Isentropic Surfaces Research Articles

The coupling between radiation and dynamics in the stratosphere

A numerical simulation of the annual cycle of the middle atmosphere is used to diagnose the coupling between radiation and dynamics in the stratosphere with particular attention to Canadian warmings and major warmings. The diagnosis is based on an adaptation of Kelvin's circulation theorem, which yields an expression for the time derivative of a circulation integral taken around isopleths of potential vorticity on an isentropic surface. Departures from radiative equilibrium produced by persistent planetary-scale disturbances in late winter lead to systematic changes in circulation integrals, which reflect systematic changes in the average distribution of potential vorticity. The main finding is that radiation is a prime contributor to the changes in circulation integrals that are characteristic of the formation in late winter of a main vortex/surf-zone structure.

A Study of Two Cases of Comma-Cloud Cyclogenesis Using a Semigeostrophic Model

Abstract The linear stability of two atmospheric flows is examined, with basic-state data taken from environments where comma clouds were observed to form. The basic states each feature a baroclinic zone associated with an upper-level jet, with conditional instability on the north side. The semigeostrophic approximation is used, along with a simple parameterization for cumulus heating, and the eigenvalue problem is solved using a Chebyshev spectral method. The instabilities can be regarded as resulting from the interaction of potential-vorticity anomalies from three sources: advection along isentropic surfaces of the basic-state potential-vorticity (PV) gradient. advection of the surface potential temperature gradient (which is equivalent to a PV gradient in a thin layer at the surface), and potential vorticity generated by release of latent heat. It is found that for the comma clouds the instability is a baroclinic interaction of upper- and lower-level PV anomalies. However, heating is an important sourc...

Toward the four dimensional assimilation of stratospheric chemical constituents

A model to assimilate stratospheric chemical constituents is proposed. The chemical transport model used is two dimensional on an isentropic surface and solves the photochemical equations using an extended family approach. Thirty‐day integrations are performed at the three isentropic surfaces given by the potential temperature values 600 K, 850 K and 1400 K starting on January 1, 1979. During the integrations, Limb Infrared Monitor of the Stratosphere (LIMS) satellite data are assimilated into the model and the results are compared with independently mapped (map archive tape (MAT)) data. The assimilation results are found to be in good agreement with the independent analyses at all three levels. Preliminary assimilations revealed significant biases in modeled HNO3 at 850 K and in O3 at 1400 K. However, by changing the adjustable parameters of the assimilation model the HNO3 bias was successfully eliminated. The O3 bias was easily eliminated by the addition of an extra source term. The comparisons reveal possible minor weaknesses in the MAT analyses. First, the chemical constituents appear to be occasionally in error in the MAT data during rapidly changing dynamical events such as stratospheric warmings. Second, the temporal continuity is sometimes lacking in the MAT analyses, leading to rapidly changing concentrations which cannot be understood on the basis of conventionally understood photochemistry. On the other hand, the assimilation model used here does not produce very satisfactory analyses when the chemical data are very noisy. The differences between the assimilated and MAT analyses are quite small and are generally less than data errors. Although the assimilation model is here applied to LIMS data, the method is generally applicable to any data source and could be especially useful for analyzing the large volume of data from the Upper Atmosphere Research Satellite.

Potential vorticity index vacillation in the 1978/79 winter: Its relation to teleconnection patterns

A potential vorticity (PV) index is defined as a measure of the zonally averaged, raid-latitude PV gradient on the 300 K isentropic surface in the northern hemisphere, and use is made of the 1978/79 wińter ECMWF FGGE Hl-b data set to study the evolution of that index and its relation to teleconnection patterns of 500 mb geopotential-height anomaly. This index vacillates with a dominant period of about two weeks. For some time lags, the isoline patterns of the cross-correlation function between the PV index and 500mb geopotential-height anomalies are similar to some teleconnection patterns studied by several investigators using large climatological data sets. Scandinavia, Greenland and the Pacific are important centres of ‘action’ in the northern hemisphere in the 1978/79 winter. the formation of blocking highs at these locations occurs at rather specific phases during the vacillatory evolution of the PV index, especially as the flow transits from a low to a high PV index. When the above study is duplicated using a zonal mean wind gradient at 500mb as a zonal index, the results do not reflect the teleconnection patterns as crisply as the PV index does for the FGGE year.

