Height Tendency Research Articles

Impact of the North Pacific Meridional Mode on the Tropical Pacific Modulated by the Interdecadal Pacific Oscillation

Abstract The North Pacific meridional mode (NPMM) peaking in boreal spring influences El Niño–Southern Oscillation (ENSO) properties in the ensuing winter. Whether the precursory impact of NPMM on the spatial diversity of ENSO has decadal variation remains unknown. Using long-term reanalysis datasets, we find that the interdecadal Pacific oscillation (IPO) significantly modulates the NPMM forcing on two types of ENSO. During the positive IPO (+IPO) phase, a strengthened background Aleutian low and southward-shifted storm track, in comparison to the negative IPO (−IPO) phase, produce stronger basin-scale negative geopotential height tendency anomalies over the North Pacific through synoptic-scale eddy–mean flow interaction. Such strong background negative tendency facilitates an Aleutian low–like pressure monopole rather than a North Pacific Oscillation (NPO)-like pressure dipole in boreal spring, leading to a weak NPMM that cannot effectively promote development of either a central Pacific (CP) or an eastern Pacific (EP) ENSO. By contrast, the NPO-like dipole enhances in boreal spring during −IPO, corresponding to stronger and more frequently occurring NPMM events that induce a robust CP-ENSO-like response in boreal winter. Moreover, the −IPO-related tropical Pacific mean states and the associated positive feedbacks cause a strong decrease in mixed layer temperature variance in the equatorial eastern Pacific, but a slight increase in the central Pacific, thus further contributing to the enhanced correlation between NPMM and CP-ENSO. Therefore, −IPO has played a role in the stronger impact of NPMM on CP-ENSO since the 1990s, and the modulation effects of IPO should be considered in understanding the extratropical–tropical climatic connection and ENSO spatial diversity.

Two Types of Mid-High-Latitude Intraseasonal Oscillations in the Southern Hemisphere during Austral Summer

Abstract Using NCEP–NCAR reanalysis data, the atmospheric intraseasonal oscillation (ISO) over the mid-high latitudes in the Southern Hemisphere during austral summer is studied. Two types of 10–30-day eastward- and westward-propagating ISO at mid-high latitudes are extracted by extended empirical orthogonal functions. The analysis of wave activity flux reveals that the energy sources of the eastward-propagating wave train are in the southwest Pacific and southern Indian Ocean, and the westward-propagating wave train is over the east coast of South America. The diagnosis of geopotential height tendency shows that the zonal gradient of ISO relative vorticity guided by mean zonal wind plays a major role in the eastward propagation of the wave train, while the meridional gradient of mean relative vorticity and geostrophic vorticity guided by ISO meridional wind plays a major role in the westward propagation of the wave train. Energy analysis shows that the amplitude enhancement over the South Pacific is due to the ISO disturbance gaining energy from the mean flow. The eastward-propagating ISO obtains energy from the mean flow through kinetic and potential energy conversion, while the westward-propagating ISO cannot obtain energy through potential energy conversion. The surface air temperature and precipitation in South America are affected by the circulation and propagation corresponding to both types of ISO. The useful forecasting skills by the subseasonal-to-seasonal forecasting project for eastward- and westward-propagating types can reach to 17 days, while the potential forecasting skills improve to 20 and 19 days, respectively. Significance Statement The intraseasonal oscillation (ISO) has important effects on regional weather and climate, and we focus on the ISO over mid-high latitudes in the Southern Hemisphere, which has been paid little attention. We reveal two leading types of the mid-high-latitude ISO during austral summer, namely, the eastward- and westward-propagating types. Also, the characteristics and effects of the two types are explored. To understand their dynamical process, the geopotential height tendency and energy conversion are diagnosed. It is also found that the two ISO types can be predicted up to 17 days in advance by the subseasonal-to-seasonal project, which may provide guidance for the extended-range forecasts.

