Potential Vorticity Perturbations Research Articles

A Moist Potential Vorticity Model for Midlatitude Long-Lived Mesoscale Convective Systems over Land

Abstract Mesoscale convective systems (MCSs) bring large amounts of rainfall and strong wind gusts to the midlatitude land regions, with significant impacts on local weather and hydrologic cycle. However, weather and climate models face a huge challenge in accurately modeling the MCS life cycle and the associated precipitation, highlighting an urgent need for a better understanding of the underlying mechanisms of MCS initiation and propagation. From a theoretical perspective, a suitable model to capture the realistic properties of MCSs and isolate the bare-bones mechanisms for their initiation, intensification, and eastward propagation is still lacking. To simulate midlatitude MCSs over land, we develop a simple moist potential vorticity (PV) model that readily describes the interactions among PV perturbations, air moisture, and soil moisture. Multiple experiments with or without various environmental factors and external forcing are used to investigate their impacts on MCS dynamics and mesoscale circulation vertical structures. The result shows that mechanical forcing can induce lower-level updraft and cooling, providing favorable conditions for MCS initiation. A positive feedback among surface winds, evaporation rate, and air moisture similar to the wind-induced surface heat exchange over tropical ocean is found to support MCS intensification. Both background surface westerlies and vertical westerly wind shear are shown to provide favorable conditions for the eastward propagation of MCSs. Last, our result highlights the crucial role of stratiform heating in shaping mesoscale circulation response. The model should serve as a useful tool for understanding the fundamental mechanisms of MCS dynamics.

Strength and Spatial Structure of the Perturbation Induced by a Tropical Cyclone to the Underlying Eddies

AbstractA tropical cyclone (TC) induces the oceanic geostrophic response, which perturbs the underlying ocean eddy field. This study investigates the strength and spatial structure of isopycnal undulations and potential vorticity (PV) changes linked with the geostrophic response by use of a linear, two‐layer theory and an OGCM. It is found that the strength and cross‐track length scale of the geostrophic response are well compared with those of background eddies, highlighting the ability of a TC to perturb underlying ocean eddies. More importantly, the TC‐induced PV perturbation is confined within the thermocline between 100 and 300 m depth, causing the 3D quasi‐geostrophic evolution of the perturbed eddies. The length scale of upwelling perturbation (which is ~130 km) is larger than that of PV response (~50 km). This scale disparity means that the patterns of the TC‐induced perturbations are subject to eddy‐TC separation distance. Consequently, the perturbation of a TC to an eddy is significant (negligible) when the eddy‐TC separation distance is less than 80 km (larger than 200 km). This study provides a starting point for understanding the TC‐induced 3D evolution of the perturbed ocean eddy field.

Integral-equation approach to resonances in circular two-layer flows around an island with bottom topography

This paper presents an integral-equation approach to the linear instability problem of two-layer quasi-geostrophic flows around circular islands with radial offshore bottom slope. The flows are composed of concentric uniform potential-vorticity (PV) rings in each layer, with the PV of each ring being opposite in sign. The study extends an earlier similar barotropic model and focuses on the degree to which the topographic waves resonate with the deformation waves at the rings’ peripheries. The integral approach poses the instability problem in a physically elucidating way, whereby the resonating waves in the system are directly identified. Four types of instabilities are identified: instability caused by the resonance of waves at the liquid contours at the edge of each PV ring, instability caused by the resonance of the wave at the upper-layer contour and the topographic waves outside the lower-layer contour, a similar resonance of the lower-layer contour with the topographic waves, and a resonance between one of the eigenmodes of the contour subsystem with the topographic waves. The three latter resonances lead to critical layer instabilities and can be identified as resonances between the contour waves and a collection of singular topographic modes with a critical layer. The PV perturbations in the outer region can be represented asymptotically (far from the origin) as a combination of barotropic and baroclinic modes. Usually, the asymptotically barotropic mode is the mode in resonance with the contours, but, for small growth rates, the asymptotically baroclinic mode may be the dominant mode.

