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Data assimilation schemes blend observational data, with limited coverage, with a short term forecast to produce an analysis, which is meant to be the best estimate of the current state of the atmosphere. Appropriately specifying observation error statistics is necessary to obtain an optimal analysis. Observation error can originate from instrument error as well as the error of representation. While representation error is most commonly associated with unresolved scales and processes, this term is often considered to include contributions from pre-processing or quality control and errors associated with the observation operator. With a focus on practical operational implementation, this chapter aims to define the components of observation error, discusses their sources and characteristics, and provides an overview of current methods for estimating observation error statistics. We highlight the implicit assumptions of these methods, as well as their shortcomings. We will detail current operational practice for diagnosing observation error and accounting for correlated observation error. Finally, we provide a practical methodology for using these diagnostics, as well as the associated innovation-based observation impact, to optimize the assimilation of meteor radar observations in the upper atmosphere.

A methodology is presented for objectively optimizing nonorographic gravity wave source parameters to minimize forecast error for target regions and forecast lead times. In this study, we employ a high-altitude version of the Navy Global Environmental Model (NAVGEM-HA) to ascertain the forcing needed to minimize hindcast errors in the equatorial lower stratospheric zonal-mean zonal winds in order to improve forecasts of the quasi-biennial oscillation (QBO) over seasonal time scales. Because subgrid-scale wave effects play a large role in driving the QBO, this method leverages the nonorographic gravity wave drag (GWD) parameterization scheme to provide the necessary forcing. To better constrain the GWD source parameters, we utilize ensembles of NAVGEM-HA hindcasts over the 2014–16 period with perturbed source parameters and develop a cost function to minimize errors in the equatorial lower stratosphere compared to analysis. Thus, we may determine the set of GWD source parameters that yields a forecast state that most closely agrees with observed QBO winds over each optimization time interval. Results show that the source momentum flux and phase speed spectrum width are the most important parameters. The seasonal evolution of optimal parameter value, specifically a robust semiannual periodicity in the source strength, is also revealed. Changes in optimal source parameters with increasing forecast lead time are seen, as the GWD parameterization takes on a more active role as QBO driver at longer forecast lengths. Implementation of a semiannually varying source function at the equator provides RMS error improvement in QBO winds over the default constant value.

Ongoing climate change in Earth’s middle and upper atmosphere will affect the rapidly expanding space and telecommunications sectors. Maintaining observations of this region is more crucial than ever.

Solar thermal (migrating) atmospheric tides play an important role in shaping the day-to-day and seasonal variability of the Mesosphere-Lower-Thermosphere (MLT) region. Due the planetary scale of the migrating tides, observations have, however, remained sparse. This study uses meteor-echo wind measurements from a longitudinal array of SuperDARN HF-radars to isolate the amplitude and phase of the migrating diurnal, semidiurnal, and terdiurnal tide. The array of SuperDARN radars, covering nearly 180 degrees longitude at 60±5 degrees North, provide hourly horizontal wind measurements at approximately 95km altitude. The migrating components of the tides are isolated by fitting wave surfaces in space and time. The results are compared with global synoptic wind analyses from the high-altitude version of the Navy Global Environmental Model (NAVGEM-HA) to validate the method. The tides are also compared against those measured at a single station by the Trondheim (66N, 10E) meteor radar. We will present the method, a comparison between (migrating) tidal components in SuperDARN, NAVGEM-HA and the Trondheim meteor radar between 2014 and 2015, and migrating tide climatologies based on 21 years of SuperDARN data.

The lower atmosphere contributes significantly to the day-to-day variability of the ionosphere, especially during solar minimum conditions. Ionosphere/atmosphere model simulations that incorporate meteorology from data assimilation analysis products can be critically important for elucidating the physical processes that have substantial impact on ionospheric weather. In this study, the NCAR Whole Atmosphere Community Climate Model, extended version with specified dynamics (SD-WACCM-X) is coupled with an ionospheric model (Sami3 is Another Model of the Ionosphere) to study day-to-day variability in the ionosphere during January 2010. Lower atmospheric weather patterns are introduced into the SAMI3/SD-WACCM-X simulations using the 6-h Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude (NOGAPS-ALPHA) data assimilation products. The same time period is simulated using the new atmospheric forecast model, the High Altitude Navy Global Environmental Model (HA-NAVGEM), a hybrid 4D-Var prototype data assimilation with the ability to produce meteorological fields at a 3-h cadence. Our study shows that forcing SD-WACCM-X with HA-NAVGEM better resolves the semidiurnal tides and introduces more day-to-day variability into the ionosphere than forcing with NOGAPS-ALPHA. The SAMI3/SD-WACCM-X/HA-NAVGEM simulation also more accurately captures the longitudinal variability associated with non-migrating tides in the equatorial ionization anomaly (EIA) region as compared to total electron content (TEC) maps derived from GPS data. Both the TEC maps and the SAMI3/SD-WACCM-X/HA-NAVGEM simulation show an enhancement in TEC over South America during 17–21 January 2010, which coincides with the commencement of a stratospheric warming event on 19 January 2010. Analysis of the SAMI3/SD-WACCM-X/HA-NAVGEM simulations indicates non-migrating tides (including DW4, DE2 and SW5) played a role during 17–21 January in shifting the phase of the wave-3 pattern in the ionosphere on these days. Constructive interference of wave-3 and wave-4 patterns in the E × B drifts contributed to the enhanced TEC in the South American longitude sector. The results of the study highlight the importance of high fidelity meteorology in understanding the day-to-day variability of the ionosphere.

There is currently an ambiguity in what controls polar mesospheric cloud (PMC) periodicities near 83 km altitude. This is primarily because satellite and ground-based data sets cannot resolve global mesospheric temperature variability over the diurnal cycle. To address this limitation, we employ a global meteorological analysis and forecast system that assimilates mesospheric satellite data with two significant advances. The first is that we use output at a more rapid one hourly cadence, allowing for a quantitative description of diurnal (24 h), semidiurnal (12 h), and terdiurnal oscillations. The second is that the output drives a simple PMC parameterization which depends only on the local temperature, pressure, and water vapor concentrations. Our study focuses on results from July 2009 in the Northern Hemisphere and January 2008 in the Southern Hemisphere. We find that the 24 h migrating temperature tide as well as the 12 h and 24 h nonmigrating tides dominate northern PMC oscillations whereas the 12 h and 24 h nonmigrating tides dominate southern oscillations. Monthly averaged amplitudes for each of these components are generally 2–6 K with the larger amplitudes at lower PMC latitudes (50°). The 2 day and 5 day planetary waves also contribute in both hemispheres, with monthly averaged amplitudes from 1 to 3 K although these amplitudes can be as high as 4–6 K on some days. Over length scales of ~1000 km and timescales of ~1 week, we find that local temperature oscillations adequately describe midlatitude PMC observations.

