scholarly journals The Characteristics of Key Analysis Errors. Part III: A Diagnosis of Their Evolution

2007 ◽  
Vol 135 (7) ◽  
pp. 2754-2777 ◽  
Author(s):  
Jean-François Caron ◽  
M. K. Yau ◽  
Stéphane Laroche
Keyword(s):  
Level Structure ◽  
Potential Temperature ◽  
Initial Time ◽  
Forecast Errors ◽  
Diagnostic Study ◽  
Low Level ◽  
Diabatic Processes ◽  
Upper Level ◽  
Analysis Error ◽  
Analysis Errors

Abstract This paper presents a diagnostic study of the evolution of initial corrections obtained from the key analysis error algorithm that minimizes the short-range (24 h) forecast errors for four specific events poorly forecasted over the eastern part of North America. A potential vorticity (PV) perspective is employed. It is shown that the modification to the low-level structure at the initial time is mainly attributed to the modification of the low-level PV distribution, while changes in the upper-level structure are attributed to the modification of the upper-level PV distribution. The low-level corrections grow mainly through background surface potential temperature advection by the wind corrections attributable to the interior PV corrections. Changes in the diabatic processes and the vertical alignment of low-level PV corrections by differential PV advection also increase the magnitude of the low-level corrections with time. The upper-level corrections grow by advection of background PV from wind corrections. However, the cause of these latter wind corrections responsible for upper-level background PV advection varies from case to case. An investigation of the relative importance of the low-level and of the upper-level initial corrections to produce the final-time corrections also reveals strong variability between cases. Finally, comparison of two cases in which the key analysis errors propagate vertically with two others without significant vertical propagation shows how the relative position of the key analysis errors with respect to the structure of the background flow can influence the evolution of the initial corrections.

Cyclogenetic Perturbations and Analysis Errors Decomposed into Singular Vectors

2005 ◽  
Vol 62 (7) ◽  
pp. 2234-2247 ◽  
Author(s):  
Chris Snyder ◽  
Gregory J. Hakim
Keyword(s):  
Potential Vorticity ◽  
Vertical Structure ◽  
Energy Norm ◽  
Vertical Shear ◽  
Initial Time ◽  
Forecast Errors ◽  
Ensemble Forecasts ◽  
Singular Vectors ◽  
Analysis Error ◽  
Analysis Errors

Abstract Singular vectors (SVs) have been applied to cyclogenesis, to initializing ensemble forecasts, and in predictability studies. Ideally, the calculation of the SVs would employ the analysis error covariance norm at the initial time or, in the case of cyclogenesis, a norm based on the statistics of initial perturbations, but the energy norm is often used as a more practical substitute. To illustrate the roles of the choice of norm and the vertical structure of initial perturbations, an upper-level wave with no potential vorticity perturbation in the troposphere is considered as a typical cyclogenetic perturbation or analysis error, and this perturbation is then decomposed by its projection onto each energy SV. All calculations are made, for simplicity, in the context of the quasigeostrophic Eady model (i.e., for a background flow with constant vertical shear and horizontal temperature gradient). Viewed in terms of the energy SVs, the smooth vertical structure of the typical perturbation, as well as its evolution, results from strong cancellation between the growing and decaying SVs, most of which are highly structured and tilted in the vertical. A simpler picture, involving less cancellation, follows from decomposition of the typical perturbation into SVs using an alternative initial norm, which is based on the relation between initial norms and the statistics of initial perturbations together with the empirical assumption that the initial perturbations are not dominated by interior potential vorticity. Differences between the energy SVs and those based on the alternative initial norm can be understood by noting that the energy norm implicitly assumes initial perturbations with second-order statistics given by the covariance matrix whose inverse defines the energy norm. Unlike the “typical” perturbation, perturbations with those statistics have large variance of potential vorticity in the troposphere and fine vertical structure. Finally, a brief assessment is presented of the extent to which the upper wave, and more generally the alternative initial norm, is representative of cyclogenetic perturbations and analysis errors. There is substantial evidence supporting deep perturbations with little vertical structure as frequent precursors to cyclogenesis, but surrogates for analysis errors are less conclusive: operational midlatitude analysis differences have vertical structure similar to that of the perturbations implied by the energy norm, while short-range forecast errors and analysis errors from assimilation experiments with simulated observations are more consistent with the alternative norm.