Explosive Cyclogenesis around the Korean Peninsula in May 2016 from a Potential Vorticity Perspective: Case Study and Numerical Simulations

An explosive cyclone event that occurred near the Korean Peninsula in early May 2016 is simulated using the Weather Research and Forecasting (WRF) model to examine the developmental mechanisms of the explosive cyclone. After confirming that the WRF model reproduces the synoptic environments and main features of the event well, the favorable environmental conditions for the rapid development of the cyclone are analyzed, and the explosive development mechanisms of the cyclone are investigated with perturbation potential vorticity (PV) fields. The piecewise PV inversion method is used to identify the dynamically relevant meteorological fields associated with each perturbation PV anomaly. The rapid deepening of the surface cyclone was influenced by both adiabatic (an upper tropospheric PV anomaly) and diabatic (a low-level PV anomaly associated with condensational heating) processes, while the baroclinic processes in the lower troposphere had the smallest contribution. In the explosive phase of the cyclone life cycle, the diabatically generated PV anomalies associated with condensational heating induced by the ascending air in the warm conveyor belt are the most important factors for the initial intensity of the cyclone. The upper-level forcing is the most important factor in the evolution of the cyclone’s track, but it is of secondary importance for the initial strong deepening.

A Climatological Assessment of Intense Extratropical Cyclones from the Potential Vorticity Perspective

Extratropical cyclones (ETCs) are known to intensify due to three vertically interacting positive potential vorticity perturbations that are associated with potential temperature anomalies close to the surface (θB), condensational heating in the lower-level atmosphere (qsat), and stratospheric intrusion in the upper-level atmosphere (qtr). This study presents the first climatological assessment of how much each of these three mechanisms contributes to the intensity of extreme ETCs. Using relative vorticity at 850 hPa as a measure of ETC intensity, results show that in about half of all cases the largest contributions during maximum ETC intensity are associated with qsat (53% of all ETCs), followed by qtr (36%) and θB (11%). The relative frequency of storms that are dominated by qsat is higher 1) during warmer months (61% of all ETCs during warmer months) compared to colder months (50%) and 2) in the Pacific (56% of all ETCs in the Pacific) compared to the Atlantic (46%). The relative frequency of ETCs that are dominated by θB is larger 1) during colder months (13%) compared to warmer months (3%), 2) in the Atlantic (15%) compared to the Pacific (8%), and 3) in western (11%–20%) compared to eastern ocean basins (4%–9%). These findings are based on piecewise potential vorticity inversion conducted for intense ETCs that occurred from 1980 to 2016 in the Northern Hemisphere (3273 events; top 7%). The results may serve as a baseline for evaluating ETC biases and uncertainties in global climate models.

Diagnosis of a North American Polar–Subtropical Jet Superposition Employing Piecewise Potential Vorticity Inversion

Abstract The polar jet (PJ) and subtropical jet (STJ) often reside in different climatological latitude bands. On occasion, the meridional separation between the two jets can vanish, resulting in a relatively rare vertical superposition of the PJ and STJ. A large-scale environment conducive to jet superposition can be conceptualized as one that facilitates the simultaneous advection of tropopause-level potential vorticity (PV) perturbations along the polar and subtropical waveguides toward midlatitudes. Once these PV perturbations are transported into close proximity to one another, interactions between tropopause-level, lower-tropospheric, and diabatically generated PV perturbations work to restructure the tropopause into the two-step, pole-to-equator tropopause structure characteristic of a jet superposition. This study employs piecewise PV inversion to diagnose the interactions between large-scale PV perturbations throughout the development of a jet superposition during the 18–20 December 2009 mid-Atlantic blizzard. While the influence of PV perturbations in the lower troposphere as well as those generated via diabatic processes were notable in this case, tropopause-level PV perturbations played the most substantial role in restructuring the tropopause prior to jet superposition. A novel PV partitioning scheme is presented that isolates PV perturbations associated with the PJ and STJ, respectively. Inversion of the jet-specific PV perturbations suggests that these separate features make distinct contributions to the restructuring of the tropopause that characterizes the development of a jet superposition.

