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The diurnal cycle of rainfall over New Guinea in convection-permitting WRF simulations

In this study, we examine the diurnal cycle of rainfall over New Guinea using a series of convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model. We focus our simulations on a period of suppressed regional-scale conditions (February 2010) during which local diurnal forcings are maximised. Additionally, we focus our study on the occurrence and dynamics of offshore-propagating convective systems that contribute to the observed early-morning rainfall maximum north-east of New Guinea.In general, modelled diurnal precipitation shows good agreement with satellite-observed rainfall, albeit with some timing and intensity differences. The simulations also reproduce the occurrence and variability of overnight convection that propagate offshore as organised squall lines north-east of New Guinea. The occurrence of these offshore systems is largely controlled by background conditions. Days with offshore-propagating convection have more middle tropospheric moisture, larger convective available potential energy, and greater low-level moisture convergence. Convection has similar characteristics over the terrain on days with and without offshore propagation.The offshore-propagating convection manifests via a multi-stage evolutionary process. First, scattered convection over land, which is remnant of the daytime maximum, moves towards the coast and becomes reorganised near the region of coastal convergence associated with the land breeze. The convection then moves offshore in the form of a squall line at ∼ 5 m<mspace linebreak="nobreak" width="0.125em"/>s−1. In addition, cool anomalies associated with gravity waves generated by precipitating land convection propagate offshore at a dry hydrostatic gravity wave speed (of ∼ 15 m<mspace linebreak="nobreak" width="0.125em"/>s−1) and act to destabilise the coastal/offshore environment prior to the arrival of the squall line. Although the gravity wave does not appear to initiate the convection or control its propagation, it should contribute to its longevity and maintenance. The results highlight the importance of terrain and coastal effects along with gravity waves in contributing to the diurnal cycle over the Maritime Continent, especially the offshore precipitation maxima adjacent to quasi-linear coastlines.
Effects of emission reductions on organic aerosol in the southeastern United States

Long-term (1999 to 2013) data from the Southeastern Aerosol Research and Characterization (SEARCH) network are used to show that anthropogenic emission reductions led to important decreases in fine-particle organic aerosol (OA) concentrations in the southeastern US On average, 45 % (range 25 to 63 %) of the 1999 to 2013 mean organic carbon (OC) concentrations are attributed to combustion processes, including fossil fuel use and biomass burning, through associations of measured OC with combustion products such as elemental carbon (EC), carbon monoxide (CO), and nitrogen oxides (NOx). The 2013 mean combustion-derived OC concentrations were 0.5 to 1.4 µg m−3 at the five sites operating in that year. Mean annual combustion-derived OC concentrations declined from 3.8 ± 0.2 µg m−3 (68 % of total OC) to 1.4 ± 0.1 µg m−3 (60 % of total OC) between 1999 and 2013 at the urban Atlanta, Georgia, site (JST) and from 2.9 ± 0.4 µg m−3 (39 % of total OC) to 0.7 ± 0.1 µg m−3 (30 % of total OC) between 2001 and 2013 at the urban Birmingham, Alabama (BHM), site. The urban OC declines coincide with reductions of motor vehicle emissions between 2006 and 2010, which may have decreased mean OC concentrations at the urban SEARCH sites by > 2 µg m−3. BHM additionally exhibits a decline in OC associated with SO2 from 0.4 ± 0.04 µg m−3 in 2001 to 0.2 ± 0.03 µg m−3 in 2013, interpreted as the result of reduced emissions from industrial sources within the city. Analyses using non-soil potassium as a biomass burning tracer indicate that biomass burning OC occurs throughout the year at all sites. All eight SEARCH sites show an association of OC with sulfate (SO4) ranging from 0.3 to 1.0 µg m−3 on average, representing ∼  25 % of the 1999 to 2013 mean OC concentrations. Because the mass of OC identified with SO4 averages 20 to 30 % of the SO4 concentrations, the mean SO4-associated OC declined by ∼  0.5 to 1 µg m−3 as SO4 concentrations decreased throughout the SEARCH region. The 2013 mean SO4 concentrations of 1.7 to 2.0 µg m−3 imply that future decreases in mean SO4-associated OC concentrations would not exceed ∼  0.3 to 0.5 µg m−3. Seasonal OC concentrations, largely identified with ozone (O3), vary from 0.3 to 1.4 µg m−3 ( ∼  20 % of the total OC concentrations).
An optimal-estimation-based aerosol retrieval algorithm using OMI near-UV observations