Objective Categorization of Heavy-Rain-Producing MCS Synoptic Types by Rotated Principal Component Analysis

2014 ◽  
Vol 142 (5) ◽  
pp. 1716-1737 ◽  
Author(s):  
John M. Peters ◽  
Russ S. Schumacher
Keyword(s):  
Principal Component Analysis ◽  
Temperature Gradient ◽  
Potential Temperature ◽  
Warm Season ◽  
Principal Component ◽  
Heavy Rain ◽  
Component Analysis ◽  
Synoptic Type ◽  
Low Level ◽  
Upper Level

Abstract In this research, rotated principal component analysis was applied to the atmospheric fields associated with a large sample of heavy-rain-producing mesoscale convective systems (MCSs). Cluster analysis in the subspace defined by the leading two resulting principal components revealed two subtypes with distinct synoptic and mesoscale characteristics, which are referred to as warm-season-type and synoptic-type events, respectively. Subsequent composite analysis showed that both subtypes typically occurred on the cool side of a quasi-stationary, low-level frontal boundary, within a region of locally maximized low-level convergence and warm advection. Synoptic-type events, which tended to exhibit greater horizontal extent than warm-season-type events, typically occurred downstream of a progressive upper-level trough, along a low-level potential temperature gradient with the warmest air to the south and southeast. Warm-season-type events, on the other hand, occurred within the right-entrance region of a minimally to anticyclonically curved upper-level jet streak, along a low-level potential temperature gradient with the warmest low-level air to the southwest. Synoptic-scale forcing for ascent was stronger in synoptic-type events, while low-level moisture was greater in warm-season-type events. Warm-season-type events were frequently preceded by the passage of a trailing-stratiform- (TS) type MCS, whereas synoptic-type events often occurred prior to the passage of a TS-type system. Analysis of the composite vertical wind profiles at the event location suggests that quasi-stationary behavior in warm-season events predominantly resulted from upstream propagation that nearly canceled advection by the mean steering flow, whereas in the case of synoptic-type events training predominantly resulted from system motion that paralleled a front.

scholarly journals The Impact of the Asian Summer Monsoon Circulation on the Tropopause

2016 ◽  
Vol 29 (24) ◽  
pp. 8689-8701 ◽  
Author(s):  
Yutian Wu ◽  
Tiffany A. Shaw
Keyword(s):  
Asian Summer Monsoon ◽  
Potential Temperature ◽  
Surface Heating ◽  
Monsoon Circulation ◽  
Low Level ◽  
Sverdrup Balance ◽  
Upper Level ◽  
Extratropical Tropopause ◽  
Asian Summer Monsoon Circulation ◽  
The Impact

Abstract Previous studies have identified two important features of summertime thermodynamics: 1) a significant correlation between the low-level distribution of equivalent potential temperature and the potential temperature θ of the extratropical tropopause and 2) a northwestward shift of the maximum tropopause θ relative to the maximum low-level . Here, the authors hypothesize these two features occur because of the Asian monsoon circulation. The hypothesis is examined using a set of idealized prescribed sea surface temperature (SST) aquaplanet simulations. Simulations with a zonally symmetric background climate exhibit a weak moisture–tropopause correlation. A significant correlation and northwestward shift occurs when a zonal wave-1 SST perturbation is introduced in the Northern Hemisphere subtropics. The equivalent zonal-mean subtropical warming does not produce a significant correlation. A mechanism is proposed to explain the moisture–tropopause connection that involves the circulation response to zonally asymmetric surface heating and its impact on the tropopause defined by the 2-potential-vorticity-unit (PVU; 1 PVU = 10−6 K kg−1 m2 s−1) surface. While the circulation response to diabatic heating is well known, here the focus is on the implications for the tropopause. Consistent with previous research, surface heating increases the low-level and produces low-level convergence and a cyclonic circulation. The low-level convergence is coupled with upper-level divergence via convection and produces an upper-level anticyclonic circulation consistent with Sverdrup balance. The anticyclonic vorticity lowers the PV northwest of the surface heating via Rossby wave dynamics. The decreased PV leads to a northwestward shift of the 2-PVU surface on fixed pressure levels. The θ value to the northwest of the surface heating is higher, and consequently the maximum tropopause θ increases.