On the Relative Contribution of Inertia–Gravity Wave Radiation to Asymmetric Instabilities in Tropical Cyclone–like Vortices

Abstract Intense atmospheric vortices such as tropical cyclones experience various asymmetric instabilities during their life cycles. This study investigates how vortex properties and ambient conditions determine the relative importance of different mechanisms that can simultaneously influence the growth of an asymmetric perturbation. The focus is on three-dimensional disturbances of barotropic vortices with nonmonotonic radial distributions of potential vorticity. The primary modes of instability are examined for Rossby numbers between 10 and 100 and Froude numbers in the broad neighborhood of unity. This parameter regime is deemed appropriate for tropical cyclone perturbations with vertical length scales ranging from the depth of the vortex to moderately smaller scales. At relatively small Froude numbers, the main cause of instability inferred from analysis typically involves the interaction of vortex Rossby waves with each other and/or critical-layer potential vorticity perturbations. As the Froude number increases from its lower bound, the main cause of instability transitions to inertia–gravity wave radiation. In some cases, the transition occurs abruptly at a critical point where a mode whose growth is driven almost entirely by radiation suddenly becomes dominant. In other cases, the transition is gradual and less direct as the fastest-growing mode continuously changes its structure. Examination of the angular pseudomomentum budget helps quantify the impact of radiation. The radiation-driven instabilities examined herein are shown to be quite fast and potentially relevant to real-world tropical cyclones. Their sensitivities to parameterized moisture and outer vorticity skirts are briefly addressed.

Mechanisms Influencing Cirrus Banding and Aviation Turbulence near a Convectively Enhanced Upper-Level Jet Stream

Abstract Mechanisms supporting a cold-season aviation turbulence outbreak over the northwest Atlantic Ocean and adjacent coastal regions of North America are investigated using high-resolution numerical simulations. Two distinct episodes of moderate-or-greater turbulence in the upper troposphere are observed, and the simulations suggest the turbulence is linked to eastward-translating mesoscale perturbations of negative potential vorticity (PV) emanating from upstream organized deep convection along the anticyclonic shear side of an upper-level jet. Within the exit region of the jet where the turbulence episodes occur, thermodynamic and kinematic fields in the vicinity of the PV perturbations exhibit structural characteristics of mesoscale inertia–gravity waves. These wavelike perturbations are shown to facilitate turbulence by influencing the vertical shear and static stability, which promotes mesoscale regions of banded cirrus clouds, near or within which the observed turbulence occurs. The simulations also suggest that the turbulence arises from fundamentally different mechanisms in the two episodes. In the first and most severe turbulence episode, mesoscale wave-related vertical shear enhancements lead to Kelvin–Helmholtz instability (KHI) near aircraft cruising altitudes (~8.9–11.2 km MSL). Simulated KHI is most prevalent near relatively isolated areas of shallow, moist convection, where smaller-scale internal gravity waves originating in the middle troposphere in response to the shallow convection may play a role in excitation of the KHI located above. The second turbulence episode is consistent with simulated thermal-shear instability related to wave-induced mesoscale reductions in upper-tropospheric static stability. However, unlike for the earlier episode of enhanced turbulence, cloud-radiative feedbacks are necessary for the instability and mesoscale regions of banded cirrus to develop.

Vortex formation in protoplanetary discs induced by the vertical shear instability

We present the results of 2D and 3D hydrodynamic simulations of idealized protoplanetary discs that examine the formation and evolution of vortices by the vertical shear instability (VSI). In agreement with recent work, we find that discs with radially decreasing temperature profiles and short thermal relaxation time-scales, are subject to the axisymmetric VSI. In three dimensions, the resulting velocity perturbations give rise to quasi-axisymmetric potential vorticity perturbations that break-up into discrete vortices, in a manner that is reminiscent of the Rossby wave instability. Discs with very short thermal evolution time-scales (i.e. {\tau}<0.1 local orbit periods) develop strong vorticity perturbations that roll up into vortices that have small aspect ratios ({\chi}<2) and short lifetimes (~ a few orbits). Longer thermal time-scales give rise to vortices with larger aspect ratios (6<{\chi}<10), and lifetimes that depend on the entropy gradient. A steeply decreasing entropy profile leads to vortex lifetimes that exceed the simulation run times of hundreds of orbital periods. Vortex lifetimes in discs with positive or weakly decreasing entropy profiles are much shorter, being 10s of orbits at most, suggesting that the subcritical baroclinic instability plays an important role in sustaining vortices against destruction through the elliptical instability. Applied to the outer regions of protoplanetary discs, where the VSI is most likely to occur, our results suggest that vortices formed by the VSI are likely to be short lived structures.

