Middle Atmosphere Temperature Research Articles

Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave radiometer

Abstract. In recent decades, theoretical studies and numerical models of thermal tides have gained attention. It has been recognized that tides have a significant influence on the dynamics of the middle and upper atmosphere; as they grow in amplitude and propagate upward, they transport energy and momentum from the lower to the upper atmosphere, contributing to the vertical coupling between atmospheric layers. The superposition of tides with other atmospheric waves leads to non-linear wave–wave interactions. However, direct measurements of thermal tides in the middle atmosphere are challenging and are often limited to satellite measurements in the tropics and at low latitudes. Due to orbit geometry, such observations provide only a reduced insight into the short-term variability in atmospheric tides. In this paper, we present tidal analysis from 5 years of continuous observations of middle-atmospheric temperatures. The measurements were performed with the ground-based temperature radiometer TEMPERA (TEMPErature RAdiometer), which was developed at the University of Bern in 2013 and was located in Bern (46.95° N, 7.45° E) and Payerne (46.82° N, 6.94° E). TEMPERA achieves a temporal resolution of 1–3 h and covers the altitude range between 25–50 km. Using an adaptive spectral filter with a vertical regularization (ASF2D) for the tidal analysis, we found maximum amplitudes for the diurnal tide of approximately 2.4 K, accompanied by seasonal variability. The maximum amplitude was reached on average at an altitude of 43 km, which also reflected some seasonal characteristics. We demonstrate that TEMPERA is suitable for providing continuous temperature soundings in the stratosphere and lower mesosphere with a sufficient cadence to infer tidal amplitudes and phases for the dominating tidal modes. Furthermore, our measurements exhibit a dominating diurnal tide and smaller amplitudes for the semidiurnal and terdiurnal tides in the stratosphere.

Design of a three-frequency Rayleigh lidar for simultaneous temperature and wind measurements

This study proposes what we believe to be a novel high-spectral-resolution three-frequency Rayleigh lidar for simultaneously measuring middle atmosphere temperature and wind. The temperature and wind could be retrieved without assuming an external reference temperature, as typical for a traditional Rayleigh Doppler lidar. Adopting a similar idea used in sodium temperature/wind lidar, this system alternatively emits laser pulses at three frequencies. It receives the corresponding Rayleigh backscattered signals filtered by an iodine cell as a frequency discriminator. The three frequencies are optimized based on the spectral characteristics resulting from the convolution of the pulse laser lineshape convolved Rayleigh scattering signal with iodine molecular absorption spectrum. A two-dimensional calibration curve for temperature and wind ratio is then generated from the theoretical calculation of the final convoluted spectra and used to retrieve temperature and wind simultaneously. Simulated with the return signals collected by a current broadband Rayleigh lidar (30-inch telescope and 15 W output laser power), the temperature and wind uncertainties with resolutions of 1 km and 1 hr are estimated to be 0.4 K and 0.35 m/s, respectively, at 30 km and increase to 16.3 K and 8.1 m/s at 70 km.

Characterization of the middle and upper atmosphere temperatures by Rayleigh scattering Lidar

The temperature plays a critical role in atmospheric research. A measurement system based on Rayleigh scattering Lidar was developed in this study to detect mid and upper-level atmospheric temperature. To analyze the error introduced by the calibration point temperature, the echo signal was simulated. The continuously measured atmospheric temperature structure is stable. The measured atmospheric temperature profile follows the same trend as the atmospheric mode, as shown by comparison with different modes and balloons. The mean difference between the measured atmospheric temperature and the balloon temperature is 2.59 K, with the error mostly concentrated below 5 K. Continuous observation of atmospheric temperature using the system revealed the occurrence of temperature-induced gravity waves at a vertical wavelength from 2 to 3 km at an altitude between 30 and 45 m. Partial fragmentation occurs when energy dissipation is performed in the low-frequency mode from 38 to 45 km. This system was demonstrated to effectively detect temperature and continuously observe gravity waves, with its accuracy confirmed by comparison with balloon temperature measurements.