scholarly journals Genesis Criteria for Diabatic Rossby Vortices: A Model Study

2013 ◽  
Vol 141 (1) ◽  
pp. 252-263 ◽  
Author(s):  
Richard W. Moore ◽  
Michael T. Montgomery ◽  
Huw Davies
Keyword(s):  
Mesoscale Model ◽  
Potential Temperature ◽  
Environmental Parameters ◽  
Time Frame ◽  
Initial Disturbance ◽  
Lower Atmosphere ◽  
Low Level ◽  
Warm Core ◽  
Model Study ◽  
Diabatic Processes

Abstract A suite of idealized mesoscale model simulations are conducted to examine the dynamic pathway to the genesis of short-scale, moist baroclinic disturbances. It is shown that in an initially subsaturated environment, two distinct stages of development are necessary before a preexisting surface-concentrated, warm-core vortex can begin to amplify. The first stage, termed environmental preconditioning, involves the moistening of the lower atmosphere via the transport of relatively high equivalent potential temperature air into the immediate environment of the translating vortex. The second stage results from continuous cloud diabatic processes and it involves the emergence of a low-level positive potential vorticity (PV) anomaly with some evidence of a further, more diffuse, negative PV anomaly at higher elevations. The PV structure is characteristic of a diabatic Rossby vortex (DRV). The disturbance does not begin to amplify until the magnitude and coherence of the low-level PV structure allows for the sufficient production of eddy available potential energy through diabatic processes to overcome frictional dissipation, and this necessitates forced convection for a finite period of time. A comparison of the simulated disturbance structure with observed DRVs lends credence to the idealized model results. Furthermore, the simulations facilitate the identification of an amplitude threshold for DRV genesis that is defined as a function of both environmental parameters (baroclinicity and moisture content) and the strength of an initial disturbance. In particular, if, given a background environment, the amplitude of an initial disturbance is not sufficiently large, frictional processes will inhibit the genesis of a growing disturbance within a realistic time frame.

scholarly journals The Characteristics of Key Analysis Errors. Part I: Dynamical Balance and Comparison with Observations

2007 ◽  
Vol 135 (2) ◽  
pp. 249-266 ◽  
Author(s):  
Jean-François Caron ◽  
M. K. Yau ◽  
Stéphane Laroche ◽  
Peter Zwack
Keyword(s):  
Forecast Error ◽  
Forecast Errors ◽  
Control Analysis ◽  
Initial Condition ◽  
Initial State ◽  
Mass Correction ◽  
Current Algorithm ◽  
Dynamical Balance ◽  
Analysis Error ◽  
Analysis Errors

Abstract The characteristics of the initial corrections obtained from the Canadian Meteorological Centre (CMC) energy-norm-based key analysis error algorithm that minimizes short-range (24 h) forecast errors were investigated for four specific CMC operational analyses. The results show that both the rotational and the divergent components of the initial corrections are strongly out of balance. Some dispersive modes are also present in the mass component of the initial corrections. The results from one experiment where the initial state errors were known suggest that the current algorithm always selects a set of unbalanced initial corrections with more mass correction than wind correction, regardless of the characteristics of the real initial condition errors. Comparison with observational data showed that the corrected analysis is systematically farther away from the observations than the control analysis even in large forecast error events where most of the forecast errors are believed to have originated from errors in the initial state.