Potential Vorticity Index Vacillation In the 1978/79 Winter: Its Relation to Teleconnection Patterns

AbstractA potential vorticity (PV) index is defined as a measure of the zonally averaged, raid‐latitude PV gradient on the 300 K isentropic surface in the northern hemisphere, and use is made of the 1978/79 wińter ECMWF FGGE Hl‐b data set to study the evolution of that index and its relation to teleconnection patterns of 500 mb geopotential‐height anomaly. This index vacillates with a dominant period of about two weeks. For some time lags, the isoline patterns of the cross‐correlation function between the PV index and 500mb geopotential‐height anomalies are similar to some teleconnection patterns studied by several investigators using large climatological data sets. Scandinavia, Greenland and the Pacific are important centres of ‘action’ in the northern hemisphere in the 1978/79 winter. the formation of blocking highs at these locations occurs at rather specific phases during the vacillatory evolution of the PV index, especially as the flow transits from a low to a high PV index.When the above study is duplicated using a zonal mean wind gradient at 500mb as a zonal index, the results do not reflect the teleconnection patterns as crisply as the PV index does for the FGGE year.

Surface Frontogenesis in Isentropic Coordinates

The semigeostrophic equations take a particularly simple form when isentropic and geostrophic coordinates are used simultaneously: the horizontal ageostrophic velocities are entirely implicit, and the entire dynamics reduces to a predictive equation for the potential pseudodensity (inverse Ertel potential vorticity) and an invertibility relation. However, a perceived disadvantage of isentropic coordinates is the difficulty of treating a lower boundary that is not an isentropic surface. Here we present the massless layer method, which allows isentropic surfaces to intersect the lowerboundary, and show that this extends the applicability of potential vorticity modeling in isentropic/geostrophic coordinates. When applied to the classic problem of surface frontogenesis by a vertically independent deformation field, the model produces realistic fronts with a surface discontinuity in finite time and tropopause folding, without the need for special treatment of the lower boundary.

Irreversible transport in the stratosphere by internal waves of short vertical wavelength

The U‐2 aircraft was instrumented and flown in the stratosphere during the Stratosphere‐Troposphere Exchange Project's experiments of April 1984 to provide a set of simultaneous measurements by fast responding sensors that would aid in the identification of the modes of cross‐jet transport. The measurements confirm the preexperimental deductions that transport is dominated by waves, not by large‐scale circulations. Monotonic gradients of trace constituents normal to the jet axis, with upper stratospheric tracers increasing poleward and tropospheric tracers increasing equatorward, are augmented by large‐scale confluence as the jet intensifies during cyclogenesis. These gradients are rotated, intensified, and significantly increased in area as their mixing ratio surfaces are folded by the differential transport of a very low frequency, transverse wave. The quasi‐horizontal transport produces a laminar structure with stable layers rich in upper stratospheric tracers alternating vertically with less stable layers rich in tropospheric tracers. The transport proceeds toward irreversibility as higher frequency, shear‐gravity waves extend the folding to smaller horizontal scales. It becomes irreversible when these short waves actually fold the isentropic surfaces and small‐scale mixing develops. The progression to higher wave numbers is a discrete, not a continuous, cascade with major gaps in the observed horizontal wavelengths. The wave modes are identified by matching the observed amplitudes and phases against those obtained by linear perturbation theory. Prior to mixing, the wave‐generated perturbations maintain the correlations produced by advecting the larger‐scale mean gradients; thus the high resolution measurements support the linear turbulence closure assumption.