An optimal-estimation(OE)-based aerosol retrieval algorithm using the OMI (Ozone Monitoring Instrument) near-ultraviolet observation was developed in this study. The OE-based algorithm has the merit of providing useful estimates of errors simultaneously with the inversion products. Furthermore, instead of using the traditional look-up tables for inversion, it performs online radiative transfer calculations with the VLIDORT (linearized pseudo-spherical vector discrete ordinate radiative transfer code) to eliminate interpolation errors and improve stability. The measurements and inversion products of the Distributed Regional Aerosol Gridded Observation Network campaign in northeast Asia (DRAGON NE-Asia 2012) were used to validate the retrieved aerosol optical thickness (AOT) and single scattering albedo (SSA). The retrieved AOT and SSA at 388 nm have a correlation with the Aerosol Robotic Network (AERONET) products that is comparable to or better than the correlation with the operational product during the campaign. The OE-based estimated error represented the variance of actual biases of AOT at 388 nm between the retrieval and AERONET measurements better than the operational error estimates. The forward model parameter errors were analyzed separately for both AOT and SSA retrievals. The surface reflectance at 388 nm, the imaginary part of the refractive index at 354 nm, and the number fine-mode fraction (FMF) were found to be the most important parameters affecting the retrieval accuracy of AOT, while FMF was the most important parameter for the SSA retrieval. The additional information provided with the retrievals, including the estimated error and degrees of freedom, is expected to be valuable for relevant studies. Detailed advantages of using the OE method were described and discussed in this paper.
Improving aerosol interaction with clouds and precipitation in a regional chemical weather modeling system

A comprehensive aerosol–cloud–precipitation interaction (ACI) scheme has been developed under a China Meteorological Administration (CMA) chemical weather modeling system, GRAPES/CUACE (Global/Regional Assimilation and PrEdiction System, CMA Unified Atmospheric Chemistry Environment). Calculated by a sectional aerosol activation scheme based on the information of size and mass from CUACE and the thermal-dynamic and humid states from the weather model GRAPES at each time step, the cloud condensation nuclei (CCN) are interactively fed online into a two-moment cloud scheme (WRF Double-Moment 6-class scheme – WDM6) and a convective parameterization to drive cloud physics and precipitation formation processes. The modeling system has been applied to study the ACI for January 2013 when several persistent haze-fog events and eight precipitation events occurred. The results show that aerosols that interact with the WDM6 in GRAPES/CUACE obviously increase the total cloud water, liquid water content, and cloud droplet number concentrations, while decreasing the mean diameters of cloud droplets with varying magnitudes of the changes in each case and region. These interactive microphysical properties of clouds improve the calculation of their collection growth rates in some regions and hence the precipitation rate and distributions in the model, showing 24 to 48 % enhancements of threat score for 6 h precipitation in almost all regions. The aerosols that interact with the WDM6 also reduce the regional mean bias of temperature by 3 °C during certain precipitation events, but the monthly means bias is only reduced by about 0.3 °C.
Can we explain the observed methane variability after the Mount Pinatubo eruption?

The CH4 growth rate in the atmosphere showed large variations after the Pinatubo eruption in June 1991. A decrease of more than 10 ppb yr−1 in the growth rate over the course of 1992 was reported, and a partial recovery in the following year. Although several reasons have been proposed to explain the evolution of CH4 after the eruption, their contributions to the observed variations are not yet resolved. CH4 is removed from the atmosphere by the reaction with tropospheric OH, which in turn is produced by O3 photolysis under UV radiation. The CH4 removal after the Pinatubo eruption might have been affected by changes in tropospheric UV levels due to the presence of stratospheric SO2 and sulfate aerosols, and due to enhanced ozone depletion on Pinatubo aerosols. The perturbed climate after the eruption also altered both sources and sinks of atmospheric CH4. Furthermore, CH4 concentrations were influenced by other factors of natural variability in that period, such as El Niño–Southern Oscillation (ENSO) and biomass burning events. Emissions of CO, NOX and non-methane volatile organic compounds (NMVOCs) also affected CH4 concentrations indirectly by influencing tropospheric OH levels. Potential drivers of CH4 variability are investigated using the TM5 global chemistry model. The contribution that each driver had to the global CH4 variability during the period 1990 to 1995 is quantified. We find that a decrease of 8–10 ppb yr−1 CH4 is explained by a combination of the above processes. However, the timing of the minimum growth rate is found 6&nash;9 months later than observed. The long-term decrease in CH4 growth rate over the period 1990 to 1995 is well captured and can be attributed to an increase in OH concentrations over this time period. Potential uncertainties in our modelled CH4 growth rate include emissions of CH4 from wetlands, biomass burning emissions of CH4 and other compounds, biogenic NMVOC and the sensitivity of OH to NMVOC emission changes. Two inventories are used for CH4 emissions from wetlands, ORCHIDEE and LPJ, to investigate the role of uncertainties in these emissions. Although the higher climate sensitivity of ORCHIDEE improves the simulated CH4 growth rate change after Pinatubo, none of the two inventories properly captures the observed CH4 variability in this period.
Evaluating the skill of high-resolution WRF-Chem simulations in describing drivers of aerosol direct climate forcing on the regional scale