Evaluation of a Heuristic Model for Tropical Cyclone Resilience

Abstract This work examines the applicability of a previously postulated heuristic model for the temporal evolution of the small-amplitude tilt of a tropical cyclone–like vortex under vertical shear forcing for both a dry and cloudy atmosphere. The heuristic model hinges on the existence of a quasi-discrete vortex Rossby wave and its ability to represent the coherent precession and tilt decay of a stable vortex in the free-alignment problem. Linearized numerical solutions for a dry and cloudy vortex confirm the model predictions that an increase in the magnitude of the radial potential vorticity (PV) gradient within the vortex skirt surrounding the core yields a more rapid evolution of a sheared vortex toward the equilibrium, left-of-shear tilt configuration. However, in the moist-neutral limit, in which the effective static stability vanishes in rising and sinking regions, the heuristic model yields a poor approximation to the simulated vortex core evolution, but a left-of-shear tilt of the near-core vortex, radially beyond the heating region, remains the preferred long-time solution. Within the near-core skirt, the PV perturbation generated by vertical shearing exhibits continuous-spectrum-type vortex Rossby waves, features that are not captured by the heuristic model. Nevertheless, the heuristic model continues to predict the rapid vertical alignment and equilibrium, left-of-shear tilt configuration of the simulated near-core vortex in the moist-neutral limit.

Wave-vortex mode coupling in neutrally stable baroclinic flows.

Rotating stratified flows in thermal wind balance are at the center of geophysical fluid dynamics. Recently, endeavors were put on studying the linear response of such flows to potential vorticity perturbations. It has been shown that the initial potential vorticity (PV) distribution is fundamental and is responsible for important transient growth of the perturbation and gravity-wave generation. Using Pfeiffer's theorem [J. Differ. Equat. 11, 145 (1972)], we give the mathematical demonstration of the stability of asymmetric perturbations k1≠0 of a uniform, unbounded flow in thermal wind balance. Incidentally, we prove that both the wave mode (that corresponds to a vanishing PV) and the vortex mode (corresponding to a nonzero PV) are stable. The emphasis is put on the nontrivial behavior of inertia-gravity waves (IGWs) when deformed by a background shear. In particular, we show that in the linear limit, sheared inertia-gravity waves asymptotically oscillate at the inertial waves frequency, but their amplitude is sensitive to shear, stratification, and rotation. Last, we study the development of the IGWs dynamics considering isotropic initial conditions. Computations indicate that both the vortex mode and the wave mode generate IGWs, but the energy of the IGWs generated by the vortex mode is more important than the energy of the IGWs generated by the wave mode. It is also found that, at large times, the energy of the IGWs generated by the vortex mode increases as the ratio kv/kh (initial vertical wavenumber over horizontal wavenumber) increases (like kv(2)/kh(2)), while the energy of the IGWs generated by the wave mode oscillates in function of kv/kh.

On the Role of Asymmetric Convective Bursts to the Problem of Hurricane Intensification: Radiation of Vortex Rossby Waves and Wave–Mean Flow Interactions

Abstract The role of asymmetric convection to the intensity change of a weak vortex is investigated with the aid of a “dry” thermally forced model. Numerical experiments are conducted, starting with a weak vortex forced by a localized thermal anomaly. The concept of wave activity, the Eliassen–Palm flux, and eddy kinetic energy are then applied to identify the nature of the dominant generated waves and to diagnose their kinematics, structure, and impact on the primary vortex. The physical reasons for which disagreements with previous studies exist are also investigated utilizing the governing equation for potential vorticity (PV) perturbations and a number of sensitivity experiments. From the control experiment, it is found that the response of the vortex is dominated by the radiation of a damped sheared vortex Rossby wave (VRW) that acts to accelerate the symmetric flow through the transport of angular momentum. An increase of the kinetic energy of the symmetric flow by the VRW is shown also from the eddy kinetic energy budget. Additional tests performed on the structure and the magnitude of the initial thermal forcing confirm the robustness of the results and emphasize the significance of the wave–mean flow interaction to the intensification process. From the sensitivity experiments, it is found that for a localized thermal anomaly, regardless of the baroclinicity of the vortex and the radial and vertical gradients of the thermal forcing, the resultant PV perturbation follows a damping behavior, thus suggesting that deceleration of the vortex should not be expected.