On the explicit versus family solution of the fully diurnal photochemical equations of the stratosphere

A hierarchy of models is developed to test the accuracy of the family solution of the photochemical equations of the stratosphere. Results of the explicit solution of the equations for a simplified mid‐winter disturbance near the 850 K isentropic surface are compared with the solutions for models containing 26, 20, 16, 14, 12, 10, and 8 families for a 12‐day integration. In general, it is found that the family approach can yield quite accurate solutions, providing enough families are retained. However, large errors, of order 20%, are produced in some radical concentrations such as OH for models with a small number of families. The minimum number of families consistent with economy in the solution of the equations while maintaining scientific accuracy is discussed. In particular, it is recommended that exact dynamical tracers be incorporated into family models to reduce computer memory requirements by parameterizing long time scale species such as H2O, CH4 and H2 in terms of the tracer. Other species such as N2O, total chlorine, and total nitrogen may also be so parameterized according to how the model is to be used. Even for models with an extensive set of families, the concentrations of radical species are significantly in error at dawn, suggesting the need for caution in comparisons with measurements made using occultation techniques. This may not lead to significant errors in the calculation of reservoir species such as O3 on time scales up to a few weeks but can seriously perturb the qualitative features of other important molecules such as ClONO2 and ClO, and this could lead to significant errors in the calculation of O3 in the longer term. The results have important implications for the development of three‐dimensional models of stratospheric chemistry. Some examples of the chemical schemes of those models which have been described in the literature are studied to determine their strengths and weaknesses. The schemes are shown to produce good results for species such as O3 and HNO3 but show signs of weaknesses, particularly in the radical species. Some changes to these schemes are suggested, with a view toward producing an improved three‐dimensional model in the future.

Observations of a Reversal in Long-Term Average Vertical Velocities near the Jet Stream Wind Maximum

Analysis of vertical velocity measurements made for four days each month over the period from 1986 to 1988 by the MU radar in Japan shows a reversal in direction near the peak in the zonal wind profile during the winter months. More specifically, the reversal is noted during periods when the peak horizontal wind speeds 60 m s−1. The vertical velocities associated with the circulation have magnitudes of 10-20 cm s−1, and the depth of the circulation is of the order of several kilometers. In 6 out of 14 cases when the feature was observed, the direction of the vertical circulation, although not the magnitude, could be explained by adiabatic ascent or subsidence along the average potential temperature surface slopes for the observation intervals. The direction of the circulation was such that it would tend to produce cooling and heating for the ascent and subsidence, respectively, that would tend to strengthen or at least maintain the jet. In the remaining eight cases, the direction of the vertical circulation could not be explained by the slope of the time-averaged potential temperature surfaces alone since the combination of the horizontal winds and the slopes of the isentropic surfaces would have led to a prediction of a circulation directly opposed to that observed. Thus, either the local tendency in the time-averaged potential temperature must have been significant, structure with scales smaller than the rawinsonde station separation must have been present, or diabatic effects may have played a role in the dynamics of the vertical velocity feature.

Initiation and Evolution of an Intense Upper-Level Front

Abstract Within confluent northwesterly flow of an intensifying baroclinic wave over North America in late October 1963, an intense frontal zone developed in 12 h near the inflection point in the middle and upper troposphere. By 24 h after its initial appearance, the zone extended roughly from 400 to 700 mb and from the inflection point to just beyond the downstream trough. Horizontal gradients of potential temperature reached 15–20 K (100 km)−1. Air within the frontal zone was extremely dry. As the accompanying trough approached the east coast of he United States, surface frontogenesis occurred offshore, remaining distinct from the upper-level front. A region of subsidence, elongated in the direction of the upper-level flow, displayed maximum descent on the warm edge of the frontal zone and played a frontogenetical role through tilting of the isentropic surfaces. Analysis of isentropic potential vorticity showed significant increase of this quantity near the cold base and a probable decrease near the top...

On the “Downward Control” of Extratropical Diabatic Circulations by Eddy-Induced Mean Zonal Forces

Abstract The situation considered is that of a zonally symmetric model of the middle atmosphere subject to a given quasi-steady zonal force F, conceived to be the result of irreversible angular momentum transfer due to the upward propagation and breaking of Rossby and gravity waves together with any other dissipative eddy effects that may be relevant. The model's diabatic heating is assumed to have the qualitative character of a relaxation toward some radiatively determined temperature field. To the extent that the force F may be regarded as given, and the extratropical angular momentum distribution is realistic, the extratropical diabatic mass flow across a given isentropic surface may be regarded as controlled exclusively by the F distribution above that surface (implying control by the eddy dissipation above that surface and not, for instance, by the frequency of tropopause folding below). This “downward control” principle expresses a critical part of the dynamical chain of cause and effect governin...

