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William J. Randel and Fei Wu

Abstract

Detailed structure of the global quasi-biennial oscillation (QBO) in ozone is analyzed using Stratospheric Aerosol and Gas Experiment II ozone and nitrogen dioxide data. Emphasis is placed on the midlatitude QBO, in particular its vertical structure and seasonal synchronization. The global QBO signal is isolated using a combination of singular-value decomposition and regression analyses, which combine to act as an accurate QBO digital filter. Results show that the midlatitude ozone QBO has a two-cell structure in the vertical (similar to that at the equator), with in-phase maxima in the lower and middle stratosphere. Both upper- and lower-level anomalies contribute important fractions to the midlatitude column amounts. The lower-level maxima have a broad latitudinal structure (˜15°–60°), and collocation with the strongest background gradients suggests that these anomalies result from mean vertical transport. The, middle stratosphere signal maximizes in the subtropics (10°–40°) and is likely due to nitrogen-related chemical effects (which are in turn due to transport variations). The vertically in-phase seasonal synchronization in midlatitudes is evidence of QBO modulation of the winter hemisphere circulation.

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William J. Randel and Fei Wu

Abstract

Temperature trends derived from historical radiosonde data often show substantial differences compared to satellite measurements. These differences are especially large for stratospheric levels, and for data in the Tropics, where results are based on relatively few stations. Detailed comparisons of one radiosonde dataset with collocated satellite measurements from the Microwave Sounding Unit reveal time series differences that occur as step functions or jumps at many stations. These jumps occur at different times for different stations, suggesting that the differences are primarily related to problems in the radiosonde data, rather than in the satellite record. As a result of these jumps, the radiosondes exhibit systematic cooling biases relative to the satellites. A large number of the radiosonde stations in the Tropics are influenced by these biases, suggesting that cooling in the tropical lower stratosphere is substantially overestimated in these radiosonde data. Comparison of trends from stations with larger and smaller biases suggests the cooling bias extends into the tropical upper troposphere. Significant biases are observed in both daytime and nighttime radiosonde measurements.

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William J. Randel and Fei Wu

Free and Seidel (2007) have considered several questions regarding the radiosonde and satellite temperature trends discussed in Randel and Wu (2006, hereafter RW). The specific points raised are all valid discussion topics, and overall their comment provides useful additional discussion and information on these issues. We agree with the majority of the comments in Free and Seidel (2007), but have specific replies to two points.

Are the discontinuities in the difference series in the stratosphere due to errors in the sonde data or the satellite data?

The authors discuss the fact that

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William J. Randel and Fei Wu

Abstract

Temperature profiles in polar latitudes during summer reveal a strong and persistent inversion layer associated with the polar summer tropopause. This inversion layer is characterized by a temperature increase of ∼8 K in the first 2–3 km above the tropopause and is observed throughout summer polar latitudes in both hemispheres. Radiosonde and GPS radio occultation temperature observations are used to document characteristics of the inversion layer, including its seasonal variability and modulation by synoptic meteorological systems (cyclones and anticyclones). Previous analyses have suggested a radiative mechanism for formation and maintenance of tropopause inversions, related to water vapor and ozone near the tropopause. Fixed dynamical heating (FDH) calculations are used herein to investigate this behavior in polar regions, based on observed seasonally varying profiles of water vapor (from satellite measurements) and ozone (from ozonesondes). Water vapor exhibits a strong seasonal cycle throughout the troposphere and lowest stratosphere, with a pronounced summer maximum, which is primarily a result of the seasonally varying tropospheric temperatures. The FDH calculations suggest that enhanced summer water vapor leads to strong radiative cooling in a narrow layer near the tropopause, so that the radiative influence of water vapor provides a primary mechanism for the summer inversion layer.

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William J. Randel and Fei Wu

Abstract

Long time records of stratospheric temperatures indicate that substantial cooling has occurred during spring over polar regions of both hemispheres. These cooling patterns are coincident with observed recent ozone depletions. Time series of temperature from radiosonde, satellite, and National Centers for Environmental Prediction reanalysis data are analyzed in order to isolate the space–time structure of the observed temperature changes. The Antarctic data show strong cooling (of order 6–10 K) in the lower stratosphere (∼12–21 km) since approximately 1985. The cooling maximizes in spring (October–December), with small but significant changes extending throughout Southern Hemisphere summer. No Antarctic temperature changes are observed during midwinter. Significant warming is found during spring at the uppermost radiosonde data level (30 mb, ∼24 km). These observed temperature changes are all consistent with model predictions of the radiative response to Antarctic polar ozone depletion. Winter and spring temperatures in Northern Hemisphere polar regions also indicate a strong cooling in the 1990s, and the temperature changes are coherent with observed ozone losses. The overall space–time patterns are similar between both hemispheres, suggesting that the radiative response to ozone depletion is an important component of the Arctic cooling as well.