Global Simulations of Multi‐Frequency HF Signal Absorption for Direct Observation of Middle Atmosphere Temperature and Composition

AbstractThis paper presents the first numerical study on a new concept for the direct measurement of D‐region absorption in the high‐frequency (HF) band. Numerical simulations based on the Appleton‐Hartree and Garrett equations of refractive index are presented. Electron temperature as a result of HF radio pumping of the ionosphere is included in the calculations using proper numerical formulation. Both O‐ and X‐mode radio wave polarizations are taken into consideration. A global map of HF absorption in the northern hemisphere is calculated. Detailed calculations of HF radio wave absorption as it propagates through the lower atmosphere are presented. The effect of several parameters on the amount of absorption is calculated. The best frequencies to be used for the purpose of this study are discussed. A machine learning model is developed and the capability of the model in estimation of D and E‐region constituents includes N2, O, O2, as well as T and Ne is examined. Such a technique can also lead to global mapping of HF absorption and improve OTHR (over‐the‐horizon‐radar) performance.

Global Middle-Atmosphere Response to Winter Stratospheric Variability in SABER and MLS Mean Temperature

Abstract Satellite observations of middle-atmosphere temperature are used to investigate the short-term global response to planetary wave activity in the winter stratosphere. The focus is on the relation between variations in the winter and summer hemispheres. The analysis uses observations from Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) for 2002–21 and Aura Microwave Limb Sounder (MLS) for 2004–21, and reanalysis temperatures and winds from MERRA-2 for 2002–21. We calculate temporal correlations of the Eliassen–Palm flux divergence in the winter stratosphere with global temperature. Results show a robust perturbation extending to midlatitudes of the Southern Hemisphere (SH) stratosphere during Northern Hemisphere (NH) winter. An increase in wave forcing is followed by a decrease in temperatures over the depth of the stratosphere in the SH, peaking at a lag of 3 days. Summer mesospheric temperature perturbations of the opposite sign are seen in many winters. Comparable signals in the NH summer middle-atmosphere are present during some SH winters but are weaker and less consistent than those in the SH during NH winter. A diagnostic evaluation of the patterns of correlation, the mesospheric zonal winds, and the stability criteria suggests that the temperature perturbations in the midlatitude summer mesosphere are more closely associated with the summer stratosphere directly below than with the wave activity in the winter stratosphere. This suggests that the interhemispheric coupling in the stratosphere is driving or contributing to the coupling between the winter stratosphere and the summer mesosphere that has been reported in several investigations. Significance Statement There are many instances in which one part of the atmosphere is found to regularly respond to perturbations occurring in a distant region. In this study, we use observations to investigate one such pattern: temperature changes at high altitude (60–100 km) in the summer that follow dynamical changes near the winter pole at 40–60 km. Such analysis is useful to understand which physical processes contribute to the global connectivity and variability of the atmosphere.

High‐Cadence Lidar Observations of Middle Atmospheric Temperature and Gravity Waves at the Southern Andes Hot Spot

AbstractThe Southern Andes are the strongest hot spot for atmospheric gravity waves (GWs) in the stratosphere. Yet, until recently, no high‐cadence measurements of GWs within the middle atmosphere were available in this region. Therefore, the COmpact Rayleigh Autonomous Lidar (CORAL) was deployed to the Estación Astrónomica Río Grande (53.7°S, 67.7°W), Argentina, to obtain temperature profiles up to 100 km altitude. CORAL operates autonomously and obtained measurements during roughly two thirds of all nights between November 2017 and October 2020. The excellent measurement coverage allows for the quantification of GW properties at the hot spot with great detail. The hot spot nature of this region is reflected in nightly mean temperature profiles showing deviations from the monthly mean in the order of 25–55 K in each winter month. This is connected to winter mean growth rates of GW potential energy (Ep), which are to our knowledge the largest ever reported in the stratosphere. The monthly mean Ep profiles show a mesospheric limit of ∼100 Jkg−1, indicating a saturated GW spectrum at altitudes above 60 km. The winter mean power spectral density also reaches the saturation limit here. Moreover, we investigated the distribution of vertical wavelengths using our novel diagnostic technique WAVELET‐SCAN. It reveals waves with vertical wavelengths that are mostly between 10 and 16 km but also can exceed 25 km in rare occasions.