Increase in summer precipitation over the Sichuan Basin in recent decades and possible causes

AbstractA wet‐to‐dry shift in summer over the Sichuan Basin (SCB) during the late 20th century was noticed by previous studies. However, this study found that summer precipitation over SCB significantly increases during the past two decades, with an abrupt change point around 2011/2012. An anomalous cyclone over Lake Baikal, accompanied by an anticyclone over southern China, is responsible for the increase in SCB precipitation by inducing anomalous upper‐level divergence, mid‐level ascending motion and moisture convergence. Such atmospheric circulation anomalies are closely associated with the sea surface temperature (SST) warming over the Barents–Kara Sea (BKS). Observational analyses and model experiments indicate that the SST warming over the BKS warms local atmosphere and weakens meridional air temperature gradients over northern Asia and induces local anomalous easterlies, which further induce the anomalous cyclone over Lake Baikal that favours SCB precipitation. Additionally, the inhomogeneous warming over Europe increases local meridional air temperature gradients, enhances atmospheric baroclinicity and intensifies local transient eddy activities over central Europe. The intensified transient eddy activities generate negative geopotential height tendency anomalies over northern Europe, which can further excite a Rossby wave train that directly influences SCB precipitation, and can warm the atmosphere above the Kara Sea that indirectly influences SCB precipitation. This study can deepen our understanding of the interdecadal variability of summer precipitation over the SCB.

Rapid response of the East Asian trough to Tibetan Plateau snow cover

AbstractTibetan Plateau snow cover (TPSC) has subseasonal variations and rapidly influences the atmosphere. In this study, we present the rapid response of the East Asian trough (EAT) within a week to subseasonal variations in TPSC during the boreal winter. Using snow cover analysis obtained from the daily interactive multisensor snow and ice mapping system and the ERA‐Interim reanalysis, a considerable relationship between TPSC and 500‐hPa geopotential height anomalies over the downstream EAT region is found. Significant negative (positive) 500‐hPa geopotential height anomalies originating from the Tibetan Plateau and moving into the EAT region appear within a week following anomalous positive (negative) TPSC events, which lead to changes in EAT strength. Thus, a significantly enhanced (reduced) intensity of the EAT occurs approximately 5–6 days after increased (decreased) TPSC. Numerical experiments confirm the causality of this relationship. Further analysis of the quasi‐geostrophic geopotential height tendency equations in numerical experiments indicates that such EAT variations result from anomalous thermal advection from the Tibetan Plateau forced by TPSC.

Interdecadal variations in winter extratropical anticyclones in East Asia and their impacts on the decadal mode of East Asian surface air temperature

The interdecadal variations in extratropical anticyclone activity over East Asia during the winters of 1958–2013 were analysed using an objective identification and tracking algorithm for anticyclones. The influence of the interdecadal variation in anticyclone activity over the Mongolian Plateau on the interdecadal variation in East Asian surface air temperature and its possible mechanism was studied. The results showed that the Mongolian Plateau is the key region of anticyclonic activity in East Asia, and the occurrence frequency of anticyclones presented significantly opposite distribution forms in the higher and lower latitudes. An abrupt change in anticyclone activity over the Mongolian Plateau occurred in the late 1970s. From 1958 to the late 1970s, the main anticyclone activity occurred farther northward, whereas the activity was farther southward from the 1980s to 2013. Further study showed that in the period when the anticyclone occurrence frequency in the higher latitudes north of 50°N was lower (higher), while that in the lower latitudes south of 50°N was higher (lower), the surface air temperature of most parts of East Asia, especially the middle and high latitudes, was abnormally high (low). This was due to the anticyclonic activities, the Siberian high faded in the northwest part and expanded in southeast, while the East Asia trough and Ural Mountain blocking high weakened. This was also manifested as the variation in the geopotential height tendency forcing by transient waves. As the anticyclonic wave-breaking intensifies from western Mongolia to Northeast China, the high-frequency momentum flux over this area was significantly enhanced, strengthening the west wind in the higher latitudes from the north side and weakening the west wind in the middle latitudes from the south side. Under the combined force of the above activity, the southward movement of polar cold air was weakened, and the warm air in the middle and low latitudes was more easily transported to the north, resulting in warmer surface air temperatures over most parts of East Asia.