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William J. Randel and Fei Wu

Abstract

Variability in tropical zonal mean temperatures over 10–30 km is analyzed based on high-quality, high-vertical-resolution GPS temperature measurements covering 2001–13. The observations are used to quantify variability spanning time scales of weeks to over a decade, with focus on behavior of the tropopause region and coupling with the upper troposphere and stratosphere. Large variations associated with the seasonal cycle, quasi-biennial oscillation (QBO), and El Niño–Southern Oscillation (ENSO) are isolated and removed, and residual time series are analyzed using principal components and spectrum analysis. The residual temperature exhibits maximum variance in the lower stratosphere, with a vertical structure similar to the seasonal cycle. Residual temperatures exhibit two dominant modes of variability: a “deep stratosphere mode” tied to high-latitude planetary wave forcing and a shallow “near-tropopause mode” linked to dynamically forced upwelling near the tropopause. Variations in the cold point tropopause (and by inference in global stratospheric water vapor) are closely tied to the near-tropopause mode. These coherent temperature patterns provide further evidence of distinct upper and lower branches of the tropical Brewer–Dobson circulation. Zonal mean temperatures in the lower stratosphere and near the cold point are most strongly coupled to the upper troposphere on time scales of ~(30–60) days, probably linked to the Madden–Julian oscillation (MJO). Enhanced temperature variance near the tropopause is consistent with the long radiative relaxation time scales in the lower stratosphere, which makes this region especially sensitive to low-frequency dynamical forcing.

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William J. Randel, Fei Wu, and Piers Forster

Abstract

Global characteristics of the extratropical tropopause inversion layer identified in radiosonde observations by Birner are studied using high vertical resolution temperature profiles from GPS radio occultation measurements. The GPS data are organized according to the height of the thermal tropopause in each profile, and a temperature inversion layer above the tropopause (with an average magnitude of 3–5 K) is found to be a ubiquitous, climatological feature. The GPS data show that the inversion layer is present for all seasons in both hemispheres, spanning the subtropics to the pole, and there is not strong longitudinal structure. Dependence of the inversion layer on upper-troposphere vorticity is studied; while anticyclones exhibit a substantially stronger inversion than cyclones (as expected from balanced dynamics), the inversion is evident for all circulation types. Radiative transfer calculations indicate that strong gradients in both ozone and water vapor near the tropopause contribute to the inversion. Significant absorption of both longwave and shortwave radiation by ozone occurs, warming the region above the tropopause. Water vapor near and immediately above the tropopause contributes to cooling, effectively enhancing the inversion.

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William J. Randel, Rolando Garcia, and Fei Wu

Abstract

The dynamical balances associated with upwelling in the tropical lower stratosphere are investigated based on climatological 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis data. Zonal mean upwelling is calculated from momentum balance and continuity (“downward control”), and these estimates in the deep tropics are found to be in reasonable agreement with stratospheric upwelling calculated from thermodynamic balance (and also with vertical velocity obtained from ERA-40). The detailed momentum balances associated with the dynamical upwelling are investigated, particularly the contributions to climatological Eliassen–Palm (EP) flux divergence in the subtropics. Results show that the equatorward extension of extratropical waves (baroclinic eddies and, in the NH, quasi-stationary planetary waves) contribute a large component of the subtropical wave driving at 100 hPa. Additionally, there is a significant contribution to subtropical forcing from equatorial planetary waves, which exhibit a strong seasonal cycle (a reversal in phase) in response to latitudinal migration of tropical convection. The observed balances suggest that the strong annual cycle in upwelling across the tropical tropopause is forced by subtropical horizontal eddy momentum flux convergence associated with waves originating in both the tropics and extratropics.

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Mengwen Wu, Yali Luo, Fei Chen, and Wai Kin Wong

Abstract

Understanding changes in subdaily rainfall extremes is critical to urban planners for building more sustainable and resilient cities. In this study, the hourly precipitation data in 1971–2016 from 61 rain gauges are combined with historical land-use change data to investigate changes in extreme hourly precipitation (EXHP) in the Pearl River delta (PRD) region of South China. Also, 120 extreme rainfall events (EXREs) during 2011–16 are analyzed using observations collected at densely distributed automatic weather stations and radar network. Statistically significant increase of hourly precipitation intensity leads to higher annual amounts of both total and extreme precipitation over the PRD urban cluster in the rapid urbanization period (about 1994–2016) than during the preurbanization era (1971 to about 1993), suggesting a possible link between the enhanced rainfall and the rapid urbanization. Those urbanization-related positive trends are closely related to more frequent occurrence of abrupt rainfall events with short duration (≤6 h) than the continuous or growing rainfall events with longer duration. The 120 EXREs in 2011–16 are categorized into six types according to the originating location and movement of the extreme-rain-producing storms. Despite the wide range of synoptic backgrounds and seasons, rainfall intensification by the strong urban heat island (UHI) effect is a clear signal in all the six types, especially over the inland urban cluster with prominent UHIs. The UHI thermal perturbation probably plays an important role in the convective initiation and intensification of the locally developed extreme-rain-producing storms during the daytime.

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William J. Randel, Mijeong Park, Fei Wu, and Nathaniel Livesey

Abstract

Near-equatorial ozone observations from balloon and satellite measurements reveal a large annual cycle in ozone above the tropical tropopause. The relative amplitude of the annual cycle is large in a narrow vertical layer between ∼16 and 19 km, with approximately a factor of 2 change in ozone between the minimum (during NH winter) and maximum (during NH summer). The annual cycle in ozone occurs over the same altitude region, and is approximately in phase with the well-known annual variation in tropical temperature. This study shows that the large annual variation in ozone occurs primarily because of variations in vertical transport associated with mean upwelling in the lower stratosphere (the Brewer–Dobson circulation); the maximum relative amplitude peak in the lower stratosphere is collocated with the strongest background vertical gradients in ozone. A similar large seasonal cycle is observed in carbon monoxide (CO) above the tropical tropopause, which is approximately out of phase with ozone (associated with an oppositely signed vertical gradient). The observed ozone and CO variations can be used to constrain estimates of the seasonal cycle in tropical upwelling.

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