Thermal structure of Mars’ middle and upper atmospheres: Understanding the impacts of dynamics and solar forcing

We report six years of observations of dayside temperatures of the middle and upper atmospheres of Mars made by the Imaging Ultraviolet Spectrograph instrument aboard the MAVEN spacecraft. Thermospheric temperatures show strong long-term variability associated with Martian season and solar cycle. Temperatures from both the Martian thermosphere and mesosphere show strong short-term variability indicating coupling from the lower atmosphere. The observed local time effect is strong in both upper and middle atmosphere temperatures. The thermosphere tends to be colder in the morning compared to the evening when temperatures are higher. Middle atmospheric temperatures show cooling during the dawn and dusk hours. Our analysis shows strong tidal activity during aphelion, whereas non-migrating tides are suppressed during perihelion, possibly due to increased dust activity. Observations during the deep minimum of solar cycle 24 reveal that thermospheric temperatures are highly variable with respect to local time if solar forcing, Mars–Sun distance, and spatial effects are removed.

Middle Atmosphere Temperature Changes Derived from SABER Observations during 2002-2020

AbstractUsing temperature data measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument from February 2002 to March 2020, the temperature linear trend and temperature responses to the solar cycle (SC), Quasi-Biennial Oscillation (QBO), and El Niño-Southern Oscillation (ENSO) were investigated from 20 km to 110 km for the latitude range of 50°S-50°N. A four-component harmonic fit was used to remove the seasonal variation from the observed monthly temperature series. Multiple linear regression (MLR) was applied to analyze the linear trend, SC, QBO, and ENSO terms. In this study, the near-global mean temperature shows consistent cooling trends throughout the entire middle atmosphere, ranging from -0.28 to -0.97 K/decade. Additionally, it shows positive responses to the solar cycle, varying from -0.05 to 4.53 K/100sfu. A solar temperature response boundary between 50°S and 50°N is given, above which the atmospheric temperature is strongly affected by solar activity. The boundary penetrates deep below the stratopause to ~ 42 km over the tropical region and rises to higher altitudes with latitude. Temperature responses to the QBO and ENSO can be observed up to the upper mesosphere and lower thermosphere. In the equatorial region, 40%-70% of the total variance is explained by QBO signals in the stratosphere and 30%-50% is explained by the solar signal in the upper middle atmosphere. Our results, obtained from 18-year SABER observations, are expected to be an updated reliable estimation of the middle atmosphere temperature variability for the stratospheric ozone recovery period.

Interannual Variability and Trends in Sea Surface Temperature, Lower and Middle Atmosphere Temperature at Different Latitudes for 1980–2019

The influence of sea-surface temperature (SST) on the lower troposphere and lower stratosphere temperature in the tropical, middle, and polar latitudes is studied for 1980–2019 based on the MERRA2, ERA5, and Met Office reanalysis data, and numerical modeling with a chemistry-climate model (CCM) of the lower and middle atmosphere. The variability of SST is analyzed according to Met Office and ERA5 data, while the variability of atmospheric temperature is investigated according to MERRA2 and ERA5 data. Analysis of sea surface temperature trends based on reanalysis data revealed that a significant positive SST trend of about 0.1 degrees per decade is observed over the globe. In the middle latitudes of the Northern Hemisphere, the trend (about 0.2 degrees per decade) is 2 times higher than the global average, and 5 times higher than in the Southern Hemisphere (about 0.04 degrees per decade). At polar latitudes, opposite SST trends are observed in the Arctic (positive) and Antarctic (negative). The impact of the El Niño Southern Oscillation phenomenon on the temperature of the lower and middle atmosphere in the middle and polar latitudes of the Northern and Southern Hemispheres is discussed. To assess the relative influence of SST, CO2, and other greenhouse gases’ variability on the temperature of the lower troposphere and lower stratosphere, numerical calculations with a CCM were performed for several scenarios of accounting for the SST and carbon dioxide variability. The results of numerical experiments with a CCM demonstrated that the influence of SST prevails in the troposphere, while for the stratosphere, an increase in the CO2 content plays the most important role.