The Papal Front of 3 May 1987: A remarkable example of frontogenesis near the Alps

AbstractAll available, routinely collected surface data are used to document the development of the ‘Papal Front’, which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall‐line characteristics, began when a distinct mid‐troposopheric anomaly of potential vorticity reached the north‐western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic‐scale forcing and the Alpine orography for the generation of the ‘Papal Front’. A westerly ‘orographic jet’ over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available.A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case‐studies. It is concluded that a series of ‘routine‐data case‐studies’ could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during special observation programmes.

Towards a PV-<i>θ</i> view of the general circulation

In recent years, there has been a resurgence of interest in using isentropic coordinates and Rossby-Ertel potential vorticity (PV) for diagnosing the behaviour of middle latitude synoptic systems. Such a PV-θ analysis may also prove important in providing insight into the global circulation of the atmosphere. Apart from the isentropic diagnostic of D. Johnson and collaborators, some quasi-geostrophic studies and recent studies of stratospheric behaviour, there has been little work in this area and our present understanding is very limited. The object of the present paper is to stimulate such studies by presenting some initial results from continuing research. A three-fold division of the atmosphere is discussed. The “Overworld” is the region encompassed by isentropic surfaces that are everywhere above the tropopause. In the “Middleworld”, the region with isentropes crossing the tropopause but not striking the Earth’s surface, the isentropic zonal and time mean of PV exhibits interesting regions of enhanced and diminished gradients. The isentropic transient eddy advection of PV exhibits a dipolar distribution about the tropopause, suggestive of PV mixing. The marked PV signature of the Asian summer monsoon on one particular Middleworld isentrope is shown and the mean isentropic advection of PV shows interesting features. For the “Underworld”, in which isentropic surfaces intercept the surface of the Earth, a PV-θ analysis yields a novel constraint linking low-level drag and diabatic heating. This constraint links “westerlies” and “cooling”, and “easterlies” and “heating” in some average sense. The result is discussed in terms of the Southern Hemisphere strong surface westerlies and the circulation associated with the Asian summer and winter monsoons.

Towards a PV-θ view of the general circulation

In recent years, there has been a resurgence of interest in using isentropic coordinates and Rossby-Ertel potential vorticity (PV) for diagnosing the behaviour of middle latitude synoptic systems. Such a PV- θ analysis may also prove important in providing insight into the global circulation of the atmosphere. Apart from the isentropic diagnostic of D. Johnson and collaborators, some quasi-geostrophic studies and recent studies of stratospheric behaviour, there has been little work in this area and our present understanding is very limited. The object of the present paper is to stimulate such studies by presenting some initial results from continuing research. A three-fold division of the atmosphere is discussed. The “Overworld” is the region encompassed by isentropic surfaces that are everywhere above the tropopause. In the “Middleworld”, the region with isentropes crossing the tropopause but not striking the Earth's surface, the isentropic zonal and time mean of PV exhibits interesting regions of enhanced and diminished gradients. The isentropic transient eddy advection of PV exhibits a dipolar distribution about the tropopause, suggestive of PV mixing. The marked PV signature of the Asian summer monsoon on one particular Middleworld isentrope is shown and the mean isentropic advection of PV shows interesting features. For the “Underworld”, in which isentropic surfaces intercept the surface of the Earth, a PV- θ analysis yields a novel constraint linking lowlevel drag and diabatic heating. This constraint links “westerlies” and “cooling”, and “easterlies” and “heating” in some average sense. The result is discussed in terms of the Southern Hemisphere strong surface westerlies and the circulation associated with the Asian summer and winter monsoons. DOI: 10.1034/j.1600-0870.1991.t01-3-00005.x

The Papal front of 3 May 1987: A remarkable example of frontogenesis near the Alps

