warm sector along the surface front. Increases in temperature and moisture within the cold front region are associated with larger high-level but lower mid-/low-level cloud frequencies and precipitation decreases in the cold sector. This behavior seems to be related to a shift from stratiform to convective clouds and precipitation. Stronger ascent in the warm conveyor belt tends to enhance cloudiness and precipitation across the cold front. A strong temperature contrast between the warm and cold sectors also encourages greater post-cold-frontal cloud occurrence. While the seasonal contrasts in environmental temperature, moisture, and ascent strength are enough to explain most of the variations in cloud and precipitation across cold fronts in both hemispheres, they do not fully explain the differences between Northern and Southern Hemisphere cold fronts. These differences are better explained when the impact of the contrast in temperature across the cold front is also considered. In addition, these large-scale parameters do not explain the relatively large frequency in springtime postfrontal precipitation.

scholarly journals Intercomparison of Spatial Forecast Verification Methods: Identifying Skillful Spatial Scales Using the Fractions Skill Score

2010 ◽  
Vol 25 (1) ◽  
pp. 343-354 ◽  
Author(s):  
Marion Mittermaier ◽  
Nigel Roberts
Keyword(s):  
Spatial Scales ◽  
Wrf Model ◽  
Skill Score ◽  
Area Ratio ◽  
Forecast Verification ◽  
Care Needs ◽  
Forecast Performance ◽  
Verification Methods ◽  
Upper Level ◽  
Formed Part

Abstract The fractions skill score (FSS) was one of the measures that formed part of the Intercomparison of Spatial Forecast Verification Methods project. The FSS was used to assess a common dataset that consisted of real and perturbed Weather Research and Forecasting (WRF) model precipitation forecasts, as well as geometric cases. These datasets are all based on the NCEP 240 grid, which translates to approximately 4-km resolution over the contiguous United States. The geometric cases showed that the FSS can provide a truthful assessment of displacement errors and forecast skill. In addition, the FSS can be used to determine the scale at which an acceptable level of skill is reached and this usage is perhaps more helpful than interpreting the actual FSS value. This spatial-scale approach is becoming more popular for monitoring operational forecast performance. The study also shows how the FSS responds to forecast bias. A more biased forecast always gives lower FSS values at large scales and usually at smaller scales. It is possible, however, for a more biased forecast to give a higher score at smaller scales, when additional rain overlaps the observed rain. However, given a sufficiently large sample of forecasts, a more biased forecast system will score lower. The use of percentile thresholds can remove the impacts of the bias. When the proportion of the domain that is “wet” (the wet-area ratio) is small, subtle differences introduced through near-threshold misses can lead to large changes in FSS magnitude in individual cases (primarily because the bias is changed). Reliable statistics for small wet-area ratios require a larger sample of forecasts. Care needs to be taken in the choice of verification domain. For high-resolution models, the domain should be large enough to encompass the length scale of the typical mesoscale forcing (e.g., upper-level troughs or squall lines). If the domain is too large, the wet-area ratios will always be small. If the domain is too small, fluctuations in the wet-area ratio can be large and larger spatial errors may be missed. The FSS is a good measure of the spatial accuracy of precipitation forecasts. Different methods are needed to determine other patterns of behavior.

scholarly journals Influential Role of Moisture Supply from the Kuroshio/Kuroshio Extension in the Rapid Development of an Extratropical Cyclone

2015 ◽  
Vol 143 (10) ◽  
pp. 4126-4144 ◽  
Author(s):  
Hidetaka Hirata ◽  
Ryuichi Kawamura ◽  
Masaya Kato ◽  
Taro Shinoda
Keyword(s):  
Diabatic Heating ◽  
Rapid Development ◽  
Kuroshio Extension ◽  
Lower Troposphere ◽  
Active Role ◽  
Conveyor Belt ◽  
Synoptic Scale ◽  
Warm Front ◽  
The Kuroshio

Abstract This study focused on an explosive cyclone migrating along the southern periphery of the Kuroshio/Kuroshio Extension in the middle of January 2013 and examined how those warm currents played an active role in the rapid development of the cyclone using a high-resolution coupled atmosphere–ocean regional model. The evolutions of surface fronts of the simulated cyclone resemble the Shapiro–Keyser model. At the time of the maximum deepening rate, strong mesoscale diabatic heating areas appear over the bent-back front and the warm front east of the cyclone center. Diabatic heating over the bent-back front and the eastern warm front is mainly induced by the condensation of moisture imported by the cold conveyor belt (CCB) and the warm conveyor belt (WCB), respectively. The dry air parcels transported by the CCB can receive large amounts of moisture from the warm currents, whereas the very humid air parcels transported by the WCB can hardly be modified by those currents. The well-organized nature of the CCB plays a key role not only in enhancing surface evaporation from the warm currents but also in importing the evaporated vapor into the bent-back front. The imported vapor converges at the bent-back front, leading to latent heat release. The latent heating facilitates the cyclone’s development through the production of positive potential vorticity in the lower troposphere. Its deepening can, in turn, reinforce the CCB. In the presence of a favorable synoptic-scale environment, such a positive feedback process can lead to the rapid intensification of a cyclone over warm currents.

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