NRLMSIS 2.0: A Whole‐Atmosphere Empirical Model of Temperature and Neutral Species Densities

AbstractNRLMSIS® 2.0 is an empirical atmospheric model that extends from the ground to the exobase and describes the average observed behavior of temperature, eight species densities, and mass density via a parametric analytic formulation. The model inputs are location, day of year, time of day, solar activity, and geomagnetic activity. NRLMSIS 2.0 is a major, reformulated upgrade of the previous version, NRLMSISE‐00. The model now couples thermospheric species densities to the entire column, via an effective mass profile that transitions each species from the fully mixed region below ~70 km altitude to the diffusively separated region above ~200 km. Other changes include the extension of atomic oxygen down to 50 km and the use of geopotential height as the internal vertical coordinate. We assimilated extensive new lower and middle atmosphere temperature, O, and H data, along with global average thermospheric mass density derived from satellite orbits, and we validated the model against independent samples of these data. In the mesosphere and below, residual biases and standard deviations are considerably lower than NRLMSISE‐00. The new model is warmer in the upper troposphere and cooler in the stratosphere and mesosphere. In the thermosphere, N2 and O densities are lower in NRLMSIS 2.0; otherwise, the NRLMSISE‐00 thermosphere is largely retained. Future advances in thermospheric specification will likely require new in situ mass spectrometer measurements, new techniques for species density measurement between 100 and 200 km, and the reconciliation of systematic biases among thermospheric temperature and composition data sets, including biases attributable to long‐term changes.

Middle-Atmosphere Temperature Monitoring Addressed with a Constellation of CubeSats Dedicated to Climate Issues

AbstractWhile meteorological numerical models extend upward to the mesopause, mesospheric observations are required for leading simulations and numerical weather forecasts and climate projections. This work reviews some of the challenges about temperature observation requirements and the limiting factors of the actual measurements associated with atmospheric tides. A new strategy is described here using a limb-scattering technique that is based on previous experiments in space. Such observations can be used with cube satellites. Technical issues are the large dynamic range (4 orders of magnitude) required for the measurements, the accuracy of the limb pointing, and the level of stray light. The technique described here will expect accuracy of 1–2 K with a vertical resolution of 1–2 km. A constellation of 100 platforms could provide temperature observations with space (100 km) and time (3 h) resolutions recommended by the World Meteorological Organization, and tidal issues could be resolved with a minimum of 3–5 platforms with specific orbit maintained to avoid drifts.

Long‐Term Variability and Tendencies in Middle Atmosphere Temperature and Zonal Wind From WACCM6 Simulations During 1850–2014

AbstractLong‐term variability of middle atmosphere temperature (T) and zonal wind (U) is investigated using a three‐member ensemble of historical simulations of NCAR's Whole Atmospheric Community Climate Model latest version 6 (WACCM6) for 1850–2014 (165 years). The model reproduces the climatological features of T and U. The contributions of Quasi Biennial Oscillation (QBO) at 10 and 30 hPa, solar cycle (SC), El Niño‐Southern Oscillation (ENSO), ozone depleting substances (ODS), carbon dioxide (CO2), and stratospheric sulfate aerosol (volcanic eruptions) to change in monthly zonal mean T and U are analyzed using multiple linear regression. The signal due to CO2 increase dominates as a predictor of the net multidecadal global annual mean temperature change at all levels in the middle atmosphere. Contributions from ODS also affect the net multidecadal global mean temperature trend in the stratosphere. Because of similarities in the time evolution of the emissions of CO2 and ODS, the analysis of existing model output cannot accurately separate the attributions of cooling to these two dominant forcing processes. On shorter time scales, solar flux variations are the largest source of variability in the mesosphere while volcanic eruptions are the largest in the stratosphere. In the stratosphere and mesosphere, both QBO and ENSO can significantly impact zonal mean temperature and zonal‐mean zonal wind depending on latitudes, but their impact on the multidecadal global mean temperature trend is very small.