Variational Bias Correction of TAMDAR Temperature Observations in the WRF Data Assimilation System

AbstractA variational bias correction (VarBC) scheme is developed and tested using regional Weather Research and Forecasting Model Data Assimilation (WRFDA) to correct systematic errors in aircraft-based measurements of temperature produced by the Tropospheric Airborne Meteorological Data Reporting (TAMDAR) system. Various bias models were investigated, using one or all of aircraft height tendency, Mach number, temperature tendency, and the observed temperature as predictors. These variables were expected to account for the representation of some well-known error sources contributing to uncertainties in TAMDAR temperature measurements. The parameters corresponding to these predictors were evolved in the model for a two-week period to generate initial estimates according to each unique aircraft tail number. Sensitivity experiments were then conducted for another one-month period. Finally, a case study using VarBC of a cold front precipitation event is examined. The implementation of VarBC reduces biases in TAMDAR temperature innovations. Even when using a bias model containing a single predictor, such as height tendency or Mach number, the VarBC produces positive impacts on analyses and short-range forecasts of temperature with smaller standard deviations and biases than the control run. Additionally, by employing a multiple-predictor bias model, which describes the statistical relations between innovations and predictors, and uses coefficients to control the evolution of components in the bias model with respect to their reference values, VarBC further reduces the average error of analyses and short-range forecasts with respect to observations. The potential impacts of VarBC on precipitation forecasts were evaluated, and the VarBC is able to indirectly improve the prediction of precipitation location by reducing the forecast error for wind-related synoptic circulation leading to precipitation.

Assessing the mechanisms governing the daytime evolution of marine stratocumulus using large‐eddy simulation

Large‐eddy simulation (LES) output for a case of thin stratocumulus off the coast of California is examined in a mixed‐layer analysis framework to identify the specific mechanisms responsible for governing the evolution of the cloud system. An equation for cloud‐base height tendency isolates the individual cloud‐modulating mechanisms that control the evolution of boundary‐layer liquid‐water static energy (Sl) and total water mixing ratio (qT). With a suitable spin‐up procedure, the control simulation performs admirably compared with observed estimates of liquid water content, vertical velocity variance, and radiative fluxes sampled during an aircraft field campaign. Investigation of the cloud response to various environmental forcing scenarios was addressed through a suite of sensitivity simulations, including variations in subsidence velocity, surface fluxes, wind shear near the inversion, and radiative forcing. In the control simulation, rising cloud‐base tendencies are associated with entrainment warming/drying and short‐wave absorption, whereas lowering cloud‐base tendencies are driven by long‐wave cooling. Even in the presence of substantial afternoon solar heating, entrainment fluxes remained active. The thin cloud demonstrated unexpected resiliency, with mixed‐layer analysis indicating that, as the short‐wave flux decreases later in the afternoon, the relative contribution of long‐wave cooling often becomes large enough to offset entrainment warming/drying and result in a reversal of cloud‐base tendency. The evolution of cloud‐base tendency is found to be insensitive to the net radiative flux divergence for most of the simulations (liquid water path ranging from ~10–50 g/m2). Error analysis in comparison with LES Sl and qT budgets suggests that our method of entrainment flux calculation could be improved by a more complete understanding of entrainment‐layer physics.

Experimental Study of Thermal Behavior of Insulation Material Rigid Polyurethane in Parallel, Symmetric, and Adjacent Building Façade Constructions.

Both experimental and theoretical methods were proposed to assess the effects of adjacent, parallel, and symmetric exterior wall structures on the combustion and flame spreading characteristics of rigid polyurethane (PUR) foam insulation. During the combustion of PUR specimens, the flame leading edge was found to transfer from a unique inverted ‘W’ shape to an inverted ‘V’ during flame propagation. This phenomenon is attributed to edge effects related to boundary layer theory. The effects of the adjacent façade angle on flame spreading rate and flame height were shown to be nonlinear, as a result of the combined influences of heat transfer, radiation angle, and the chimney restriction effects. A critical angle around 90 degree with maximum thermal hazards outwards by parallel fire was observed and consistent with the mass loss rate and flame height tendencies. For narrow spacing configurations or angles (e.g., 60 and 90 degrees), phenomenological two-pass processing in conjunction showed that increased preheating lengths were associated with enhanced heat transfer. The results of this study have implications concerning the design of safe façade structures for high-rise buildings, and provide a better understanding of downward flame spreading over PUR.

