Water Vapor Mixing Ratio Research Articles

Observation system simulation experiments (OSSEs) for assimilation of the planetary boundary‐layer height (PBLH) using the EnSRF technique

AbstractThe planetary boundary‐layer height (PBLH) is a key parameter that is very important in numerical weather and air quality predictions. The current LiDAR networks make it possible to provide potential PBLH observations, and assimilating the parameter will be helpful to improve the forecast of variables within the planetary boundary layer (PBL). This study first carried out idealized experiments on PBLH assimilation through observation system simulation experiments (OSSEs). The ensemble square root filter (EnSRF) is applied to assimilate the simulated PBLH based on the Weather Research and Forecast (WRF) model. This study mainly focused on two issues: which variables can be effectively improved by assimilating the PBLH, and whether there are differences in the assimilation effects above and within the PBL in the vertical direction. The results show that during the daytime within the PBL, PBLH assimilation could effectively improve the simulation and forecasting of model variables, including perturbation potential temperature (pt), water vapour mixing ratio (qv) and perturbation geopotential (ph); meanwhile, PBLH assimilation could strengthen turbulence mixing and result in a warmer and drier PBL. While the vertical and horizontal wind speeds (w, u and v) cannot be effectively updated by assimilating the PBLH, they could be indirectly improved by model dynamical adjustment when pt, qv and ph are improved. Above the PBL top, most of the above six variables cannot be effectively improved by assimilating the daytime PBLH. In contrast, at night, only u and v within the PBL could be directly updated by the low nocturnal PBLH. In this study, the simplest and most idealized schemes are selected based on idealized experiments; nevertheless, the results lay a foundation for later vertical and horizontal localization research and for the study of LiDAR‐retrieved PBLH assimilation in the future, which is the ultimate goal of this series of works.

The Relationship between High-Presentation Asthma Days in Melbourne, Australia, and Modeled Thunderstorm Environments

Abstract Epidemic asthma events represent a significant risk to emergency services as well as the wider community. In southeastern Australia, these events occur in conjunction with relatively high amounts of grass pollen during the late spring and early summer, which may become concentrated in populated areas through atmospheric convergence caused by a number of physical mechanisms including thunderstorm outflow. Thunderstorm forecasts are therefore important for identifying epidemic asthma risk factors. However, the representation of thunderstorm environments using regional numerical weather prediction models, which are a key aspect of the construction of these forecasts, have not yet been systematically evaluated in the context of epidemic asthma events. Here, we evaluate diagnostics of thunderstorm environments from historical simulations of weather conditions in the vicinity of Melbourne, Australia, in relation to the identification of epidemic asthma cases based on hospital data from a set of controls. Skillful identification of epidemic asthma cases is achieved using a thunderstorm diagnostic that describes near-surface water vapor mixing ratio. This diagnostic is then used to gain insights on the variability of meteorological environments related to epidemic asthma in this region, including diurnal variations, long-term trends, and the relationship with large-scale climate drivers. Results suggest that there has been a long-term increase in days with high water vapor mixing ratio during the grass pollen season, with large-scale climate drivers having a limited influence on these conditions. Significance Statement We investigate the atmospheric conditions associated with epidemic thunderstorm asthma events in Melbourne, Australia, using historical model simulations of the weather. Conditions appear to be associated with high atmospheric moisture content, which relates to environments favorable for severe thunderstorms, but also potentially pollen rupturing as suggested by previous studies. These conditions are shown to be just as important as the concentration of grass pollen for a set of epidemic thunderstorm asthma events in this region. This means that weather model simulations of thunderstorm conditions can be incorporated into the forecasting process for epidemic asthma in Melbourne, Australia. We also investigate long-term variability in atmospheric conditions associated with severe thunderstorms, including relationships with the large-scale climate and long-term trends.