Long-term trend and solar cycle in the middle atmosphere temperature revealed from merged HALOE and SABER datasets

The middle atmosphere temperature trend and solar cycle have been studied with long-term ground-based and satellite datasets and/or model simulations. Single satellite observations for middle atmosphere temperature are not long enough (less than 2 solar cycles) to derive long-term trend and solar cycle effects with great confidence. Using a nearly 30-year dataset merged from the Halogen Occultation Experiment (HALOE) observations between 1991 and 2005 and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations between 2002 and 2019, we study the long-term trend and solar cycle in the middle atmosphere temperature between 45°S and 45°N. The cooling trend is significant below 75 km with a maximum value of ~ -1.2 K/decade near 60–70 km in the southern hemisphere (SH) tropical and subtropical region. The SH cooling trend is larger than its counterpart in the northern hemisphere (NH). The trend derived from the SABER only dataset is ~1.5 times smaller than that derived from the merged dataset, which is consistent with the upper stratospheric ozone recovery after 1995. The positive temperature response to the 11-year solar cycle is ~1 K/100 solar flux units (sfu) in the upper stratosphere and lower mesosphere and increase to more than 2 K/100 sfu in the upper mesosphere of the middle latitudes. The temperature responses show clear hemispheric asymmetry with stronger responses below 70 km but weaker above in the SH than those in the NH. This is likely related to hemispheric asymmetry of wave activities induced by indirect solar radiation.

Response of middle atmospheric temperature to the 27 d solar cycle: an analysis of 13 years of microwave limb sounder data

Abstract. This work focuses on studying the presence and characteristics of 27 d solar signatures in middle atmospheric temperature observed by the microwave limb sounder (MLS) on NASA's Aura spacecraft. The 27 d signatures in temperature are extracted using the superposed epoch analysis (SEA) technique. We use time-lagged linear regression (sensitivity analysis) and a Monte Carlo test method (significance test) to explore the dependence of the results on latitude and altitude, solar activity, and season, as well as on different parameters (e.g., smoothing filter, window width and epoch centers). Using different parameters does impact the results to a certain degree, but it does not affect the overall results. Analyzing the 13-year data set shows that highly significant 27 d solar signatures in middle atmospheric temperature are present at many altitudes and latitudes. A tendency to higher temperature sensitivity to solar forcing in the winter hemisphere compared to the summer hemisphere is found. In addition, the sensitivity of temperature to 27 d solar forcing tends to be larger at high latitudes than at low latitudes. For 11-year solar minimum conditions no statistically significant identification of a 27 d solar signature is possible at most altitudes and latitudes. Several results we obtained suggest that processes other than solar variability drive atmospheric temperature variability at periods around 27 d. Comparisons of the obtained sensitivity values with earlier experimental and model studies show good overall agreement.