Context. The mirror tracking system of the Canadian Automated Meteor Observatory (CAMO) can track meteors in real time, providing an effective angular resolution of 1 arc second and a temporal resolution of 100 frames per second. Aims. We describe the upgraded hardware and give details of the data calibration and reduction pipeline. We investigate the influence of meteor morphology on radiant and velocity measurement precision, and use direct observations of meteoroid fragmentation to constrain their compressive strengths. Methods. On July 21, 2017, CAMO observed a ~4 second meteor on a JFC orbit. It had a shallow entry angle ~8 deg and 12 fragments were visible in the narrow-field video. The event was manually reduced and the exact moment of fragmentation was determined. The aerodynamic ram pressure at the moment of fragmentation was used as a proxy for compressive strength, and strengths of an additional 19 fragmenting meteoroids were measured in the same way. The uncertainty in the atmosphere mass density was estimated to be +/-25% using NAVGEM-HA data. Results. We find that meteor trajectory accuracy significantly depends on meteor morphology. The CAMO radiant and initial velocity precision for non-fragmenting meteors with short wakes is ~0.5' and 1 m/s, while that for meteors with fragments or long wakes is similar to non-tracking, moderate field of view optical systems (5', ~50 m/s). Measured compressive strengths of 20 fragmenting meteoroids (with less precise radiants due to their morphology) was in the range of 1-4 kPa, which is in excellent accord with Rosetta in-situ measurements of 67P. Fragmentation type and strength do not appear to be dependent on orbit. The mass index of the 12 fragments in the July 21 meteoroid was very high (s = 2.8), indicating possible progressive fragmentation., Comment: Accepted for publication in Icarus

Recent studies have shown that day-to-day variability in thermospheric winds (100–300 km altitude) driven by meteorological variability from below affects ionospheric E and lower F regions, highlighting the need for accurate, continuous specification of day-to-day variability throughout the entire atmosphere for geospace weather prediction systems. To better understand the nature of forcing from below on the coupled thermosphere/ionosphere system, this study uses the Specified Dynamics Whole Atmosphere Community Climate Model eXtended (SD-WACCM-X) to quantify how the meteorology of the underlying atmosphere impacts the thermosphere. For this study, global meteorological specifications are produced by a high-altitude version of the Navy Global Environmental Model (NAVGEM-HA), which assimilates standard meteorological observations from the surface through the lower atmosphere, and satellite-based observations of temperature and constituents in middle atmosphere (MA) region 10–90 km altitude. Two SD-WACCM-X simulations for the January–February 2013 period are performed using NAVGEM-HA specifications produced with and without assimilation of MA observations. Results show that the availability of MA observations strongly constrains the modeled spectrum of planetary scale waves (zonal wavenumbers 1–3) in the thermosphere. Specifically, the amplitudes of the solar non-migrating DE3 tide and westward quasi-two day wave (Q2DW) are nearly twice as large in SD-WACCM-X simulations without MA observations compared to simulations with MA observations. Model diagnostics show that these differences are related to non-linear wave-wave interactions impacting the DE3 mode and to sources of baroclinic/barotropic instability near the summer mesospheric easterly jet impacting the Q2DW. This study highlights the importance of MA observations for constraining whole atmosphere models needed for next-generation space weather prediction capabilities.

Many operational numerical weather prediction (NWP) systems now extend into the stratosphere and are beginning to be used to generate forecasts beyond conventional 5–10-day periods out to seasonal time scales. Past observational and modeling studies have shown that the quasi-biennial oscillation (QBO) in equatorial stratospheric winds can play an important role in stratosphere–troposphere dynamical coupling over these longer time scales. Consequently, stratosphere-resolving NWP models used to generate seasonal forecasts should contain the necessary physics to generate and maintain the QBO. This study describes several key modifications that were necessary to produce a QBO in a high-altitude NWP model, which include an increase in model vertical resolution, implementation of a computationally efficient stochastic gravity wave drag parameterization, and reductions in the amount of horizontal and vertical diffusion in the stratosphere. Results from a 10-yr free-running model simulation with these modifications show that the westerly QBO phase produces lower temperatures and stronger westerly flow in the Northern Hemisphere (NH) winter polar stratosphere compared to the easterly QBO phase. Ensembles of 120-day simulations over the December–March period show that these modifications replace persistent easterly flow in the equatorial lower stratosphere with a more realistic transition from easterly to westerly flow. The resulting changes in planetary wave propagation produce a statistically significant response in the dynamics of the NH extratropical stratosphere consistent with the Holton–Tan relationship. The westerly shift in equatorial winds also produces a significant response in the NH extratropical troposphere, where the sea level pressure differences in winter resemble the positive phase of the northern annular mode.

The descent of the westerly phase of the quasi-biennial oscillation (QBO) in equatorial stratospheric zonal wind was interrupted by the development of easterlies near 40 hPa (~23 km altitude) in early 2016. We use tropical meteorological analyses of wind and temperature to describe in detail the special circumstances by which equatorward-propagating planetary waves produced this unprecedented disruption in the QBO. Our findings show that the subtropical easterly jet in the winter lower stratosphere during the 2015-2016 winter was anomalously weak owing to (1) the timing of the QBO relative to the annual cycle and (2) an extreme El Nino event. The weak jet allowed an unusually large flux of westward momentum to propagate from the extratropical Northern Hemisphere to the equator near the 40 hPa level. Consequently, the QBO westerlies at that level experienced sustained easterly acceleration from extratropical wave breaking, leading to the observed wind reversal.