Secondary 12‐Day Planetary Wave in the Mesospheric Water Vapor During the 2016/2017 Unusual Canadian Stratospheric Warming

AbstractThe anomalous strengthening of the stratospheric Aleutian High during the early boreal winter of 2016/2017 led to a rare Canadian stratospheric warming (CSW) event, during which the extent of the wind reversal in the Arctic region is the strongest among CSWs since 2001. Unlike sudden stratospheric warmings, the polar vortex during this CSW is merely distorted, not displaced off the pole or split into two pieces, and the polar mean temperature only has a modest increase of a few kelvins. Using the Aura satellite Microwave Limb Sounder measurements, an exceptionally strong 12‐day wave (12DW) with westward zonal wavenumber 1 is captured in the mesospheric water vapor mixing ratio during the recovery phase of this CSW. Further diagnostic analysis suggests that the baroclinic/barotropic instability related to this CSW triggers the 12DW in the Arctic stratosphere, and meridional advection plays a leading role in the modulation of 12DW on mesospheric water vapor.

Comparing 3DVAR and hybrid radar data assimilation methods for heavy rain forecast

Radar data are assimilated into numerical prediction models to improve precipitation forecasts using three-dimensional (3D) variational (VAR) and hybrid data assimilation (DA) methods. The effect of radar DA (RDA) and prediction accuracy according to DA methods are evaluated by conducting experiments of three heavy rainfall events and analyzing the spatial pattern of accumulated precipitation, profiles of hydrometeors, and cloud microphysical processes. Increments of the initial analysis revealed that the hybrid method simulated the convective band, wind convergence in front of the cold front and a higher amount of water vapor mixing ratio compared to 3DVAR. Based on numerical experiments, the root mean square error of the total cumulative precipitation in 3DVAR was 9.68 mm lower than that of the no-RDA experiment (CTRL), whereas that of the Hybrid was 11.12 mm lower than that of CTRL. RDA improved precipitation forecasts, and Hybrid showed better precipitation forecast accuracy than 3DVAR. Although RDA indirectly changed the water vapor, rain, snow, and graupel mixing ratios, the change in the water vapor amount was the major factor affecting microphysical processes. The increase (decrease) in the water vapor mixing ratio had the greatest impact on precipitation formation (dissipation), whereas the effect of snow, rain, and graupel mixing ratios was relatively small. The results indicate that it is critical to create an environment in which water vapor can transform into precipitating hydrometeors and the hydrometeors can grow through cloud microphysical processes. The accuracy of heavy rain forecast can be improved by hybrid DA method, which can consider the model error in real time, making it more adaptable to the day-to-day changing weather conditions.

Numerically consistent budgets of potential temperature, momentum, and moisture in Cartesian coordinates: application to the WRF model

Abstract. Numerically accurate budgeting of the forcing terms in the governing equations of a numerical weather prediction model is hard to achieve. Because individual budget terms are generally 2 to 3 orders of magnitude larger than the resulting tendency, exact closure of the budget can only be achieved if the contributing terms are calculated consistently with the model numerics. We present WRFlux, an open-source software that allows precise budget evaluation for the WRF model and, in comparison to existing similar tools, incorporates new capabilities. WRFlux transforms the budget equations from the terrain-following grid of the model to the Cartesian coordinate system, permitting a simplified interpretation of budgets obtained from simulations over non-uniform orography. WRFlux also decomposes the resolved advection into mean advective and resolved turbulence components, which is useful in the analysis of large-eddy simulation output. The theoretical framework of the numerically consistent coordinate transformation is also applicable to other models. We demonstrate the performance and a possible application of WRFlux with an idealized simulation of convective boundary layer growth over a mountain range. We illustrate the effect of inconsistent approximations by comparing the results of WRFlux with budget calculations using a lower-order advection operator and two alternative formulations of the coordinate transformation. With WRFlux, the sum of all forcing terms for potential temperature, water vapor mixing ratio, and momentum agrees with the respective model tendencies to high precision. In contrast, the approximations lead to large residuals: the root mean square error between the sum of the diagnosed forcing terms and the actual tendency is 1 to 3 orders of magnitude larger than with WRFlux.