scholarly journals Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

2015 ◽  
Vol 143 (7) ◽  
pp. 2459-2484 ◽  
Author(s):  
Andrew B. Penny ◽  
Patrick A. Harr ◽  
Michael M. Bell
Keyword(s):  
Tropical Cyclone ◽  
Large Scale ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Low Level ◽  
The North ◽  
Cloud Clusters ◽  
Potential Formation ◽  
Shearing Deformation ◽  
Upper Level

Abstract Large uncertainty still remains in determining whether a tropical cloud cluster will develop into a tropical cyclone. During The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC)/Tropical Cyclone Structure-2008 (TCS-08) field experiment, over 50 tropical cloud clusters were monitored for development, but only 4 developed into a tropical cyclone. One nondeveloping tropical disturbance (TCS025) was closely observed for potential formation during five aircraft research missions, which provided an unprecedented set of observations pertaining to the large-scale and convective environments of a nondeveloping system. The TCS025 disturbance was comprised of episodic convection that occurred in relation to the diurnal cycle along the eastern extent of a broad low-level trough. The upper-level environment was dominated by two cyclonic cells in the tropical upper-tropospheric trough (TUTT) north of the low-level trough in which the TCS025 circulation was embedded. An in-depth examination of in situ observations revealed that the nondeveloping circulation was asymmetric and vertically misaligned, which led to larger system-relative flow on the mesoscale. Persistent environmental vertical wind shear and horizontal shearing deformation near the circulation kept the system from becoming better organized and appears to have allowed low equivalent potential temperature () air originating from one of the TUTT cells to the north (upshear) to impact the thermodynamic environment of TCS025. This in turn weakened subsequent convection that might otherwise have improved alignment and contributed to the transition of TCS025 to a tropical cyclone.

scholarly journals Ensemble Forecasts and the Properties of Flow-Dependent Analysis-Error Covariance Singular Vectors

2003 ◽  
Vol 131 (8) ◽  
pp. 1741-1758 ◽  
Author(s):  
Thomas M. Hamill ◽  
Chris Snyder ◽  
Jeffrey S. Whitaker
Keyword(s):  
Total Energy ◽  
Linear Combination ◽  
Singular Vector ◽  
Energy Norm ◽  
Initial Time ◽  
Forecast Errors ◽  
Ensemble Forecasts ◽  
Singular Vectors ◽  
Error Covariance ◽  
Analysis Error

Abstract Approximations to flow-dependent analysis-error covariance singular vectors (AEC SVs) were calculated in a dry, T31 L15 primitive-equation global model. Sets of 400-member ensembles of analyses were generated by an ensemble-based data assimilation system. A sparse network of simulated rawinsonde observations were assimilated, and a perfect model was assumed. Ensembles of 48-h forecasts were also generated from these analyses. The structure of evolved singular vectors was determined by finding the linear combination of the forecast ensemble members that resulted in the largest forecast-error variance, here measured in a total-energy norm north of 20°N latitude. The same linear combination of analyses specifies the initial-time structure that should evolve to the forecast singular vector under assumptions of linearity of error growth. The structures of these AEC SVs are important because they represent the analysis-error structures associated with the largest forecast errors. If singular vectors using other initial norms have very different structures, this indicates that these structures may be statistically unlikely to occur. The European Centre for Medium-Range Weather Forecasts currently uses singular vectors using an initial total-energy norm [“total-energy singular vectors” or (TE SVs)] to generate perturbations to initialize their ensemble forecasts. Approximate TE SVs were also calculated by drawing an initial random ensemble with perturbations that were white in total energy and applying the same approach as for AEC SVs. Comparing AEC SVs and approximate TE SVs, the AEC SVs had maximum amplitude in midlatitudes near the tropopause, both at the initial and evolved times. The AEC SVs were synoptic in scale, deep, and did not appear to be geographically localized nor tilted dramatically upshear. This contrasts with TE SVs, which started off relatively smaller in scale, were tilted upshear, and had amplitudes typically largest in the lower to midtroposphere. The difference between AEC SVs and TE SVs suggests that operational ensemble forecasts based on TE SVs could be improved by changing the type of singular vector used to generate initial perturbations. This is particularly true for short-range ensemble forecasts, where the structure of the forecast ensemble is more closely tied to the analysis ensemble.