On February 13, 2013, the US Navy's weather forecast system reached a milestone when the NAVy Global Environmental Model (NAVGEM) replaced the Navy Operational Global Atmospheric Prediction System (NOGAPS) for operational global weather prediction. The new operational system NAVGEM 1.1 combines a semi-Lagrangian/semi-implicit dynamical core together with advanced parameterizations of subgrid-scale moist processes, convection, ozone, and radiation. The NAVGEM dynamical core allows for much higher spatial resolutions without the need for the small time steps that would be necessary in NOGAPS. The increased computational efficiency is expected to enable significant increases in resolution in future NAVGEM releases. Model physics improvements in the NAVGEM 1.1 transition include representations of cloud liquid water, cloud ice water, and ozone as fully predicted constituents. Following successful testing of a new mass flux scheme, a second transition to NAVGEM 1.2 occurred on November 6, 2013. Addition of this mass flux parameterization to the eddy diffusion vertical mixing parameterization resulted in a reduction of the cold temperature bias of the lower troposphere over ocean and further increased the forecast skill of NAVGEM.

This study uses global synoptic meteorological fields from a high-altitude data assimilation system to investigate the spatial and temporal characteristics of the quasi-2 day wave (Q2DW) and migrating diurnal tide during the Northern Hemisphere summers of 2007, 2008, and 2009. By applying a 2-dimensional fast Fourier transform to meridional wind and temperature fields, we are able to identify Q2DW source regions and to diagnose propagation of Q2DW activity into the upper mesosphere and lower thermosphere. We find that Q2DW is comprised primarily of westward propagating zonal wavenumber 3 and wavenumber 4 components that originate from within baroclinically unstable regions along the equatorward flank of the summer midlatitude easterly jet. Amplitude variations of wavenumbers 3 and 4 tend to be anti-correlated throughout the summer, with wavenumber 3 maximizing in July and wavenumber 4 maximizing in late June and early August. Monthly mean Q2DW amplitudes between 30 50N latitude are largest when diurnal tidal amplitudes are smallest and vice versa. However, there is no evidence of any rapid amplification of the Q2DW via nonlinear interaction with the diurnal tide. Instead, variations of Q2DW amplitudes during July are closely linked to variations in the strength and location of the easterly jet core from one summer to the next, with a stronger jet producing larger Q2DW amplitudes. Linear instability model calculations based on the assimilated wind fields find fast growing zonal wavenumber 3 and 4 modes with periods near 2 days in the vicinity of the easterly jet.

We have explored the sensitivity of the thermosphere and ionosphere to dynamical forcing from altitudes near the mesopause (~95 km). We performed five simulations, all for the year 2009, with the National Center for Atmospheric Research (NCAR)/Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). Two simulations were driven with the NCAR Global Scale Wind Model, and three used output from the Advanced Level Physics High Altitude (ALPHA) version of the Navy's Operational Global Atmospheric Prediction System (NOGAPS). Use of NOGAPS-ALPHA allows for realistic meteorological variability from the lower atmosphere to propagate up into the TIEGCM, including a rich spectrum of nonmigrating tides. We find that the additional vertical transport from these tides causes a significant reduction in the calculated peak electron density of the ionospheric F2 layer (NmF2). The mechanism for this effect is the enhanced downward transport of atomic oxygen to the base of the thermosphere. In turn, this yields a greater relative abundance of N2 and hence enhanced recombination of ions and electrons. To get improved agreement with observed electron densities, we must reduce (Kzz) by a factor of 5. However, even with lower Kzz, our calculation still underestimates the NmF2 compared with radio occultation observations by the Constellation Observing System for Meteorology, Ionosphere and Climate satellite system. This underestimate of NmF2 may be linked to an overestimate of the nonmigrating tides in the coupled TIEGCM-NOGAPS calculations or to uncertainties in the bottom boundary for atomic oxygen in the TIEGCM.

High-altitude meteorological analyses are used to study the interannual variability of mesospheric weather in the Southern extratropics over five recent January's (2005, 2006, 2008, 2009, and 2010). Two features are apparent. First, there is significant variability in the quasi 2 day wave (Q2DW) with the largest amplitudes in January 2006 and also the last half of January 2005. Second, these periods coincide with high-latitude temperature enhancements of 8–12 K. Previous studies have linked summer mesospheric warmings to interhemispheric coupling (IHC); however, the observed temperature and zonal wind anomalies do not agree with the predictions of IHC. Rather it appears that the westward momentum forcing from these Q2DW enhancements counteracts the gravity wave drag forcing which normally produces the cold summer mesopause. Since these temperature increases have been linked to Polar Mesospheric Cloud (PMC) disappearance, the present study supports the suggestion that the Q2DW may be an important factor governing PMC variability.

Past investigations have documented large divergent wind anomalies in stratospheric reanalyses over steep terrain, which were attributed to discretization errors produced by the terrain-following (sigma) vertical coordinate in the forecast model. However, forecasting experiments have reported negligible differences in skill between sigma- and hybrid-coordinate models. This leads to the paradoxical conclusion that discretization errors in the forecast model yield significant stratospheric analysis errors, but insignificant stratospheric forecast errors. The authors reexamine this issue by performing two forecast-assimilation experiments that are identical except for the vertical coordinate: one uses a sigma coordinate and the other uses a hybrid coordinate. The sigma-coordinate analyses exhibit large divergent wind anomalies over terrain that extend from the surface to the model top and distort explicitly resolved orographic gravity waves. Above the tropopause, divergent wind errors are suppressed by an order of magnitude or more in the hybrid-coordinate analyses. Over a 3-month period, stratospheric skill scores in the hybrid experiment show statistically significant improvements relative to the sigma experiment. Previous studies, which found no such differences, all used forecasts initialized from a common archived analysis. The results show that the dominant pathway for error growth and net skill impacts is via 0–9-h forecast backgrounds cycling successively through the data assimilation phase without significant observational correction. The skill impacts noted here should further motivate weather and climate models to adopt a hybrid coordinate with the best error suppression characteristics for a given modeling application.

[1] A middle atmosphere general circulation model is used to examine the effects of zonally asymmetric ozone (ZAO) on the Northern Hemisphere planetary waveguide (PWG) during winter (December–February). The morphology of the PWG is measured by a refractive index, Eliassen-Palm flux vectors, the latitude of the subtropical zero wind line, and the latitude of the subtropical jet. ZAO causes the PWG to contract meridionally in the upper stratosphere, expand meridionally in the lower stratosphere, and expand vertically in the upper stratosphere and lower mesosphere. The ZAO-induced changes in the PWG are the result of increased upward and poleward flux of planetary wave activity into the extratropical stratosphere and lower mesosphere. These changes cause an increase in the Eliassen-Palm flux convergence at high latitudes, which produces a warmer and weaker stratospheric polar vortex and an increase in the frequency of stratospheric sudden warmings. The ability of ZAO to alter the flux of planetary wave activity into the polar vortex has important implications for accurately modeling wave-modulated and wave-driven phenomena in the middle atmosphere, including the 11-year solar cycle, stratospheric sudden warmings, and the phase of the Northern Hemisphere annular mode.