A case study on the impact of severe convective storms on the water vapor mixing ratio in the lower mid-latitude stratosphere observed in 2019 over Europe

Abstract. Extreme convective events in the troposphere not only have immediate impacts on the surface, but they can also influence the dynamics and composition of the lower stratosphere (LS). One major impact is the moistening of the LS by overshooting convection. This effect plays a crucial role in climate feedback, as small changes of water vapor in the upper troposphere and lower stratosphere (UTLS) have a large impact on the radiative budget of the atmosphere. In this case study, we investigate water vapor injections into the LS by two consecutive convective events in the European mid-latitudes within the framework of the MOSES (Modular Observation Solutions for Earth Systems) measurement campaign during the early summer of 2019. Using balloon-borne instruments, measurements of convective water vapor injection into the stratosphere were performed. Such measurements with a high vertical resolution are rare. The magnitude of the stratospheric water vapor reached up to 12.1 ppmv (parts per million by volume), with an estimated background value of 5 ppmv. Hence, the water vapor enhancement reported here is of the same order of magnitude as earlier reports of water vapor injection by convective overshooting over North America. However, the overshooting took place in the extratropical stratosphere over Europe and has a stronger impact on long-term water vapor mixing ratios in the stratosphere compared to the monsoon-influenced region in North America. At the altitude of the measured injection, a sharp drop in a local ozone enhancement peak makes the observed composition of air very unique with high ozone up to 650 ppbv (parts per billion by volume) and high water vapor up to 12.1 ppmv. ERA-Interim does not show any signal of the convective overshoot, the water vapor values measured by the Microwave Limb Sounder (MLS) in the LS are lower than the in situ observations, and the ERA5 overestimated water vapor mixing ratios. Backward trajectories of the measured injected air masses reveal that the moistening of the LS took place several hours before the balloon launch. This is in good agreement with the reanalyses, which shows a strong change in the structure of isotherms and a sudden and short-lived increase in potential vorticity at the altitude and location of the trajectory. Similarly, satellite data show low cloud-top brightness temperatures during the overshooting event, which indicates an elevated cloud top height.

Long-term characterisation of the vertical structure of the Saharan Air Layer over the Canary Islands using lidar and radiosonde profiles: implications for radiative and cloud processes over the subtropical Atlantic Ocean

Abstract. Every year, large-scale African dust outbreaks frequently pass over the Canary Islands (Spain). Here we describe the seasonal evolution of atmospheric aerosol extinction and meteorological vertical profiles on Tenerife over the period 2007–2018 using long-term micropulse lidar (MPL-3) and radiosonde observations. These measurements are used to categorise the different patterns of dust transport over the subtropical North Atlantic and, for the first time, to robustly describe the dust vertical distribution in the Saharan Air Layer (SAL) over this region. Three atmospheric scenarios dominate the aerosol climatology: dust-free (clean) conditions, the Saharan summer scenario (summer-SAL) and the Saharan winter scenario (winter-SAL). A relatively well-mixed marine boundary layer (MBL) was observed in the case of clean (dust-free) conditions; it was associated with rather constant lidar extinction coefficients (α) below 0.036 km−1 with minimum α (< 0.022 km−1) in the free troposphere (FT). The summer-SAL has been characterised as a dust-laden layer strongly affecting both the MBL (Δα = +48 % relative to clean conditions) and the FT. The summer-SAL appears as a well-stratified layer, relatively dry at lower levels (Δr∼-44 % at the SAL’s base, where r is the water vapour mixing ratio) but more humid at higher levels compared with clean FT conditions (Δr∼+332 % at 5.3 km), with a peak of α> 0.066 km−1 at ∼ 2.5 km. Desert dust is present up to ∼ 6.0 km, the SAL top based on the altitude of SAL's temperature inversion. In the winter-SAL scenario, the dust layer is confined to lower levels below 2 km altitude. This layer is characterised by a dry anomaly at lower levels (Δr∼ −38 % in comparison to the clean scenario) and a dust peak at ∼ 1.3 km height. Clean FT conditions were found above 2.3 km. Our results reveal the important role that both dust and water vapour play in the radiative balance within the summer-SAL and winter-SAL. The dominant dust-induced shortwave (SW) radiative warming in summer (heating rates up to +0.7 K d−1) is found slightly below the dust maximum. However, the dominant contribution of water vapour was observed as a net SW warming observed within the SAL (from 2.1 to 5.7 km) and as a strong cold anomaly near the SAL's top (−0.6 K d−1). The higher water vapour content found to be carried on the summer-SAL, despite being very low, represents a high relative variation in comparison to the very dry clean free troposphere in the subtropics. This relevant aspect should be properly taken into account in atmospheric modelling processes. In the case of the winter-SAL, we observed a dust-induced radiative effect dominated by SW heating (maximum heating of +0.7 K d−1 at 1.5 km, near the dust peak); both dust and atmospheric water vapour impact heating in the atmospheric column. This is the case of the SW heating within the SAL (maximum near the r peak), the dry anomaly at lower levels (Δr∼ −38 % at 1 km) and the thermal cooling (∼ 0.3 K d−1) from the temperature inversion upwards. Finally, we hypothesise that the SAL can impact heterogeneous ice nucleation processes through the frequent occurrence of mid-level clouds observed near the SAL top at relatively warm temperatures. A dust event that affected Tenerife on August 2015 is simulated using the regional DREAM model to assess the role of dust and water vapour carried within SAL in the ice nucleation processes. The modelling results reproduce the arrival of the dust plume and its extension over the island and confirm the observed relationship between the summer-SAL conditions and the formation of mid- and high-level clouds.