Analysis of the initiation of an extreme North Pacific jet retraction using piecewise tendency diagnosis

An unusually prolonged retraction of the tropopause‐level jet transformed the circulation in the North Pacific for weeks in mid‐February and March 2006 and preceded the development of a long‐lived negative Pacific–North American pattern. The initiation of this jet retraction, associated with a series of anticyclonic/LC1 wave‐breaking events at high latitude, is investigated through the lens of quasi‐geostrophic (QG) piecewise tendency analysis. Two key anticyclonic anomalies divert and retract the jet through serial LC1 wave‐breaking events, mostly confined to the 315–330 K isentropic layer and occurring in regions of strong deformation. The rapid development of the first anomaly coinciding with the beginning of retraction is diagnosed in a QG framework, in order to quantify contributions to QG height tendencies from various processes. The resultant analysis reveals that growth was induced by a deep potential vorticity intrusion that perturbed the jet, while the remainder of the ridge's life cycle was largely governed by upper‐level deformation in the jet exit region. Deformation facilitated both growth and decay, as the phasing between the ridge and deformation changed. Baroclinic development also contributed to growth but, unlike prior published analyses of anticyclones in the North Pacific, was not the dominant term.

Entrainment rate diurnal cycle in marine stratiform clouds estimated from geostationary satellite retrievals and a meteorological forecast model

AbstractThe mean diurnal cycle of cloud entrainment rate (we) over the northeast Pacific region is for the first time computed by combining, in a mixed‐layer model framework, the hourly composited GOES‐15 satellite‐based cloud top height (HT) tendency, advection, and large‐scale vertical velocity (w) during May to September 2013, with horizontal winds and w taken from the European Centre for Medium‐Range Weather Forecasts (ECMWF) model. The tendency term dominates the magnitude and phase of the we diurnal cycle, with a secondary role of w, and a modest advective contribution. The peak and minimum in we occur between 20:00–22:00 LT and 9:00–11:00 LT, respectively, in close agreement with the diurnal cycle of turbulence driven by cloud top longwave cooling. Uncertainties in HT and ECMWF fields are assessed with in situ observations and three meteorological reanalysis data sets. This study provides the basis for constructing nearly global climatologies of we by combining a suite of well‐calibrated geostationary satellites.

OZO v.1.0: software for solving a generalised omega equation and the Zwack–Okossi height tendency equation using WRF model output

Abstract. A software package (OZO, Omega–Zwack–Okossi) was developed to diagnose the processes that affect vertical motions and geopotential height tendencies in weather systems simulated by the Weather Research and Forecasting (WRF) model. First, this software solves a generalised omega equation to calculate the vertical motions associated with different physical forcings: vorticity advection, thermal advection, friction, diabatic heating, and an imbalance term between vorticity and temperature tendencies. After this, the corresponding height tendencies are calculated with the Zwack–Okossi tendency equation. The resulting height tendency components thus contain both the direct effect from the forcing itself and the indirect effects (related to the vertical motion induced by the same forcing) of each physical mechanism. This approach has an advantage compared with previous studies with the Zwack–Okossi equation, in which vertical motions were used as an independent forcing but were typically found to compensate the effects of other forcings.The software is currently tailored to use input from WRF simulations with Cartesian geometry. As an illustration, results for an idealised 10-day baroclinic wave simulation are presented. An excellent agreement is found between OZO and the direct WRF output for both the vertical motion and the height tendency fields. The individual vertical motion and height tendency components demonstrate the importance of both adiabatic and diabatic processes for the simulated cyclone. OZO is an open-source tool for both research and education, and the distribution of the software will be supported by the authors.