All available, routinely collected surface data are used to document the development of the ‘Papal Front’, which crossed southern Germany on 3 May 1987 causing casualities and damage. Objective analyses on isentropic surfaces, initialized with the European aerological data, indicate that the development of the violent mesoscale front, which exhibited squall-line characteristics, began when a distinct mid-troposopheric anomaly of potential vorticity reached the north-western rim of the Alpine arc. The forced uplift ahead of the associated shortwave trough combined with ageostrophic motions due to a jet streak at higher levels apparently provided a trigger for deep convection as was observed over the Alpine foreland. Numerical simulations with a dry hydrostatic model indicate the relative importance of the synoptic-scale forcing and the Alpine orography for the generation of the ‘Papal Front’. A westerly ‘orographic jet’ over the foreland below crest height only develops in the simulation with full Alps and is corroborated by the few mountain wind observations that are available. A comparison with studies of frontal propagation along other mountain ranges reveals that the routine network in the Alpine region and its surroundings is at least of similar density, but so far hardly used for frontal case-studies. It is concluded that a series of ‘routine-data case-studies’ could help to determine the variability of frontal progression along the Alps and, thus, provide guidance when assessing the significance of single events, particularly those sampled during special observation programmes.

The coarse particle aerosols in the free troposphere around Japan

Airborne measurements of solar aureole intensities were carried out around Japan with a spectral scanning radiometer. Horizontal and vertical stratifications and volume spectra of tropospheric aerosols were retrieved from the measured solar aureole intensities. The results show that bimodal volume spectra of aerosols with a saddle point radius around 0.5 μm are generally observed in the troposphere, both within and above the planetary boundary layer. A submicrometer particle mode of the spectra, known as the accumulation mode, dominates the coarse particle mode within the planetary boundary layer, especially in summer, whereas coarse particles were observed frequently in the free troposphere. Synoptic analyses with air mass trajectories, isentropic surfaces, and weather charts suggest that the windblown dust particles injected into the atmosphere in arid regions of eastern Asia are the origin of the coarse particles observed in the free troposphere around Japan. Windblown dust particles are elevated to the free troposphere by ascending air motion in a warm front and then transported by the westerlies. The arid regions to the east of the Tibetan Plateau, where disturbances due to the frontogenesis occur frequently, are capable of supplying coarse particles to the free troposphere over the northwestern Pacific Ocean throughout the year.

On the Conservation and Impermeability Theorems for Potential Vorticity

If the partial analogy between the behavior of Rossby-Ertel potential vorticity (PV) and the behavior of chemical tracers is to be correctly used in the general case of diabatic, frictional motion, then certain fundamental differences, as well as similarities, between the behavior of PV and that of chemical tracers must be recognized. These differences stem from the well-known kinematical relationship between PV and isentropic circulation (via Stokes' theorem), which has no counterpart for chemical substances. One way of stating the analogy while recognizing the differences is to say first that PV behaves like the mixing ratio of a peculiar chemical “substance” that has zero source; i.e., is exactly conserved, away from boundaries (conserved not in the material or Lagrangian sense, but in the general sense associated with the idea of an indestructible chemical substance), and second that isentropic surfaces behave exactly as if they were impermeable to this “PV-substance” or “PVS,” even when diabatic heating or cooling, including that associated with turbulent mixing, makes them permeable to mass and chemical substances. In this respect isentropic surfaces can be said to act like semipermeable membranes. The PV itself can of course change, as can the mixing ratio of an exactly conserved chemical substance or decay-corrected radioactive tracer. For instance, all these mixing ratios can change by dilution when cumulonimbus clouds penetrate isentropic surfaces in a tropopause fold. The net flux or transport of PVS along isentropic surfaces can be either up or down any pre-existing isentropic gradient of PV. For instance the typical effect of the small-scale turbulence due to breaking internal gravity waves is to transport PVS along isentropes in a gradient-independent sense, while transporting chemical substances across isentropes in a downgradient sense. It is the turbulent transport of PVS along isentropes that gives rise to the phenomenon of gravity-wave drag. Such a transport is absent from the formulation given in Danielsen (1990), which supposes that PV always behaves like the mixing ratio of a chemical even in three-dimensionally turbulent flow. The latter supposition is demonstrably incorrect.