Air temperature equation derived from sonic temperature and water vapor mixing ratio for turbulent airflow sampled through closed-path eddy-covariance flux systems

Abstract. Air temperature (T) plays a fundamental role in many aspects of the flux exchanges between the atmosphere and ecosystems. Additionally, knowing where (in relation to other essential measurements) and at what frequency T must be measured is critical to accurately describing such exchanges. In closed-path eddy-covariance (CPEC) flux systems, T can be computed from the sonic temperature (Ts) and water vapor mixing ratio that are measured by the fast-response sensors of a three-dimensional sonic anemometer and infrared CO2–H2O analyzer, respectively. T is then computed by use of either T=Ts1+0.51q-1, where q is specific humidity, or T=Ts1+0.32e/P-1, where e is water vapor pressure and P is atmospheric pressure. Converting q and e/P into the same water vapor mixing ratio analytically reveals the difference between these two equations. This difference in a CPEC system could reach ±0.18 K, bringing an uncertainty into the accuracy of T from both equations and raising the question of which equation is better. To clarify the uncertainty and to answer this question, the derivation of T equations in terms of Ts and H2O-related variables is thoroughly studied. The two equations above were developed with approximations; therefore, neither of their accuracies was evaluated, nor was the question answered. Based on first principles, this study derives the T equation in terms of Ts and the water vapor molar mixing ratio (χH2O) without any assumption and approximation. Thus, this equation inherently lacks error, and the accuracy in T from this equation (equation-computed T) depends solely on the measurement accuracies of Ts and χH2O. Based on current specifications for Ts and χH2O in the CPEC300 series, and given their maximized measurement uncertainties, the accuracy in equation-computed T is specified within ±1.01 K. This accuracy uncertainty is propagated mainly (±1.00 K) from the uncertainty in Ts measurements and a little (±0.02 K) from the uncertainty in χH2O measurements. An improvement in measurement technologies, particularly for Ts, would be a key to narrowing this accuracy range. Under normal sensor and weather conditions, the specified accuracy range is overestimated, and actual accuracy is better. Equation-computed T has a frequency response equivalent to high-frequency Ts and is insensitive to solar contamination during measurements. Synchronized at a temporal scale of the measurement frequency and matched at a spatial scale of measurement volume with all aerodynamic and thermodynamic variables, this T has advanced merits in boundary-layer meteorology and applied meteorology.

Sea Breeze Geoengineering to Increase Rainfall over the Arabian Red Sea Coastal Plains

Abstract The Red Sea (RS) has a high evaporation rate, exceeding 2 m of water per year. The water vapor is transported from the shorelines by sea breezes as far as 200 km landward. Relative humidity in the vicinity of the RS exceeds 80% in summer. Nevertheless, precipitation is scarce in most of the Arabian RS coastal plain. In this work we use the Weather Research and Forecasting (WRF) regional model to assess how deliberate changes (geoengineering) in the surface albedo or conversion of bare land to wide-leaf forests over a vast coastal plain region affect precipitation over the Arabian RS coast. Our simulations show that geoengineering of land surface characteristics perturbs coastal circulation; alters temperature, moisture, and momentum exchange between the land surface and atmosphere; and changes the breeze intensity, cloud cover, and eventually the amount of precipitation. We find that extended afforestation and increased surface albedo are not effective in triggering rainfall over the RS coastal plains. Conversely, decreasing surface albedo to 0.2, assuming installation of solar panels over the coastal plains, increases surface air temperature by 1–2 K, strengthens horizontal surface temperature differences between sea and land, intensifies breezes, increases water vapor mixing ratio in the boundary layer above 3 km by about 0.5 g kg−1, enhances vertical mixing within the planetary boundary layer, and generates 1.5 Gt of extra rainwater, equivalent to the annual consumption of five million people. Thus, this form of regional land surface geoengineering, along with advanced methods of collection and underground storage of freshwater, provides a feasible solution to mitigation of the existing water crisis in the arid coastal regions.