A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001 hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed.

This paper describes CHEM2D-H2O, a new parameterization of H2O photochemical production and loss based on the CHEM2D photochemical-transport model of the middle atmosphere. This parameterization accounts for the altitude, latitude, and seasonal variations in the photochemical sources and sinks of water vapor over the pressure region from 100–0.001 hPa (~16–90 km altitude). A series of free-running NOGAPS-ALPHA forecast model simulations offers a preliminary assessment of CHEM2D-H2O performance over the June 2007 period. Results indicate that the CHEM2D-H2O parameterization improves global 10-day forecasts of upper mesospheric water vapor compared to forecasts using an existing one-dimensional (altitude only) parameterization. Most of the improvement is seen at high winter latitudes where the one-dimensional parameterization specifies photolytic H2O loss year round despite the lack of sunlight in winter. The new CHEM2D-H2O parameterization should provide a better representation of the downwelling of dry mesospheric air into the stratospheric polar vortex in operational analyses that do not assimilate middle atmospheric H2O measurements.

The forecast model and three-dimensional variational data assimilation components of the Navy Operational Global Atmospheric Prediction System (NOGAPS) have each been extended into the upper stratosphere and mesosphere to form an Advanced Level Physics High Altitude (ALPHA) version of NOGAPS extending to ~100 km. This NOGAPS-ALPHA NWP prototype is used to assimilate stratospheric and mesospheric temperature data from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments. A 60-day analysis period in January and February 2006, was chosen that includes a well documented stratospheric sudden warming. SABER and MLS temperatures indicate that the SSW caused the polar winter stratopause at ~40 km to disappear, then reform at ~80 km altitude and slowly descend during February. The NOGAPS-ALPHA analysis reproduces this observed stratospheric and mesospheric temperature structure, as well as realistic evolution of zonal winds, residual velocities, and Eliassen-Palm fluxes that aid interpretation of the vertically deep circulation and eddy flux anomalies that developed in response to this wave-breaking event. The observation minus forecast (O-F) standard deviations for MLS and SABER are ~2 K in the mid-stratosphere and increase monotonically to about 6 K in the upper mesosphere. Increasing O-F standard deviations in the mesosphere are expected due to increasing instrument error and increasing geophysical variance at small spatial scales in the forecast model. In the mid/high latitude winter regions, 10-day forecast skill is improved throughout the upper stratosphere and mesosphere when the model is initialized using the high-altitude analysis based on assimilation of both SABER and MLS data.

The innovations or observation minus forecast (O–F) residuals produced by a data assimilation system provide a convenient metric of evaluating global analyses. In this study, O–F statistics from the Global Ozone Assimilation Testing System (GOATS) are used to examine how ozone assimilation products and their associated O–F statistics depend on input data biases and ozone photochemistry parameterizations (OPP). All the GOATS results shown are based on a 6-h forecast and analysis cycle using observations from SBUV/2 (Solar Backscatter UltraViolet instrument-2) during September–October 2002. Results show that zonal mean ozone analyses are more independent of observation biases and drifts when using an OPP, while the mean ozone O–Fs are more sensitive to observation drifts when using an OPP. In addition, SD O–Fs (standard deviations) are reduced in the upper stratosphere when using an OPP due to a reduction of forecast model noise and to increased covariance between the forecast model and the observations. Experiments that changed the OPP reference state to match the observations by using an "adaptive" OPP scheme reduced the mean ozone O–Fs at the expense of zonal mean ozone analyses being more susceptible to data biases and drifts. Additional experiments showed that the upper boundary of the ozone DAS can affect the quality of the ozone analysis and therefore should be placed well above (at least a scale height) the region of interest.

This paper evaluates the performance of various linear ozone photochemistry parametrizations using the stratosphere-troposphere data assimilation system of the Met Office. A set of experiments were run for the period 23 September 2003 to 5 November 2003 using the Cariolle (v1.0 and v2.1), LINOZ and Chem2D-OPP (v0.1 and v2.1) parametrizations. All operational meteorological observations were assimilated, together with ozone retrievals from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). Experiments were validated against independent data from the Halogen Occultation Experiment (HALOE) and ozonesondes. Additionally, a simple offline method for comparing the parametrizations is introduced. It is shown that in the upper stratosphere and mesosphere, outside the polar night, ozone analyses are controlled by the photochemistry parametrizations and not by the assimilated observations. The most important factor in getting good results at these levels is to pay attention to the ozone and temperature climatologies in the parametrizations. There should be no discrepancies between the climatologies and the assimilated observations or the model, but there is also a competing demand that the climatologies be objectively accurate in themselves. Conversely, in the lower stratosphere outside regions of heterogeneous ozone depletion, the ozone analyses are dominated by observational increments and the photochemistry parametrizations have little influence. We investigate a number of known problems in LINOZ and Cariolle v1.0 in more detail than previously, and we find discrepancies in Cariolle v2.1 and Chem2D-OPP v2.1, which are demonstrated to have been removed in the latest available versions (v2.8 and v2.6 respectively). In general, however, all the parametrizations work well through much of the stratosphere, helped by the presence of good quality assimilated MIPAS observations.

The new CHEM2D-Ozone Photochemistry Parameterization (CHEM2D-OPP) for high-altitude numerical weather prediction (NWP) systems and climate models specifies the net ozone photochemical tendency and its sensitivity to changes in ozone mixing ratio, temperature and overhead ozone column based on calculations from the CHEM2D interactive middle atmospheric photochemical transport model. We evaluate CHEM2D-OPP performance using both short-term (6-day) and long-term (1-year) stratospheric ozone simulations with the prototype high-altitude NOGAPS-ALPHA forecast model. An inter-comparison of NOGAPS-ALPHA 6-day ozone hindcasts for 7 February 2005 with ozone photochemistry parameterizations currently used in operational NWP systems shows that CHEM2D-OPP yields the best overall agreement with both individual Aura Microwave Limb Sounder ozone profile measurements and independent hemispheric (10°–90° N) ozone analysis fields. A 1-year free-running NOGAPS-ALPHA simulation using CHEM2D-OPP produces a realistic seasonal cycle in zonal mean ozone throughout the stratosphere. We find that the combination of a model cold temperature bias at high latitudes in winter and a warm bias in the CHEM2D-OPP temperature climatology can degrade the performance of the linearized ozone photochemistry parameterization over seasonal time scales despite the fact that the parameterized temperature dependence is weak in these regions.