A model of stratospheric chemistry and transport on an isentropic surface

First results from a new type of photochemical model are presented. The model has an Eulerian grid with latitude and longitude coordinates on a single isentropic surface, which in this work is specified at a potential temperature of 846.6 K. The photolysis rates in the model are determined using a specified distribution of the total ozone column above the surface. The model contains 107 reactions and 40 species whose concentrations are solved for explicitly with an algorithm that fully accounts for diurnal variations. Species are advected implicitly using a semi‐Lagrangian technique in which 12 hour air particle trajectories are computed and then the particles are reinitialized back onto the Eulerian grid. The photochemical model is integrated for 12 days with winds and temperatures supplied from a three‐dimensional model integration of an idealized wavenumber one disturbance. The results for the long‐lived model tracers such as N2O show excellent correlation with the potential vorticity distribution, providing confidence in the transport scheme. The distribution of the tracers and the night‐time distribution of the reservoir species show the effect of the wave disturbance in enhancing gradients at the edge of the vortex region and in low to mid‐latitudes where a tongue of low potential vorticity air is advected. The effect of the photochemistry is to produce ringed structures surrounding the vortex in reservoir species such as HNO3 and ClONO2. For the radical species, the differences in the steepness of the gradients at the nighttime terminator and the daytime terminator are clearly apparent. Steep high‐latitude gradients in NO2 (the Noxon cliff) are also seen in the results. The results demonstrate the highly complex nature of the photochemistry resulting from the movement in the dynamical vortex and its interaction with the diurnal variation near the terminator. The model provides an interesting way of viewing the full complexity of the photo‐chemistry in the presence of large‐scale advection. Calculations with zonally averaged wind and temperature fields reveal discrepancies in the calculation of the zonal mean of less than 10% for O3 and HNO3 compared with the zonal mean of the previous results. However, the discrepancies for NO2 and N2O5 were much larger, especially in high latitudes. This suggests the need to exercise caution in the interpretation of the results of some species calculated by latitude‐height models.

Simulated transport of NOy to Hawaii during August: A synoptic study

Using data sets generated by the Geophysical Fluid Dynamics Laboratory general circulation/transport model's U.S.‐Canada combustion nitrogen source experiment, a detailed analysis of the simulated transport mechanisms producing the observed August NOy, maximum at Hawaii is presented. Combustion nitrogen is not simply advected from the United States to Hawaii by the winds circulating around the climatological subtropical anticyclone. Rather, its transport results from a complicated three‐way interaction of surface advection from source regions, enhanced vertical diffusion due to dry convection, and winds in the “free troposphere.” Backward trajectories from Hawaii using model pressure and isentropic surfaces were insufficient in explaining the transport. Model‐consistent three‐dimensional trajectories revealed that the transport originated in the “free troposphere” along a path from northern Baja to the Texas Gulf coast. Combustion nitrogen from the source regions of southern California and as distant as the Texas Gulf area is advected along the surface toward the arid areas of Baja, the desert southwest, northern Mexico, and west Texas. Dry convection then vertically mixes the air to pressures of 800–650 mbar, where the subsiding wind flow from the east‐northeast transports the NOy to Hawaii. Observed wind fields and heights of dry convection are compared to the model where data are available.

Determination of upstream bora layer depth

Bora layer depth is defined as a layer of NE bora flow usually capped by an inversion. The inversion height can be determined by the hydrostatic stability parameters calculated from the point values of pressure and temperature data in the Zagreb's sounding. The method is based on an objective polynomial vertical interpolation scheme in isentropic coordinates and includes the computation of energetic changes across the isentropic surface, as presented by a nonhydrostatic coefficient and demonstrated by the application in an ALPEX bora case. Smith's (1985) hydraulic model is applied to several bora cases. The results indicate differences in the solution when the incoming mean wind velocity is replaced by the NE (“bora”) wind component perpendicular to the coastal mountain ridge. This is consistent with the two-dimensional modelling approximation, and stresses the importance of considering a three-dimensional atmospheric structure in the real bora flow.