Atmospheric rivers drive exceptional Saharan dust transport towards Europe

This study highlights the occurrence of atmospheric rivers (ARs) over northwest Africa towards Europe, which were accompanied by intense episodes of Saharan dust transport all the way to Scandinavia, in the winter season. Using a combination of observational and reanalysis data, we investigate two extreme dusty AR events in February 2021 and assess their impact on snow melt in the Alps. The warm, moist, and dusty air mass (spatially-averaged 2-meter temperature and water vapour mixing ratio anomalies of up to 8 K and 3 g kg−1, and aerosol optical depths and dust loadings of up to 0.85 and 11 g m−2, respectively) led to a 50% and 40% decrease in snow depth and surface albedo, respectively, in less than one month during the winter season. ARs over northwest Africa show increasing trends over the past 4 decades, with 78% of AR events associated with severe dust episodes over Europe.

Cool-roof effects on thermal and wind environments during heat waves: A case modeling study in Seoul, South Korea

This study examines cool-roof effects on thermal and wind environments in Seoul, South Korea during the 2018 heat wave through Weather Research and Forecasting (WRF) model simulations. Two 37-day simulations, one with cool roofs that have a higher albedo and the other with conventional roofs, are conducted. In the daytime, cool roofs decrease the 2-m temperature, 10-m wind speed, surface sensible and latent heat fluxes, and planetary boundary layer height but increase the 2-m water vapor mixing ratio. The daytime maximum decreases are 1.0 °C in 2-m temperature and 0.5 m s−1 in 10-m wind speed. The cool roof-induced reduction of near-surface temperature tends to decrease with increasing heat wave intensity. The decrease in near-surface temperature weakens convergence of near-surface wind, indicating the weakening of daytime urban breeze. Also, the decrease in near-surface temperature, thus decreased land-sea air temperature difference, leads to the weakening of sea breeze. Interestingly, there are occasions on which the cool-roof case produces higher near-surface temperature at late afternoon times in a region of Seoul than the conventional-roof case. This can occur when the cooling caused by the sea breeze in the conventional-roof case is dominant over the cooling due to cool roofs in the cool-roof case.

Scalable multiple changepoint detection for functional data sequences

AbstractWe propose the multiple changepoint isolation (MCI) method for detecting multiple changes in the mean and covariance of a functional process. We first introduce a pair of projections to represent the variability “between” and “within” the functional observations. We then present an augmented fused lasso procedure to split the projections into multiple regions robustly. These regions act to isolate each changepoint away from the others so that the powerful univariate CUSUM statistic can be applied region‐wise to identify the changepoints. Simulations show that our method accurately detects the number and locations of changepoints under many different scenarios. These include light and heavy tailed data, data with symmetric and skewed distributions, sparsely and densely sampled changepoints, and mean and covariance changes. We show that our method outperforms a recent multiple functional changepoint detector and several univariate changepoint detectors applied to our proposed projections. We also show that MCI is more robust than existing approaches and scales linearly with sample size. Finally, we demonstrate our method on a large time series of water vapor mixing ratio profiles from atmospheric emitted radiance interferometer measurements.