Warm Conveyor Belts in Idealized Moist Baroclinic Wave Simulations

AbstractThis idealized modeling study of moist baroclinic waves addresses the formation of moist ascending airstreams, so-called warm conveyor belts (WCBs), their characteristics, and their significance for the downstream flow evolution. Baroclinic wave simulations are performed on the f plane, growing from a finite-amplitude upper-level potential vorticity (PV) perturbation on a zonally uniform jet stream. This nonmodal approach allows for dispersive upstream and downstream development and for studying WCBs in the primary cyclone and the downstream cyclone. A saturation adjustment scheme is used as the only difference between the dry and moist simulations, which are systematically compared using a cyclone-tracking algorithm, with an eddy kinetic energy budget analysis, and from a PV perspective. Using trajectories and a selection criterion of maximum ascent, forward- and rearward-sloping WCBs in the moist simulation are identified. No WCB is identified in the dry simulation. Forward-sloping WCBs originate in the warm sector, move into the frontal fracture region, and ascend over the bent-back front, where maximum latent heating occurs in this simulation. The outflow of these WCBs is located at altitudes with prevailing zonal winds; they hence flow anticyclonically (“forward”) into the downstream ridge. In case of a slightly weaker ascent, WCBs curve cyclonically (“rearward”) above the cyclone center. A detailed analysis of the PV evolution along the WCBs reveals PV production in the lower troposphere and destruction in the upper troposphere. Consequently, WCBs transport low-PV air into their outflow region, which contributes to the formation of distinct negative PV anomalies. They, in turn, affect the downstream flow and enhance downstream cyclogenesis.

Potential vorticity diagnosis of rapid intensification of very severe cyclone GIRI (2010) over the Bay of Bengal

The life cycle of Bay of Bengal cyclone GIRI, characterized by a rapid intensification during 36-h interval, is investigated. The cyclone under study underwent a period of explosive cyclogenesis from 0000 UTC 21 October to 1200 UTC 22 October 2010. During this period, the sea level pressure minimum at the center of cyclone dropped by 52 hPa. European Centre for Medium Range Weather Forecasts (ECMWF) model data is used to perform the analysis of Q-vectors, K-Index and potential vorticity (PV) perturbation in order to diagnose the life cycle of this unusual cyclone. The analysis reveals that during the period of explosive development, the 500–700 hPa column-averaged Q-vector convergence (regions of quasi-geostrophic forcing for ascent) directly above the surface cyclone had strengthened, which in turn affected the lower to middle-tropospheric ascent and associated surface cyclogenesis. The analysis also reveals that the presence of lower-tropospheric cyclogenetic forcing in the environment, characterized by reduced static stability as measured by very high values of the K-Index produced a burst of heavy precipitation during the development stage of the cyclone. The associated latent heat release produced a substantial diabatic positive PV anomaly in the middle and lower troposphere that caused lower-tropospheric height falls associated with the explosive cyclogenesis. Thus, diabatic consequence of the latent heat release fueled the explosive development of the cyclone. The intensification mechanism of the cyclone occurred in two stages. A diabatically generated lower-tropospheric positive PV anomaly dominated the rapid intensification stage after initial triggering by a positive upper-level PV anomaly. A limited verification of ECMWF model shows that the model could predict the rapid intensification of the cyclone to a large extent and landfall near observed landfall point and time. It predicted lowest central pressure of 970.5 hPa 24-h in advance with landfall near 19.7°N and 93.7°E around 1400 UTC 22 October 2010 against the lowest estimated central pressure of 950 hPa and observed landfall near 20.0°N and 93.5°E around 1400 UTC 22 October 2010.