Coastal Clouds at the Eastern Margin of the Southeast Pacific: Climatology and Trends

Abstract The climatology and recent trends of low-level coastal clouds at three sites along the northern Chilean coast (18.3°–23.4°S) are documented based upon up to 45 years of hourly observations of cloud type, coverage, and heights. Consistent with the subtropical location, cloud types are dominated by stratocumuli having greatest coverage (>7 oktas) and smaller heights (600–750 m) during the nighttime of austral winter and spring. Meridionally, nighttime cloud fraction and cloud-base heights increase from south to north. Long-term trends in mean cloud cover are observed at all sites albeit with a seasonal modulation, with increasing (decreasing) coverage in the spring (fall). Consistent trend patterns are also observed in independent sunshine hour measurements at the same sites. Cloud heights show negative trends of about 100 m decade−1 (1995–2010), although the onset time of this tendency differs between sites. The positive cloud fraction trends during the cloudy season reported here disagree with previous studies, with discrepancies attributed to differences in datasets used or to methodological differences in data analysis. The cloud-base height tendency, together with a less rapid lowering of the subsidence inversion base height, suggests a deepening of the coastal cloud layer. While consistent with the tendency toward greater low-level cloud cover and the known cooling of the marine boundary layer in this region, these tendencies are at odds with a drying trend of the near-surface air documented here as well. Assessing whether this intriguing result is caused by physical factors or by limitations of the data demands more detailed observations, some of which are currently under way.

The MJO Cycle Forcing of the North Atlantic Circulation: Intervention Experiments with the Community Earth System Model

Abstract A three-dimensional evolution of Madden–Julian oscillation (MJO) diabatic heating for October–March from satellite data is constructed: the heating propagates eastward for three cycles, modulated by the likelihood for a given MJO phase to occur on a given calendar day. This heating is added to the temperature tendencies of each member of an ensemble of 48 (1 October–31 March) simulations with the Community Earth System Model. The leading two most predictable modes of the planetary wave vertically integrated total (added plus model generated) heating capture 81% of the ensemble-mean variance and form an eastward-propagating oscillation with very high signal-to-noise ratio. The two most predictable modes of the extratropical Northern Hemisphere 200-hPa height form an oscillation, as do those of the 300-hPa height tendency due to synoptic vorticity flux convergence, the 200-hPa Rossby wave source, and the envelope transient kinetic energy. The North Atlantic Oscillation (NAO+) occurs 15–25 days after the MJO convection crosses the 90°E meridian, supported by synoptic vorticity flux convergence and a distinct pattern of Rossby wave source. The daily North Atlantic circulation anomalies are categorized into four circulation regimes with a cluster analysis. The NAO+ and NAO− are equally likely in the control model runs, but the NAO+ is 10% more likely in the model runs with heating, compared to a difference of 14% in reanalyses. The daily occurrence of the NAO+ regime in the heating ensemble shows maxima at times when the leading two optimal modes of height also indicate NAO+ but also shows maxima at other times.

Storm-Centered Ensemble Data Assimilation for Tropical Cyclones

Abstract A significant challenge for tropical cyclone ensemble data assimilation is that storm-scale observations tend to make analyses that are more asymmetric than the prior forecasts. Compromised structure and intensity, such as an increase of amplitude across the azimuthal Fourier spectrum, are a routine property of ensemble-based analyses, even with accurate position observations and frequent assimilation. Storm dynamics in subsequent forecasts evolve these states toward axisymmetry, creating difficulty in distinguishing between model-induced and actual storm asymmetries for predictability studies and forecasting. To address this issue, a novel algorithm using a storm-centered approach is proposed. The method is designed for use with existing ensemble filters with little or no modification, facilitating its adoption and maintenance. The algorithm consists of 1) an analysis of the environment using conventional coordinates, 2) a storm-centered analysis using storm-relative coordinates, and 3) a merged analysis that combines the large-scale and storm-scale fields together at an updated storm location. This algorithm is evaluated in two sets of observing system simulation experiments (OSSEs): first, no-cycling tests of the update step for idealized three-dimensional storms in radiative–convective equilibrium; second, full cycling tests of data assimilation applied to a shallow-water model for a field of interacting vortices. Results are compared against a control experiment based on a conventional ensemble Kalman filter (EnKF) scheme as well as an alternative EnKF scheme proposed by Lawson and Hansen. The storm-relative method yields vortices that are more symmetric and exhibit finer inner-core structure than either approach, with errors reduced by an order of magnitude over a control case with prior spread consistent with the National Hurricane Center (NHC)’s mean 5-yr forecast track error at 12 h. Azimuthal Fourier error spectra exhibit much-reduced noise associated with data assimilation as compared to both the control and the Lawson and Hansen approach. An assessment of free-surface height tendency of model forecasts after the merge step reveals a balanced trend between the storm-centered and conventional approaches, with storm-centered values more closely resembling the reference state.