Pathways of precipitation formation in different thermodynamic and aerosol environments over the Indian Peninsula

This study examines precipitation pathways in different thermodynamic and aerosol environments over the Indian peninsula. A three-dimensional idealized Large-Eddy Simulation (LES) model with spectral bin microphysics is used to simulate convective clouds growing under different monsoon environments. Numerical simulations were evaluated using in-situ measurements from the Cloud-Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX). Sensitivity studies were carried out using varied combinations of water vapor mixing ratio (by ±10 and ± 20%) and cloud condensation nuclei number concentrations (100 and 3000 cm−3). The response of various microphysical process rates to the changes in moisture and aerosol is analyzed. The clouds developed in a moist environment have a greater cloud depth, more supercooled liquid, broader cloud drop spectra, earlier onset of mixed-phase regimes, and enhanced precipitation. The amount of moisture is an important factor in determining the aerosol effects on cloud phase and precipitation, with a more significant aerosol impact in the moist environment (+ 20%). An increase in aerosol number concentration results in an enhanced mixed-phase fraction, updraft velocity, and precipitation, mainly through enhancing deposition and aggregation rates. The overall change in moisture by ±20% in cloud properties and precipitation is much more significant than the ten-times change in aerosol concentrations. The secondary ice formation via the Hallet-Mossop (H-M) process plays a significant role in determining the total ice crystal concentration, and its importance increases with the increases of both moisture and aerosol number concentration. The impact of this process on precipitation is minimal. These results provide an important understanding of how co-varied moisture and aerosols over the Indian region would affect the monsoon clouds and precipitation.

Impact of Water-latent Heat on the Thermal Structure of Ultra-cool Objects: Brown Dwarfs and Free-floating Planets

Abstract Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper and as observing capabilities continue to improve, the number of known Y dwarfs—the coldest class of sub-stellar objects, with effective temperatures below about 600 K—is rapidly growing. Critically, this class of ultra-cool objects has atmospheric conditions that overlap with solar-system worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn, and other nearby worlds are well suited for application to sub-stellar atmospheres. To that end, we developed a one-dimensional (vertical) atmospheric structure model for ultra-cool objects that includes moist adiabatic convection, as this is an important process for many solar-system planets. Application of this model across a range of effective temperatures (350, 300, 250, 200 K), metallicities ([M/H] of 0.0, 0.5, 0.7, 1.5), and gravities (log g of 4.0, 4.5, 4.7, 5.0) demonstrates strong impact of water-latent heat release on simulated temperature-pressure profiles. At the highest metallicities, water-vapor mixing ratios reach an Earth-like 3% with associated major alterations to the thermal structure in the atmospheric regions where water condenses. Spectroscopic and photometric signatures of metallicity and moist convection should be readily detectable at near- and mid-infrared wavelengths, especially with James Webb Space Telescope observations, and can help indicate the formation history of an object.

Differential absorption lidar for water vapor isotopologues in the 1.98 µm spectral region: sensitivity analysis with respect to regional atmospheric variability

Abstract. Laser active remote sensing of tropospheric water vapor is a promising technology to complement passive observational means in order to enhance our understanding of processes governing the global hydrological cycle. In such a context, we investigate the potential of monitoring both water vapor H216O and its isotopologue HD16O using a differential absorption lidar (DIAL) allowing for ground-based remote measurements at high spatio-temporal resolution (150 m and 10 min) in the lower troposphere. This paper presents a sensitivity analysis and an error budget for a DIAL system under development which will operate in the 2 µm spectral region. Using a performance simulator, the sensitivity of the DIAL-retrieved mixing ratios to instrument-specific and environmental parameters is investigated. This numerical study uses different atmospheric conditions ranging from tropical to polar latitudes with realistic aerosol loads. Our simulations show that the measurement of the main isotopologue H216O is possible over the first 1.5 km of atmosphere with a relative precision in the water vapor mixing ratio of <1 % in a mid-latitude or tropical environment. For the measurement of HD16O mixing ratios under the same conditions, relative precision is found to be slightly lower but still sufficient for the retrieval of range-resolved isotopic ratios with precisions in δD of a few per mil. We also show that expected precisions vary by an order of magnitude between tropical and polar conditions, the latter giving rise to poorer sensitivity due to low water vapor content and low aerosol load. Such values have been obtained for a commercial InGaAs PIN photodiode, as well as for temporal and line-of-sight resolutions of 10 min and 150 m, respectively. Additionally, using vertical isotopologue profiles derived from a previous field campaign, precision estimates for the HD16O isotopic abundance are provided for that specific case.