Middle Atmosphere Temperature Trends in the Twentieth and Twenty‐First Centuries Simulated With the Whole Atmosphere Community Climate Model (WACCM)

AbstractWe use Whole Atmosphere Community Climate Model simulations made under various climate change scenarios to study the evolution of the global‐mean temperature trend in the late twentieth century and the twenty‐first century. Results are compared with available satellite observations, including new trend estimates derived from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on NASA's TIMED spacecraft. Modeled and observed trends are shown to be consistent throughout the entire middle atmosphere, from near the tropopause (~16 km) to the lower thermosphere (~95 km) in the period covered by the satellite data. Simulations are extended into the twenty‐first century to document the evolution of the global‐mean temperature trend profile. We find, consistent with previous studies, a marked change in the trend profile at the turn of the twenty‐first century, which is driven by the recovery of stratospheric ozone following the adoption of the Montreal Protocol. In the twenty‐first century, the trend profile becomes more uniform with altitude, but its overall shape and magnitude are conditioned by the scenario adopted for future emissions of greenhouse gases. Our results suggest that the vertical profile of temperature trends in the middle atmosphere will remain an important signature of global climate change, and they underscore the importance of global, continuous monitoring of this region of the atmosphere.

Long‐Term Trends in the Low‐Latitude Middle Atmosphere Temperature and Winds: Observations and WACCM‐X Model Simulations

AbstractIn recent years, the middle atmosphere has evoked great scientific interest as long‐term changes can be clearly captured owing to the large perturbation amplitudes at these altitudes. In the present study, more than 25 years of the data are used to investigate the long‐term trends in the middle atmosphere by suitably combining the observations from different techniques (Rocketsonde, High‐Resolution Doppler Imager (HRDI)/Upper Atmosphere Research Satellite (UARS), Halogen Occultation Experiment (HALOE)/UARS, Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/Thermosphere Ionosphere Mesosphere Energetic Dynamics (TIMED), and Mesosphere‐stratosphere‐troposphere (MST) radar) from Indian region. As different instruments/techniques are used and the time periods are not the same, extreme care has been taken while merging various data sets to obtain meaningful long‐term trends. To understand the observed long‐term trends, Whole Atmosphere Community Climate Model–eXtended model simulations for the Indian low‐latitude regions are also used. A significant cooling trend of ~−1.7 ± 0.5 K/decade between 30 and 80 km is noticed. Large decreasing trend (~5 m/s/decade) in the eastward winds is noticed which is significant between 70 and 80 km only changing from strong eastward wind in 1970s to weak westward wind in recent decade. No significant trends are observed in the meridional wind. These observations are well captured by the Whole Atmosphere Community Climate Model–eXtended model simulations while considering changes in concentrations of greenhouse gases including carbon dioxide (CO2), methane (CH4), water vapor (H2O), and chlorofluorocarbon species that cause depletion of stratospheric ozone (O3). Thus, it is prudent to conclude that long‐term decreasing trends in the zonal winds and cooling trends in the temperature in the middle atmosphere are caused to a large extent by greenhouse gases suggesting the role of anthropogenic changes in the dynamics of the middle atmosphere.

On the assembly and calibration of a spatial heterodyne interferometer for limb sounding of the middle atmosphere

Spatial heterodyne interferometers are an enabling technology to build highly miniaturized optical instruments for the observation of faint emissions in the atmosphere. They are particularly suited for the deployment on nano- or micro-satellite constellations. One application of the SHI technology is a middle atmosphere temperature sounder based on the measurement of relative intensities emitted by the O2 atmospheric band system. Beside basic design considerations, aspects of the opto-mechanical design and assembly of a monolithic SHI for a space application are addressed. For the characterization of such an instrument, a light stimulus based on a Köhler illuminator is presented.

Evaluating Different Techniques for Constraining Lower Atmospheric Variability in an Upper Atmosphere General Circulation Model: A Case Study During the 2010 Sudden Stratospheric Warming