Two-dimensional radiance maps from Channel 9 (~60–90 hPa) of the Advanced Microwave Sounding Unit (AMSU-A), acquired over southern Scandinavia on 14 January 2003, show plane-wave-like oscillations with a wavelength λh of ~400–500 km and peak brightness temperature amplitudes of up to 0.9 K. The wave-like pattern is observed in AMSU-A radiances from 8 overpasses of this region by 4 different satellites, revealing a growth in the disturbance amplitude from 00:00 UTC to 12:00 UTC and a change in its horizontal structure between 12:00 UTC and 20:00 UTC. Forecast and hindcast runs for 14 January 2003 using high-resolution global and regional numerical weather prediction (NWP) models generate a lower stratospheric mountain wave over southern Scandinavia with peak 90 hPa temperature amplitudes of ~5–7 K at 12:00 UTC and a similar horizontal wavelength, packet width, phase structure and time evolution to the disturbance observed in AMSU-A radiances. The wave's vertical wavelength is ~12 km. These NWP fields are validated against radiosonde wind and temperature profiles and airborne lidar profiles of temperature and aerosol backscatter ratios acquired from the NASA DC-8 during the second SAGE III Ozone Loss and Validation Experiment (SOLVE II). Both the amplitude and phase of the stratospheric mountain wave in the various NWP fields agree well with localized perturbation features in these suborbital measurements. In particular, we show that this wave formed the type II polar stratospheric clouds measured by the DC-8 lidar. To compare directly with the AMSU-A data, we convert these validated NWP temperature fields into swath-scanned brightness temperatures using three-dimensional Channel 9 weighting functions and the actual AMSU-A scan patterns from each of the 8 overpasses of this region. These NWP-based brightness temperatures contain two-dimensional oscillations due to this resolved stratospheric mountain wave that have an amplitude, wavelength, horizontal structure and time evolution that closely match those observed in the AMSU-A data. These comparisons not only verify gravity wave detection and horizontal imaging capabilities for AMSU-A Channel 9, but provide an absolute validation of the anticipated radiance signals for a given three-dimensional gravity wave, based on the modeling of Eckermann and Wu (2006).

A high-altitude version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) spectral forecast model is used to simulate the unusual September 2002 Southern Hemisphere stratospheric major warming. Designated as NOGAPS-Advanced Level Physics and High Altitude (NOGAPS-ALPHA), this model extends from the surface to 0.005 hPa (∼85 km altitude) and includes modifications to multiple components of the operational NOGAPS system, including a new radiative heating scheme, middle-atmosphere gravity wave drag parameterizations, hybrid vertical coordinate, upper-level meteorological initialization, and radiatively active prognostic ozone with parameterized photochemistry. NOGAPS-ALPHA forecasts (hindcasts) out to 6 days capture the main features of the major warming, such as the zonal mean wind reversal, planetary-scale wave amplification, large upward Eliassen–Palm (EP) fluxes, and splitting of the polar vortex in the middle stratosphere. Forecasts beyond 6 days have reduced upward EP flux in the lower stratosphere, reduced amplitude of zonal wavenumbers 2 and 3, and a middle stratospheric vortex that does not split. Three-dimensional EP-flux diagnostics in the troposphere reveal that the longer forecasts underestimate upward-propagating planetary wave energy emanating from a significant blocking pattern over the South Atlantic that played a large role in forcing the major warming. Forecasts of less than 6 days are initialized with the blocking in place, and therefore are not required to predict the blocking onset. For a more thorough skill assessment, NOGAPS-ALPHA forecasts over 3 weeks during September–October 2002 are compared with operational NOGAPS 5-day forecasts made at the time. NOGAPS-ALPHA forecasts initialized with 2002 operational NOGAPS analyses show a modest improvement in skill over the NOGAPS operational forecasts. An additional, larger improvement is obtained when NOGAPS-ALPHA is initialized with reanalyzed 2002 fields produced with the currently operational (as of October 2003) Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS). Thus the combination of higher model top, better physical parameterizations, and better initial conditions all yield improved forecasting skill over the NOGAPS forecasts issued operationally at the time.

This paper presents three-dimensional prognostic O3 simulations with parameterized gas-phase photochemistry from the new NOGAPS-ALPHA middle atmosphere forecast model. We compare 5-day NOGAPS-ALPHA hindcasts of stratospheric O3 with satellite and DC-8 aircraft measurements for two cases during the SOLVE II campaign: (1) the cold, isolated vortex during 11-16 January 2003; and (2) the rapidly developing stratospheric warming of 17-22 January 2003. In the first case we test three different photochemistry parameterizations. NOGAPS-ALPHA O3 simulations using the NRL-CHEM2D parameterization give the best agreement with SAGE III and POAM III profile measurements. 5-day NOGAPS-ALPHA hindcasts of polar O3 initialized with the NASA GEOS4 analyses produce better agreement with observations than do the operational ECMWF O3 forecasts of case 1. For case 2, both NOGAPS-ALPHA and ECMWF 114-h forecasts of the split vortex structure in lower stratospheric O3 on 21 January 2003 show comparable skill. Updated ECMWF O3 forecasts of this event at hour 42 display marked improvement from the 114-h forecast; corresponding updated 42-hour NOGAPS-ALPHA prognostic O3 fields initialized with the GEOS4 analyses do not improve significantly. When NOGAPS-ALPHA prognostic O3 is initialized with the higher resolution ECMWF O3 analyses, the NOGAPS-ALPHA 42-hour lower stratospheric O3 fields closely match the operational 42-hour ECMWF O3 forecast of the 21 January event. We find that stratospheric O3 forecasts at high latitudes in winter can depend on both model initial conditions and the treatment of photochemistry over periods of 1-5 days. Overall, these results show that the new O3 initialization, photochemistry parameterization, and spectral transport in the NOGAPS-ALPHA NWP model can provide reliable short-range stratospheric O3 forecasts during Arctic winter.