Intermediate frequency atmospheric disturbances: A dynamical bridge connecting western U.S. extreme precipitation with East Asian cold surges

AbstractIn this study, an atmospheric river (AR) detection algorithm is developed to investigate the downstream modulation of the eastern North Pacific ARs by another weather extreme, known as the East Asian cold surge (EACS), in both reanalysis data and high‐resolution global model simulations. It is shown that following the peak of an EACS, atmospheric disturbances of intermediate frequency (IF; 10–30 day period) are excited downstream. This leads to the formation of a persistent cyclonic circulation anomaly over the eastern North Pacific that dramatically enhances the AR occurrence probability and the surface precipitation over the western U.S. between 30°N and 50°N. A diagnosis of the local geopotential height tendency further confirms the essential role of IF disturbances in establishing the observed persistent anomaly. This downstream modulation effect is then examined in the two simulations of the National Center for Atmospheric Research Community Climate System Model version 4 with different horizontal resolutions (T85 and T341) for the same period (1979–2005). The connection between EACS and AR is much better captured by the T341 version of the model, mainly due to a better representation of the scale interaction and the characteristics of IF atmospheric disturbances in the higher‐resolution model. The findings here suggest that faithful representations of scale interaction in a global model are critical for modeling and predicting the occurrences of hydrological extremes in the western U.S. and for understanding their potential future changes.

Interannual Variability of the East Asian Winter Monsoon and Related Modulations of the Planetary Waves

Interannual variability of the East Asian winter monsoon is investigated through composite analysis applied to observational data for 50 recent years. Although the monsoon activity itself is confined into the lower troposphere, its midwinter variability tends to accompany upper-tropospheric geopotential height anomalies similar to the Eurasian (EU) and western Pacific (WP) teleconnection patterns. The “EU-like” pattern is characterized by a wavy signature over the Eurasian continent and the North Atlantic, with surface temperature anomalies over the Far East and North America. In the “WP-like” pattern, a meridional dipole of upper-level height anomalies is evident over the Far East. These anomaly patterns related to the anomalous winter monsoon activity are found to accompany marked modulations of the climatological development of the upper-tropospheric planetary waves from late autumn to midwinter. Enhanced monsoon activity in January associated with the WP-like pattern involves anomalous seasonal development of a planetary wave ridge with enhanced positive height tendencies from November to January over eastern Siberia and Alaska, while the corresponding tendencies are anomalously negative under the weakened monsoon activity. The stronger monsoon also accompanies an enhanced seasonal decline of geopotential height over the midlatitude North Pacific, corresponding to the enhanced southeastward development of a planetary wave trough. Similar modulations of the planetary wave evolution are observed with the anomalous monsoon activity associated with the EU-like pattern. In addition, the anomalous midwinter activity of the monsoon is also accompanied by noticeable variability of the seasonal development of the planetary waves over the Euro-Atlantic sector.