Distinct impacts of humidity profiles on physical properties and secondary formation of aerosols in Shanghai

As a key component in the atmosphere, the increase in water vapor content can aggravate air pollution by promoting hygroscopic growth and secondary formation of aerosols. In this study, water vapor mixing ratio (WVMR) profiles retrieved from Raman lidar at night in Shanghai were used to evaluate the accuracy of a maintained microwave radiometer (MWR) and reanalysis datasets (ERA5 from ECMWF and MERRA-2 from NASA). The relative humidity (RH) profiles from radiosonde and reanalysis datasets were used to explore the influence of humidity vertical distributions on aerosol physical properties and secondary formation. There was a large WVMR difference between MWR and Raman lidar, with a mean bias (MB) and a root mean square error (RMSE) being 2.47 g/kg and 3.29 g/kg, respectively. However, the WVMR from ERA5 and MERRA-2 showed excellent agreement (correlation coefficients were both 0.94, N = 11288 and 3619, respectively) with that from Raman lidar and high accuracy especially MERRA-2, which are more suitable for meteorological and climatic applications than MWR. It was found that RH had a significant effect on the vertical distribution of aerosol extinction coefficient (AEC). Aerosol hygroscopic growth caused by high RH resulted in an obvious enhancement of AEC at 355 nm below 610 m in summer. As a good indicator for determining contribution of aerosol secondary formation, large ratios of PM2.5 to CO (PM2.5/CO > 0.08) corresponded to high nitrogen oxidation ratios (NOR), sulfur oxidation ratios (SOR), and also high RH below 800 m during nighttime in winter, suggesting that RH and its vertical distribution played an important role in the aerosol secondary formation. The RH was high in the vertical direction when nitrate and sulfate mainly come from the secondary formation (NOR >0.1 or SOR >0.1), and in contrast to nitrate, the secondary formation for sulfates tended to depend on higher RH (>70%), which indicated that more stringent measures for nitrogen oxides emission reduction should be implemented in Shanghai to reduce the pollution of secondary aerosols in the future.

The outflow of Asian biomass burning carbonaceous aerosol into the upper troposphere and lower stratosphere in spring: radiative effects seen in a global model

Abstract. Biomass burning (BB) over Asia is a strong source of carbonaceous aerosols during spring. From ECHAM6–HAMMOZ model simulations and satellite observations, we show that there is an outflow of Asian BB carbonaceous aerosols into the upper troposphere and lower stratosphere (UTLS) (black carbon: 0.1 to 6 ng m−3 and organic carbon: 0.2 to 10 ng m−3) during the spring season. The model simulations show that the greatest transport of BB carbonaceous aerosols into the UTLS occurs from the Indochina and East Asia region by deep convection over the Malay Peninsula and Indonesia. The increase in BB carbonaceous aerosols enhances atmospheric heating by 0.001 to 0.02 K d−1 in the UTLS. The aerosol-induced heating and circulation changes increase the water vapor mixing ratios in the upper troposphere (by 20–80 ppmv) and in the lowermost stratosphere (by 0.02–0.3 ppmv) over the tropics. Once in the lower stratosphere, water vapor is further transported to the South Pole by the lowermost branch of the Brewer–Dobson circulation. These aerosols enhance the in-atmosphere radiative forcing (0.68±0.25 to 5.30±0.37 W m−2), exacerbating atmospheric warming, but produce a cooling effect on climate (top of the atmosphere – TOA: -2.38±0.12 to -7.08±0.72 W m−2). The model simulations also show that Asian carbonaceous aerosols are transported to the Arctic in the troposphere. The maximum enhancement in aerosol extinction is seen at 400 hPa (by 0.0093 km−1) and associated heating rates at 300 hPa (by 0.032 K d−1) in the Arctic.

Nitrogen oxides (NO and NO2) pollution in the Accra metropolis: Spatiotemporal patterns and the role of meteorology