AbstractWe analyze the effects specified dynamics (SD) and 4D Tendency nudging have on accurately reproducing the middle and upper atmospheric variability induced by the 2010 sudden stratospheric warming (SSW) event in the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM). TIME‐GCM numerical experiments were performed using constrained middle atmospheric winds and temperatures from a high‐altitude version of the Navy Global Environmental Model to compare the previously implemented SD scheme, with the newly implemented 4D Tendency scheme. Model comparisons focused on zonal mean winds, composition, planetary waves, and tides in the thermosphere‐ionosphere system. Through 4D Tendency nudging we reveal that coupling coefficients of the one‐way SD coupling approach between the TIME‐GCM and observed SSW conditions were too strong. Prior implementations produced unusually strong vertical shears in the zonal mean winds in the mesosphere and lower thermosphere (MLT), where the model is free running. Differences in zonal mean MLT winds between SD and 4D Tendency nudging simulations resulted in migrating diurnal (DW1) and semidiurnal (SW2) tidal amplitude differences at lower thermospheric altitudes. The consequences of simulating different MLT dynamics using SD and 4D Tendency nudging in the overlaying ionosphere are reported and validated using electron density data from the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites. Although we demonstrate that SD and 4D Tendency nudging techniques are approximately equivalent, results presented herein establish that 4D Tendency nudging has the added potential to identify physical model parameters that contribute to data‐model differences during the 2010 SSW.

Effects of aerosol-radiation feedback and topography during an air pollution event over the North China Plain during December 2017

The online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model was used to investigate a regional haze event, which occurred over the North China Plain (NCP) from 27 to 30 December 2017. Modeling scenarios with and without aerosol-radiation feedback within the planetary boundary layer (PBL) were investigated. By adding aerosol-radiation feedback to the model, we captured the spatial and temporal characteristics of the observed temperature and relative humidity (RH), as well as surface PM2.5, SO2, and NO2 concentrations during this event. The primary meteorological driver of this event was stable meteorological conditions, namely, a low PBL, strong temperature inversion, high RH, and a weak wind field. Aerosol-radiation feedback mechanisms affected these meteorological fields, causing reductions in maximum surface solar radiation, the surface energy budget, PBL height, surface 2-m temperature and middle atmosphere RH, while increasing surface 2-m RH, middle atmosphere temperature, and atmospheric stability. Changes in meteorological variables in turn affected air pollutant distributions and concentrations, with PM2.5 increasing by more than 20 μg/m3 over the NCP during this event. Another sensitivity experiment was carried out over the Taihang and Yanshan mountain areas, in which topography was flattened to a 30-m height to explore the impacts of topography on air pollution in the NCP region. Modeling revealed various topographic effects on meteorological variables related to these two mountains, including a lowered PBL height, reduced wind speed, blocked uniform surface winds, and more intense temperature inversion. Consequently, surface air pollutant concentrations increased unevenly over the NCP in this experiment. Implementation of more detailed PBL processes, as well as radiosonde and satellite data products into the WRF-Chem model would improve simulations of haze formation over the NCP.

Comparing ECMWF high‐resolution analyses with lidar temperature measurements in the middle atmosphere

Middle atmospheric lidar temperature observations conducted above Sodankylä, Finland (67.4°N, 26.6°E), during December 2015 are compared with two estimates of the atmospheric state computed by the integrated forecast system (IFS) of the European Centre for Medium‐Range Weather Forecasts (ECMWF). The first set corresponds to an hourly sampling of the middle atmosphere by high‐resolution analyses and very short‐range forecasts produced by the operational IFS cycle 41r1 at a horizontal resolution of 16 km. The second set is retrieved from the upgraded IFS cycle 41r2 (horizontal resolution 9 km), which was running in parallel with cycle 41r1 during the validation before it became operational. A remarkable agreement between both IFS datasets and the lidar temperature observations above Sodankylä is found below 45 km altitude. Above 45 km altitude, within the sponge layer of the IFS, both IFS datasets depict lower temperatures than the observations, with the 9 km runs showing the coldest temperatures. Various sensitivity experiments conducted with the IFS are analyzed and compared with the lidar observations to investigate the impact of the different changes implemented in IFS cycle 41r2. It is found that both the scientific changes and the horizontal resolution upgrade contribute to the colder mesosphere above Sodankylä. The data assimilation seems to amplify this effect even further.