Extreme ozone minima represent localized and temporally brief (several days) reductions in column ozone amounts below some chosen absolute level. Although such minima at middle to high northern latitudes are known to be primarily dynamical in origin, a remaining issue is whether heterogeneous chemical loss processes may also contribute significantly to their formation. A case in point is the record low 165 Dobson units (DU) minimum occurring on November 30, 1999, when temperatures near 30 hPa at the location of the minimum were lower than the threshold for the formation of type I polar stratospheric clouds (PSC). An examination of Polar Ozone and Aerosol Measurement III data for times surrounding the event shows that PSCs were indeed present in the vicinity where the ozone minimum was observed. However, archived data show that a similar extreme minimum of 167 DU with characteristics comparable to those of the November 30, 1999, minimum occurred on October 30, 1985, when no PSCs were present. An ensemble of 71 extreme ozone minima with amplitudes under 215 DU exhibit a nearly linear relationship between ozone minimum deviations from the zonal mean and corresponding 30-hPa temperature deviations. Such a relationship is predicted by analytic transport models which assume that vertical motions (i.e., upwelling) are responsible for the ozone minima. Temperature deviations near 30-hPa were unusually large for both the November 30, 1999, and the October 30, 1985, events, implying unusually rapid upward transport for these events. All 71 minima occur in regions where deviations from the zonal mean of 330 K potential vorticity are negative, implying an additional contribution to their formation by quasi-horizontal transport. The timescale for column ozone reductions during extreme ozone minima events is also determined and found to be at least 20 times more rapid than expected from known chemical loss processes. The data are therefore most consistent with a purely dynamical origin for extreme ozone minima in general and the November 30, 1999, event in particular. As was shown by earlier work, the basic dynamical process involves a combination of isentropic transport of ozone-poor air from the tropical upper troposphere and rapid upwelling over upper tropospheric anticyclonic disturbances resulting from poleward Rossby wave breaking events.

Based on 19 years (1979–1998) of NCEP/NCAR reanalyses data and potential vorticity (PV) area diagnostics, we found that in the southern hemisphere (SH) the polar vortex has lasted about two weeks longer in the 1990s than in the early 1980s and the northern hemisphere (NH) polar vortex has lasted four weeks longer. The SH vortex persisted within the layer (12–22 km) with almost complete ozone loss, but did not persist at higher altitudes where ozone was not depleted. However, the NH vortex persisted in a broader vertical range not limited to the ozone-depletion layer. We show that wave activity has weakened in recent years in the NH, but not in the SH. The springtime Antarctic ozone hole seems to be the main cause for the SH polar vortex persistence, while the cause for the NH vortex persistence involves changes in polar ozone as well as changes in dynamics.

The source of increased water vapor mixing ra- tios over the central and eastern tropical Pacic region during the 1992 El Ni~ no event is examined using measurements of upper tropospheric water vapor provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Re- search Satellite. Horizontal winds on isentropic surfaces are combined with ISCCP cloud information to provide back- trajectory calculations free of high clouds. These calculations show that the water vapor mixing ratio of an air parcel in the cloud-free regions of the eastern Pacic decreases to approxi- mately one-half of its original value within the rst 30 hours after encountering deep convection. This analysis also nds a larger number of air parcels encountering deep convection within 30 hours of observation, and therefore having higher mixing ratios, in March-April 1992 compared to March-April 1994. Hence, increased deep convection over the equatorial central and eastern Pacic in 1992 contributed to the moist- ening of the downstream tropical upper troposphere.

Quasi-stationary planetary waves induce zonal asymmetries in wintertime extratropical total ozone fields with typical magnitudes of 30 to 60 Dobson Units. Year-to-year changes in the zonally asymmetric component of monthly mean total ozone fields are examined using both the Nimbus 7 TOMS record (1979 - 1993) and the combined Nimbus 7, NOAA II, and NOAA 9 SBUV(2) records (1979 - 1996), focusing on northern extratropical latitudes in February and southern extratropical latitudes in October. A simple model based on the linearized ozone mixing ratio continuity equation, in which the dynamics are specified from NCEP reanalysis geopotential height and temperature fields, is used to simulate the interannual variability in the spatial distribution of total ozone. Model estimates of the wave-induced variability in total ozone fields agree well, both qualitatively and quantitatively, with the variability observed in lower-resolution SBUV(2) and higher-resolution TOMS records, with the exception of certain years at high southern latitudes in October. Disagreement during these years is related to the displacement of the springtime ozone minimum from over the pole by enhanced planetary wave activity.

The combined Nimbus 7 SBUV, NOAA 11, and preliminary NOAA 9 SBUV/2 monthly zonal mean total ozone data set, extending from January 1979 to December 1995, is analyzed with a multiple regression statistical model that includes a term to describe the direct effect of volcanic aerosols on total ozone. Outside of polar regions, which are not included in this analysis, the volcanic regression coefficients in the northern hemisphere are negative (consistent with heterogeneous chemical losses of ozone on volcanic sulfate aerosols) and reach their maximum values between 40°N - 60°N latitude. Although inclusion of the aerosol term in the statistical model introduces some additional uncertainty to the derived solar response, statistically significant values of the solar coefficients are found in northern and southern subtropical regions throughout the year, with maximum amplitudes of ∼2.5% at 30° latitude in both hemispheres during winter. We conclude that the 11-year signal in the SBUV-SBUV/2 total ozone record is approximately separable from volcanic aerosol effects.

Ozone “mini-holes” represent regions of low total ozone (

A form of the linearized steady state ozone continuity equation, expressed in terms of perturbations in lower stratospheric temperature and geopotential height, is used to model the spatial distribution (i.e., deviations from the zonal mean) of total ozone in northern hemisphere (NH) winter for the period 1979-1991. Lower stratospheric temperatures from channel 4 of the microwave sounding unit (MSU), geopotential height and temperature analyses provided by the Stratospheric Research Group at the Free University of Berlin (FUB), and climatological values of the zonal mean ozone mixing ratio are used to calculate the ozone mixing ratio perturbation for a given location at three levels in the lower stratosphere: 100, 50, and 30 mbar. These values are then summed together to provide an estimate of the total ozone perturbation at northern extratropical latitudes for the months of January and February. By applying this modeling approach to individual months on a year-to-year basis, much of the observed year-to-year variability in the spatial distribution of total ozone is approximately simulated. A significant decrease of the wave-induced variance in the wintertime total ozone distribution over the 1979-1991 period is indicated, consistent with the observed longitude dependences of total ozone trends in NH winter.