Ensemble prediction of Mediterranean high-impact events using potential vorticity perturbations. Part II: Adjoint-derived sensitivity zones

In Part I of this work, an ensemble prediction system (EPS) based on different combinations of model physical parameterizations was compared against another ensemble based on perturbing initial and boundary conditions through the Potential Vorticity (PV) field. This comparison was done for western Mediterranean cyclonic situations associated with high-impact weather phenomena such as heavy rain and showed a better performance of the PV-perturbed ensemble over the more traditional multiphysics approach. The current study extends the comparison to another ensemble based on perturbing initial and boundary conditions through the PV field but guided by the MM5 adjoint derived sensitivity zones (PV-adjoint) instead of by the three-dimensional PV features showing intense values and gradients as was done in Part I (PV-gradient).The PV-adjoint and PV-gradient EPSs perturb specific areas of the cyclonic development using a PV error climatology that typifies PV errors in the initial and boundary conditions to provide the appropriate error range. The non-hydrostatic MM5 mesoscale model nested in the ECMWF forecast fields is used to provide all predictions.For the studied cases, 19 cyclonic events associated with heavy rain, the verification results show that both PV-perturbed are skillful, the PV-gradient being the best. Therefore, for our testbed, the extra computational cost of running the MM5 adjoint model does not provide a significant ensemble skill improvement.

A polar low named Vera: the use of potential vorticity diagnostics to assess its development

Abstract20 November 2008 the Norwegian Meteorological Institute issued an extreme weather warning for Trøndelag County. A storm was expected in the afternoon. The storm, a polar low (PL), was named ‘Vera’. Vera was the second of two polar lows that developed along a wedge of warm air at the rear of a synoptic‐scale low that moved northeastwards into northern Norway. The dynamical development may be explained by classic dynamical theory: low‐level warm air seclusion and shallow secondary circulation in a frontal zone that couples to transient upper level disturbances. The study focuses on the potential vorticity (PV) perspective of the PL development and we have modified the initial PV field to study the effect of each individual PV perturbation. Two PV and two low‐level temperature anomalies have been chosen. The PV anomalies retained their structure during the development while the temperature anomalies became less coherent and difficult to define by the end of the cyclogenesis period. By modifying the initial conditions in the upper troposphere over Greenland, it is shown that in this case the upper PV anomaly had the strongest effect on the development, and that this effect remained strong throughout the cyclogenesis until the polar low made landfall. The effect from the low level PV anomaly became large after onset of cyclogenesis, reflecting the positive feedback, although its impact on the development remained smaller than that of the upper PV anomaly. It is also shown that the topography of Greenland was important in determining the correct position of this polar low development. Copyright © 2011 Royal Meteorological Society

Ensemble prediction of Mediterranean high-impact events using potential vorticity perturbations. Part I: Comparison against the multiphysics approach

The western Mediterranean is a very cyclogenetic area and many of the cyclones developed over this region are associated with high-impact weather phenomena that affect the society of the coastal countries. Two ensemble prediction systems (EPSs) based on multiphysics and perturbed initial and boundary conditions (IBC) are designed in order to improve the forecast of these heavy rain episodes. The MM5 mesoscale model nested in the ECMWF forecast fields provides the simulations, run at 22.5 km resolution for a two-day period. The multiphysics ensemble combines different model physical parameterization schemes while the other ensemble perturbs the initial state and boundary forcing of the model with the aid of a PV inversion scheme. A PV error climatology derived from the large-scale fields allows to perturb the ECMWF PV fields using the appropriate error range. The verification procedure indicates that even though both EPSs are skillful, the perturbed IBC ensemble is more proficient than the multiphysics EPS for the 19 Mediterranean cyclonic events with heavy rain considered in the study. Therefore the results show a more dominant role of the uncertainties in the initial and boundary conditions than the model error, although both of them contribute significantly to improve the predictability of Western Mediterranean high impact weather situations.

Analysis and forecasting of heavy-rainfall event by strong convection

For the case of severe convection occurring in the southern region of North China on July 8, 2009, the three dynamical factors, namely, vertical component of convective vorticity vector, moist thermodynamic advection parameter and wave-activity density are diagnosed. The result shows that the anomalous-value regions of dynamical factors are located mainly in middle and lower troposphere over the strong precipitation area. Since the dynamical factors can effectively describe the synthetical characteristics of strong convective system, such as vertical shear of horizontal wind, advection of potential temperature, vorticity perturbation and baroclinicity of moist atmosphere, they correspond to the observation of 6-h accumulative rainfall in horizontal distribution patterns. Using the 6-h, 12-h, 18-h and 24-h forecasting dataset of U.S. NCEP/NCAR 0.5-Degree GFS, the dynamical factors are calculated to analyze their indications of precipitation forecasting during a longer period. The results show that during June 2 – Oct 1, 2009, the 6-h, 12-h, 18-h and 24-h forecasted dynamical factors each have an indication of strong precipitation. The moist thermodynamic advection parameter suggests that the precipitation is best among the three dynamical factors.