scholarly journals The Characteristics of Key Analysis Errors. Part II: The Importance of the PV Corrections and the Impact of Balance

2007 ◽  
Vol 135 (2) ◽  
pp. 267-280 ◽  
Author(s):  
Jean-François Caron ◽  
M. K. Yau ◽  
Stéphane Laroche ◽  
Peter Zwack
Keyword(s):  
Short Range ◽  
Vertical Motion ◽  
Forecast Errors ◽  
Divergent Part ◽  
Essential Information ◽  
Original Analysis ◽  
Analysis Error ◽  
Motion Corrections ◽  
The Impact ◽  
Analysis Errors

Abstract This study examines a few approaches to isolate the balanced component of the initial corrections from the Canadian Meteorological Centre energy-norm-based key analysis error algorithm, in an attempt to capture the part of the key analysis errors responsible for short-range forecast errors. The best results were obtained with the nonlinear balance potential vorticity (PV) inversion technique. It was shown that the PV component of the initial corrections contains the essential information for reducing short-range forecast errors. The remaining imbalance part of the initial corrections does not grow in time and does not contribute to the improvement of the forecast. The removal of the imbalance part of the initial corrections makes the corrected analysis slightly closer to the observations, but remains systematically farther away as compared with the original analysis. Thus the balanced part of the key analysis errors cannot justifiably be associated to analysis errors. A methodology to balance the divergent part of the initial corrections, which reduces significantly the spinup in the vertical motion corrections, is also presented. Finally, in light of the results presented in this paper, some recommendations to improve the key analysis error algorithm are proposed.

scholarly journals The Influence of Antecedent Atmospheric River Conditions on Extratropical Cyclogenesis

2021 ◽  
Author(s):  
Zhenhai Zhang ◽  
F. Martin Ralph
Keyword(s):  
Water Vapor ◽  
Initial Conditions ◽  
Detection Methods ◽  
Initial Time ◽  
Latent Heating ◽  
Low Level ◽  
Atmospheric River ◽  
The North ◽  
Close Proximity ◽  
Upper Level

AbstractSome extratropical cyclones (ETC) begin their development in close proximity to a pre-existing atmospheric river (AR). This study investigates the differences in the cyclogenesis stage between these cyclogenesis events and those that begin without an AR nearby. Well-established ETC and AR detection methods are applied to reanalysis over the North Pacific during the 1979-2009 cool seasons (November-March). Of the 3137 cyclogenesis cases detected, 35% are associated with a nearby AR at the time of initial cyclogenesis. Of all 448 cyclones that deepened explosively in the 24 h after their initiation, 60% began with a pre-existing AR nearby.The roles of both dry and diabatic processes that contribute to cyclogenesis are examined, specifically, low-level baroclinicity, upper-level forcing, water vapor inflow and latent heating. ETCs that develop associated with a pre-existing AR receive nearly 80% more water vapor inflow on average, enhancing latent heating and intensifying cyclone deepening in the genesis stage. In contrast, neither low-level baroclinicity nor upper-level potential vorticity exhibit statistically significant differences between cyclogenesis events with and without an AR. Cyclogenesis events associated with an exceptionally strong AR at the ETC initial time deepen even more rapidly in the genesis stage, indicating that the intensity of an antecedent AR can modulate cyclogenesis. About half of the cyclogenesis cases off the U.S. West Coast are associated with ARs at their initial time.These results imply that errors in initial conditions related to ARs can contribute to errors in both AR and ETC predictions, as well as their concomitant impacts.

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