Impact of ENSO on the Atmospheric Variability over the North Atlantic in Late Winter—Role of Transient Eddies

Abstract The dynamical mechanism for the late-winter teleconnection between El Niño–Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) is examined using the output from a 2000-yr integration of a coupled general circulation model (GCM). The coupled model captures many salient features of the observed behavior of both ENSO and NAO, as well as their impact on the surface climate in late winter. Both the observational and model data indicate more occurrences of negative phase of NAO in late winter during El Niño events, and positive NAO in La Niña episodes. The potential role of high-frequency transient eddies in the above teleconnection is diagnosed. During El Niño winters, the intensified transient disturbances along the equatorward-shifted North Pacific storm track extend their influences farther downstream. The eddy-induced negative height tendencies are found to be more coherent and stronger over North Atlantic than that over North Pacific. These negative height tendencies over the North Atlantic are coincident with the southern lobe of NAO, and thus favor more occurrences of negative NAO events. During those El Niño winters with relatively strong SST warming in eastern equatorial Pacific, the eastward extension of eddy activity is reinforced by the enhanced near-surface baroclinicity over the subtropical eastern Pacific. This flow environment supports a stronger linkage between the Pacific and Atlantic storm tracks, and is more conducive to a negative NAO phase. These model results are supported by a parallel analysis of various observational datasets. It is further demonstrated that these transient eddy effects can be reproduced in atmospheric GCM integrations subjected to ENSO-related SST forcing in the tropical Pacific.

Method for efficient prevention of gravity wave decoupling on rectangular semi-staggered grids

Generation of short gravity wave noise often occurs on semi-staggered rectangular grids as a result of sub-grid decoupling when there is a strong forcing in the mass field. In this study a numerical scheme has been proposed to prevent the generation of the gravity wave decoupling. The proposed numerical method provides efficient communication between decoupled gravity wave finite-difference solutions by a simple averaging of a term in the height tendency in the continuity equation. The proposed method is tested using a shallow water sink model developed for the purpose of this study. It has been demonstrated that this method outperforms other existing approaches. The new scheme is time efficient, based on explicit time integration and can be easily implemented. The proposed method is applicable in hydrodynamic models specified on semi-staggered B or E grids.

Stratospheric Influences on Tropospheric Weather Systems

Abstract Four cases of tropospheric weather systems (two sea level cyclones, one sea level anticyclone, and one blocking midtropospheric anticyclone) are investigated with the goal of understanding the role of stratospheric versus tropospheric processes in their developments. The relative contributions of the stratosphere and troposphere to geopotential height tendency fields (1000 mb for the sea level systems, 500 mb for the midtropospheric system) are quantified through vertical integration of thermodynamic processes (advective, adiabatic, and diabatic) over and following tendency centers associated with these systems. Previously known or suspected tropospheric contributions to system development, as well as the influence of stratospheric warm-air advection in the sea level cyclogenesis cases, are confirmed by the diagnoses. New findings include identification of the influence of stratospheric adiabatic cooling in the sea level and midtropospheric anticyclogenesis cases. It is further found that the stratospheric contribution to tropospheric development can be larger in magnitude than the tropospheric contribution and can even overwhelm an opposing tropospheric effect. In particular, it is shown that the midtropospheric anticyclogenesis associated with the blocking case critically depended on the stratospheric contribution and could not be solely explained by tropospheric processes. The 500-mb height tendencies associated with quasigeostrophic potential vorticity (QGPV) changes above the 500-mb level in this case were twice as large those associated with QGPV changes at and below 500 mb during blocking onset.

Piecewise Potential Vorticity Inversion and Vortex Interaction

Abstract Piecewise potential vorticity inversion (PPVI) aims at an interpretation of the interaction of potential vorticity (PV) anomalies q. The total flow domain D is divided into subdomains D1 and D2 with q1 and q2 as PV anomalies. PPVI estimates the height tendency in D that results if q is transported by the winds obtained by an inversion of a PV field with q = q1 in D1 and q = 0 in D2. Tests compare the actual height tendency to that obtained via PPVI. This method is applied to the interaction of vortices in quasigeostrophic shallow water, a problem generally accepted as a paradigm for PPVI. Indeed, results are excellent for vortex pairs. Combinations of more vortices, including vortex clusters akin to Rossby waves, are also exposed to tests. The utility of PPVI tendencies depends strongly on the arrangement of the vortices in that case. Very good agreement is found as well as cases with quite low skill. The reasons for this scatter are discussed and a dynamic interpretation of PPVI is proposed.