Sensitivity of Typhoon Track Predictions in a Regional Prediction System to Initial and Lateral Boundary Conditions

Abstract Tropical cyclone (TC) track predictions from the operational regional nonhydrostatic TC forecast system of the Taiwanese Central Weather Bureau (CWB) are examined for their sensitivities to initial and lateral boundary conditions. Five experiments are designed and discussed, each using a combination of different initial and lateral boundary conditions coming either from the CWB or the National Centers for Environmental Prediction (NCEP) global forecast system. Eight typhoons in the western Pacific Ocean with 51 cases in 2004 and 2005 are tested with the five designed experiments for the 3-day forecast. The average track forecasts are the best when both the initial and lateral boundary conditions are from the NCEP global forecast system. This reflects the generally superior performance of the NCEP global forecast system relative to that of the CWB. Using different lateral boundary conditions has a greater impact on the track than using different initial conditions. Diagnostics using piecewise inversion of potential vorticity perturbations are carried out to identify synoptic features surrounding the featured typhoon that impact the track the most in each experiment. For the two cases demonstrated with the largest track improvement using NCEP global fields, the diagnostics indicate that the prediction of the strength and extent of the subtropical high in the western Pacific plays the major role in affecting these storm tracks. Using the analysis and predictions of the CWB global forecast system as the initial and lateral boundary conditions produces an overintensified subtropical ridge in the regional TC forecast model. Because most of the typhoons studied are located in the southwestern peripheral of the western Pacific subtropical high, the stronger steering from the more intense and extended high system is the main cause of the poleward bias in the predicted typhoon tracks in the operational run, which uses the CWB global forecast fields. The study suggests that, when efforts are made to improve a regional TC forecast model, it is also critically important to improve the global forecast system that provides the lateral boundary and initial conditions to the regional system.

Inclusion of potential vorticity uncertainties into a hydrometeorological forecasting chain: application to a medium size basin of Mediterranean Spain

Abstract. The improvement of the short- and mid-range numerical runoff forecasts over the flood-prone Spanish Mediterranean area is a challenging issue. This work analyses four intense precipitation events which produced floods of different magnitude over the Llobregat river basin, a medium size catchment located in Catalonia, north-eastern Spain. One of them was a devasting flash flood – known as the "Montserrat" event – which produced 5 fatalities and material losses estimated at about 65 million euros. The characterization of the Llobregat basin's hydrological response to these floods is first assessed by using rain-gauge data and the Hydrologic Engineering Center's Hydrological Modeling System (HEC-HMS) runoff model. In second place, the non-hydrostatic fifth-generation Pennsylvania State University/NCAR mesoscale model (MM5) is nested within the ECMWF large-scale forecast fields in a set of 54 h period simulations to provide quantitative precipitation forecasts (QPFs) for each hydrometeorological episode. The hydrological model is forced with these QPFs to evaluate the reliability of the resulting discharge forecasts, while an ensemble prediction system (EPS) based on perturbed atmospheric initial and boundary conditions has been designed to test the value of a probabilistic strategy versus the previous deterministic approach. Specifically, a Potential Vorticity (PV) Inversion technique has been used to perturb the MM5 model initial and boundary states (i.e. ECMWF forecast fields). For that purpose, a PV error climatology has been previously derived in order to introduce realistic PV perturbations in the EPS. Results show the benefits of using a probabilistic approach in those cases where the deterministic QPF presents significant deficiencies over the Llobregat river basin in terms of the rainfall amounts, timing and localization. These deficiences in precipitation fields have a major impact on flood forecasts. Our ensemble strategy has been found useful to reduce the biases at different hydrometric sections along the watershed. Therefore, in an operational context, the devised methodology could be useful to expand the lead times associated with the prediction of similar future floods, helping to alleviate their possible hazardous consequences.