A one-dimensional radiative transfer model with fixed dynamical heating is used to calculate the approximate latitude and seasonal dependences of lower stratospheric temperature changes associated with observed ozone trends. The spatial and temporal distribution of ozone profile trends in the lower stratosphere is estimated from a combination of Nimbus 7 Solar Backscattered Ultraviolet (SBUV) global measurements of the ozone column below 32 mbar for the period 1979-1990 and balloon ozonesonde profile trends at northern middle latitudes. The calculated temperature trends near 100 mbar compare favorably with those recently derived by Randel and Cobb (1994) using data from Channel 4 of the Microwave Sounding Unit (MSU) on the NOAA operational satellites, although a number of quantitative differences are found. An independent analysis reported here of 100 mbar temperatures derived from northern hemisphere radiosonde data at the Free University of Berlin (FUB) supports the validity of the satellite-derived lower stratospheric temperature trends. These results are therefore generally consistent with the hypothesis that observed lower stratospheric cooling trends are predominantly determined by reductions in radiative heating associated with stratospheric ozone depletion.

[1] Previous modeling studies have found significant differences in winter extratropical stratospheric temperatures depending on the presence or absence of zonally asymmetric ozone heating (ZAOH), yet the physical mechanism causing these differences has not been fully explained. The present study describes the effect of ZAOH on the dynamics of the Northern Hemisphere extratropical stratosphere using an ensemble of free-running atmospheric general circulation model simulations over the 1 December - 31 March period. We find that the simulations including ZAOH produce a significantly warmer and weaker stratospheric polar vortex in mid-February due to more frequent major stratospheric sudden warmings compared to the simulations using only zonal mean ozone heating. This is due to regions of enhanced Eliassen-Palm flux convergence found in the region between 40°N–70°N latitude and 10–0.05 hPa. These results are consistent with changes in the propagation of planetary waves in the presence of ZAOH predicted by an ozone-modified refractive index.

A multiple regression statistical model is applied to estimate the altitude, latitude, and seasonal dependences of stratospheric ozone trends using 11.5 years of Nimbus 7 SBUV data for the period November 1978 to June 1990. In the upper stratosphere, the derived trends agree in both latitude dependence and approximate amplitude with published predictions from stratospheric models that consider gas-phase chemical processes together with the observed [approximately]0.1 ppbV per year increase in tropospheric chlorine. The dominant contribution to column ozone trends occurs in the lower stratosphere where significant negative trends are present at latitudes >20[degrees] in both hemispheres. The observed latitude dependence is qualitatively consistent with model predictions that include the effects of heterogeneous chemical ozone losses on lower stratospheric aerosols.

[1] We have studied the mesospheric response to two recent stratospheric warmings by performing short-term forecasts at medium (1.5°) and high (0.5°) spatial resolution under different gravity wave drag (GWD) scenarios. We validated our models with our high-altitude analysis that extends from 0 to 90 km. For the minor warming of January 2008, reduced upper-level orographic GWD weakened the downward residual circulation and cooled the mesosphere. Parameterized nonorographic GWD increased the simulated mesospheric cooling. For the prolonged major warming of 2006, heavily attenuated orographic GWD led to pronounced cooling near 50 km. During the extended phase of this event, an unusually strong westerly polar vortex reformed in the lower mesosphere, which allowed westward propagating nonorographic gravity waves to reach the mesosphere and break, with net westward accelerations of over 50 m s−1 d−1. This, in turn, forced a strong residual circulation, yielding descent velocities over 2 cm s−1 between 65°N and 85°N, consistent with previous reports of enhanced downward transport of trace constituents. The resulting adiabatic heating, as evidenced by the unusually vertically displaced stratopause at 80 km, is likely a direct consequence of this enhanced gravity wave driven descent. High-resolution simulations without parameterized GWD were closer to the analysis than medium-resolution simulations with parameterized orographic GWD only, but still did not fully simulate the mesospheric thermal response. Specifically, the 80 km temperature enhancement was still underestimated in these simulations. This suggests that higher spatial resolution is needed to adequately resolve extratropical gravity wave momentum fluxes.

[1] A variety of spaceborne experiments have observed polar mesospheric clouds (PMC) since the late 20th century. Many of these experiments are on satellites in Sun-synchronous orbits and therefore allow observations only at fixed local times (LT). Temperature oscillations over the diurnal cycle are an important source of PMC variability. In order to quantify long-term natural or anthropogenic changes in PMCs, it is therefore essential to understand their variation over the diurnal cycle. To this end, we employ a prototype global numerical weather prediction system that assimilates satellite temperature and water vapor observations from the ground to ∼90 km altitude. We assemble the resulting 6 hourly high-altitude meteorological assimilation fields from June 2007 in both LT and latitude and use them to drive a one-dimensional PMC formation model with cosmic smoke serving as nucleation sites. We find that there is a migrating diurnal temperature tide at 69°N with a variation of ±4 K at 83 km, which controls the variation of PMC total ice water content (IWC) over the diurnal cycle. The calculated IWC is normalized to observations at 2300 LT by the Solar Occultation for Ice Experiment and allowed to vary with temperature over the diurnal cycle. We find that the IWC at 69°N has a single maximum between 0700 and 0800 LT and a minimum between 1900 and 2200 LT and varies by at least a factor of 5. The calculated variation of IWC with LT is substantially larger at 57°N, with a single prominent peak near 0500 LT.

[1] We have investigated the 5 day wave in both temperature and water vapor in the stratosphere and mesosphere as seen in the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics High Altitude (NOGAPS-ALPHA) analysis fields for summer 2007. We have compared these fields and the derived saturation ratios with polar mesospheric cloud (PMC) measurements from the AIM satellite. We find that the 5 day wave is variable in both time and space, with significant amplitudes in the temperature wave in August (up to ∼6 K). By contrast, the 5 day wave–induced water vapor anomalies remain at a near-constant level throughout the season. During August, the 5 day wave in the NOGAPS-ALPHA saturation ratio and in the occurrence of clouds in the AIM data shows a clear anticorrelation with bright PMCs forming in the trough of the temperature wave. The analysis shows that the August enhancement in the 5 day wave amplitude acts to extend the PMC season past the time when zonal mean temperatures are saturated with respect to ice. The increased wave amplitude in August is attributed to in situ wave generation and amplification due to baroclinic instability of mean winds at around 0.1–0.01 hPa. The late-season extension of cloud occurrence due to the 5 day wave may explain previous ground-based reports of bright noctilucent cloud displays in August.