Economic and urban development in sub-Saharan Africa (SSA) may be shifting the dominant air pollution sources in cities from biomass to road traffic. Considered as a marker for traffic-related air pollution in cities, we conducted a city-wide measurement of NOx levels in the Accra Metropolis and examined their spatiotemporal patterns in relation to land use and meteorological factors. Between April 2019 to June 2020, we collected weekly integrated NOx (n = 428) and NO2 (n = 472) samples at 10 fixed (year-long) and 124 rotating (week-long) sites. Data from the same time of year were compared to a previous study (2006) to assess changes in NO2 concentrations. NO and NO2 concentrations were highest in commercial/business/industrial (66 and 76 μg/m3, respectively) and high-density residential areas (47 and 59 μg/m3, respectively), compared with peri-urban locations. We observed annual means of 68 and 70 μg/m3 for NO and NO2, and a clear seasonal variation, with the mean NO2 of 63 μg/m3 (non-Harmattan) increased by 25–56% to 87 μg/m3 (Harmattan) across different site types. The NO2/NOx ratio was also elevated by 19–28%. Both NO and NO2 levels were associated with indicators of road traffic emissions (e.g. distance to major roads), but not with community biomass use (e.g. wood and charcoal). We found strong correlations between both NO2 and NO2/NOx and mixing layer depth, incident solar radiation and water vapor mixing ratio. These findings represent an increase of 25–180% when compared to a small study conducted in two high-density residential neighborhoods in Accra in 2006. Road traffic may be replacing community biomass use (major source of fine particulate matter) as the prominent source of air pollution in Accra, with policy implication for growing cities in SSA.

Impacts of land use and land cover change and reforestation on summer rainfall in the Yangtze River basin

Abstract. Land use and cover have been significantly changed all around the world during the last decade. In particular, the Grain for Green (GG) program has resulted in significant changes in regional land use and cover, especially in China. Land use and cover change (LULCC) may lead to changes in regional climate. In this study, we take the Yangtze River basin as a case study and analyze the impacts of LULCC and reforestation on summer rainfall amounts and extremes based on the Weather Research and Forecasting model. Firstly, two observed land use and cover scenarios (1990 and 2010) were chosen to investigate the impacts of LULCC on summer rainfall during the last decade. Secondly, two hypothetical reforestation scenarios (i.e., scenarios of 20 % and 50 % cropland changed to forest) were taken based on the control year of 2010 to test the sensitivity of summer rainfall (amounts and extremes) to reforestation. The results showed that average summer rainfall and extreme summer daily rainfall decreased in the Yangtze River basin between 1990 and 2010 due to LULCC. Reforestation could increase summer rainfall amount and extremes, and the effects were more pronounced in populated areas than over the whole basin. Moreover, the effects of reforestation were influenced by the reforestation proportion. In addition, the summer rainfall increased less conversely, with the transform proportion of cropland to forest increased from 20 % to 50 %. By analyzing the changes in water vapor mixing ratio, upward moisture flux, and 10 m wind, it is suggested that this result might be caused by the horizontal transportation processes of moisture. Although a comprehensive assessment of the impacts of LULCC on summer rainfall amounts and extremes was conducted, further studies are needed to investigate the uncertainty better.

Evaluation of the CAM6 Climate Model Using Cloud Observations at McMurdo Station, Antarctica

AbstractA comparative analysis between observational data from McMurdo Station, Antarctica and the Community Atmosphere Model version 6 (CAM6) simulation is performed focusing on cloud characteristics and their thermodynamic conditions. Ka‐band Zenith Radar (KAZR) and High Spectral Resolution Lidar (HSRL) retrievals are used as the basis of cloud fraction and cloud phase identifications. Radiosondes released at 12‐h increments provide atmospheric profiles for evaluating the simulated thermodynamic conditions. Our findings show that the CAM6 simulation consistently overestimates (underestimates) cloud fraction above (below) 3 km in four seasons of a year. Normalized by total in‐cloud samples, ice and mixed phase occurrence frequencies are underestimated and liquid phase frequency is overestimated by the model at cloud fractions above 0.6, while at cloud fractions below 0.6 ice phase frequency is overestimated and liquid‐containing phase frequency is underestimated by the model. The cloud fraction biases are closely associated with concurrent biases in relative humidity (RH), that is, high (low) RH biases above (below) 2 km. Frequencies of correctly simulating ice and liquid‐containing phase increase when the absolute biases of RH decrease. Cloud fraction biases also show a positive correlation with RH biases. Water vapor mixing ratio biases are the primary contributor to RH biases, and hence, likely a key factor controlling the cloud biases. This diagnosis of the evident shortfalls of representations of cloud characteristics in CAM6 simulation at McMurdo Station brings new insight in improving the governing model physics therein.