[1] We present a case study of the non-linear interaction between the quasi-two day wave (Q2DW) and the migrating diurnal tide based on global synoptic meridional wind fields for January 2006 and January 2008 from a high-altitude data assimilation/forecast system. We find large quasi-two day wave amplitudes, small diurnal tide amplitudes, and phase locking of the Q2DW with the diurnal cycle during January 2006. In January 2008 the amplitudes of the Q2DW were much smaller, with no evidence of phase locking, while the tidal amplitudes were larger than in the 2006 case. Space-time spectral analysis reveals an enhancement in a diurnal zonal wavenumber 6 feature in the January 2006 case, which can be attributed to a non-linear interaction between the Q2DW and migrating diurnal tide. The relatively strong summer easterly jet in the extratropical upper mesosphere during early January 2006 appears to have created conditions favoring this interaction.

This work was supported in part by grants from the Office of Naval Research and from the National Aeronautics and Space Administration under grants NNX06AC06G (L. Hood, P.I.) and NNH08AI67 (J. McCormack, P.I.) issued through the Living With a Star research TR&T program. NOGAPS‐ALPHA simulations were made possible by a grant of computer time from the DoD High Performance Computing Modernization Program at the U.S. Air Force Research Laboratory.

[1] Water vapor and methane mixing ratios measured by the Halogen Occultation Experiment (HALOE), the Atmospheric Chemistry Experiment (ACE), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) along with simulations from the NRL CHEM2D middle atmosphere model are used to study the hydrogen budget in the equatorial upper stratosphere. Multiyear time series of equatorial upper stratospheric H2O + 2*CH4 show temporal variations during periods of relative long-term stability in water vapor and methane entering the stratosphere. These variations, which are anticorrelated to CH4, are quasibiennial and seasonal in nature, and peak near 2 hPa with a magnitude of 3% of the H2O + 2*CH4 mixing ratio. We find that the ratio of the changes in water vapor to the changes in methane is consistently >2 over the entire equatorial upper stratosphere in HALOE, ACE, and MIPAS data. Ratios of the H2O to CH4 changes calculated from the CHEM2D model are similar, but slightly smaller, and the variations in H2O + 2*CH4 are balanced by nearly equivalent variations in molecular hydrogen. We use this relationship to infer molecular hydrogen mixing ratios from the observations which show that its mixing ratio decreases with altitude above 5 hPa. This net loss in molecular hydrogen drives additional water vapor production so that multiyear average profiles of H2O + 2*CH4 from HALOE, ACE, and MIPAS show an ∼0.4 ppmv increase between 5 hPa and 1.5 hPa, so that total hydrogen is conserved. Collectively, these results illustrate the importance of molecular hydrogen in the equatorial upper stratospheric hydrogen budget.

A multiple regression statistical model is applied to estimate the latitude and seasonal dependences of the solar cycle, quasi-biennial oscillation (QBO), and anthropogenic trend components of stratospheric total ozone change using 13.2 years of Nimbus 7 TOMS data. The characteristics of the linear trend component are in agreement with earlier studies. The QBO regression coefficient is significantly different from zero at high southern latitudes in the Austral spring supporting earlier evidence that the Antarctic ozone depletion is modulated by the QBO. The existence of a solar cycle component is indicated by empirical studies of model residuals and by the approximate agreement of the derived global mean solar coefficient amplitude with photochemical calculations. Initial estimates for the latitude dependence of the solar coefficient suggest higher amplitudes with increasing latitude, especially in the Southern Hemisphere in spring. The statistical model predicts a return to more rapid ozone depletions during the next 4 years as solar minimum is approached.

[1] We examine the evolution of the quasi 2-day wave in the middle atmosphere during the period from 5 January to 5 February 2006 using global synoptic meteorological fields from the high-altitude Navy Operational Global Atmospheric Prediction System Advanced Level Physics, High Altitude (NOGAPS-ALPHA) forecast-assimilation system. This period is characterized by a high level of planetary wave activity in the Northern Hemisphere (winter) extratropical stratosphere prior to a sudden stratospheric warming (SSW) on 20 January 2006. Space-time spectral analysis of 6-hourly NOGAPS-ALPHA fields finds the largest quasi 2-day wave amplitudes in the Southern Hemisphere (summer) extratropical upper mesosphere. Eliassen-Palm flux diagnostics indicate that this extratropical quasi 2-day wave is related to baroclinic instability along the equatorward flank of the summer easterly jet. The quasi 2-day wave is also evident in NOGAPS-ALPHA water vapor fields near the tropical stratopause and is related to barotropic instability. We find that the strong planetary wave activity leading up to the SSW produced an enhanced northward component of the residual meridional circulation that influenced the background zonal winds and, by extension, the quasi 2-day wave forcing in both the tropical and extratropical regions. In the tropical region, the combination of enhanced horizontal momentum advection by the residual meridional circulation and inertially unstable circulations related to planetary wave breaking in the subtropics produced conditions favoring barotropic instability. In the extratropical region, the enhanced residual meridional circulation altered the zonal wind tendency through increased Coriolis torque.

The atmosphere s response to the total solar eclipse of 4 December 2002 is studied using a prototype high-altitude global numerical weather prediction model (NOGAPS-ALPHA). Local reductions in solar ultraviolet (UV) radiation during the eclipse are estimated using astronomical calculations of umbral and penumbral surface trajectories and observed solar limb darkening at ~ 200-300 nm. In NOGAPS-ALPHA these UV eclipse shadows yield stratospheric radiative cooling rate footprints peaking near 27 K day 1, a value 2 3 times larger than assumed in previous modeling. Difference fields between NOGAPS-ALPHA runs with and without this eclipse forcing reveal vertically deep middle atmospheric responses, with three-dimensional horizontal structures very similar to the large-scale bow-wave response first proposed by Chimonas (1970). Such structure appears clearly only at later times when total eclipses have abated and gravity waves generated in the stratosphere have had time to propagate vertically. Bow-wave amplitudes and direct thermal cooling responses are both small (]1 K for temperature and ]2 3 m s 1 for horizontal winds), contradicting some rocketsonde measurements that suggest much larger responses near 50 60 km altitude. We also find clear evidence of a bow-wave-like response in the model s surface pressure fields, with an amplitude 0.1 0.5 hPa, while surface air temperatures in NOGAPS-ALPHA show 4 K cooling over Africa during the eclipse. Both findings are consistent with surface atmospheric data acquired during previous eclipse passages.