Assessing the ability of global and regional models to describe aerosol optical properties is essential to reducing uncertainty in aerosol direct radiative forcing in the contemporary climate and to improving confidence in future projections. Here we evaluate the performance of high-resolution simulations conducted using the Weather Research and Forecasting model with coupled with Chemistry (WRF-Chem) in capturing spatiotemporal variability of aerosol optical depth (AOD) and the Ångström exponent (AE) by comparison with ground- and space-based remotely sensed observations. WRF-Chem is run over eastern North America at a resolution of 12 km for a representative year (2008). A systematic positive bias in simulated AOD relative to observations is found (annual mean fractional bias (MFB) is 0.15 and 0.50 relative to MODIS (MODerate resolution Imaging Spectroradiometer) and AERONET, respectively), whereas the spatial variability is well captured during most months. The spatial correlation of observed and simulated AOD shows a clear seasonal cycle with highest correlation during summer months (r = 0.5–0.7) when the aerosol loading is large and more observations are available. The model is biased towards the simulation of coarse-mode aerosols (annual MFB for AE  =  −0.10 relative to MODIS and −0.59 for AERONET), but the spatial correlation for AE with observations is 0.3–0.5 during most months, despite the fact that AE is retrieved with higher uncertainty from the remote-sensing observations. WRF-Chem also exhibits high skill in identifying areas of extreme and non-extreme aerosol loading, and its ability to correctly simulate the location and relative intensity of extreme aerosol events (i.e., AOD  >  75th percentile) varies between 30 and 70 % during winter and summer months, respectively.
Nitrogen speciation in various types of aerosols in spring over the northwestern Pacific Ocean

The cumulative atmospheric nitrogen deposition has been found to profoundly impact the nutrient stoichiometry of the eastern China seas (ECSs: the Yellow Sea and East China Sea) and the northwestern Pacific Ocean (NWPO). In spite of the potential significance of dry deposition in those regions, shipboard observations of atmospheric aerosols remain insufficient, particularly regarding the compositions of water-soluble nitrogen species (nitrate, ammonium and water-soluble organic nitrogen – WSON). We conducted a cruise covering the ECSs and the NWPO during the spring of 2014 and observed three types of atmospheric aerosols. Aluminum content, air mass backward trajectories, weather conditions, and ion stoichiometry allowed us to discern dust aerosol patches and sea-fog-modified aerosols (widespread over the ECSs) from background aerosols (open ocean). Among the three types, sea-fog-modified aerosols contained the highest concentrations of nitrate (536 ± 300 nmol N m−3), ammonium (442 ± 194 nmol N m−3) and WSON (147 ± 171 nmol N m−3); furthermore, ammonium and nitrate together occupied ∼  65 % of the molar fraction of total ions. The dust aerosols also contained significant amounts of nitrate (100 ± 23 nmol N m−3) and ammonium (138 ± 24 nmol N m−3) which were obviously larger than those in the background aerosols (26 ± 32 for nitrate and 54 ± 45 nmol N m−3 for ammonium), yet this was not the case for WSON. It appeared that dust aerosols had less of a chance to come in contact with WSON during their transport. In the open ocean, we found that sea salt (e.g., Na+, Cl−, Mg2+), as well as WSON, correlated positively with wind speed. Apparently, marine dissolved organic nitrogen (DON) was emitted from breaking waves. Regardless of the variable wind speeds from 0.8 to as high as 18 m s−1, nitrate and ammonium, by contrast, remained in narrow ranges, implying that some supply and consumption processes of nitrate and ammonium were required to maintain such a quasi-static condition. Mean dry deposition of total dissolved nitrogen (TDN) for sea-fog-modified aerosols (1090 ± 671 µmol N m−2 d−1) was 5 times higher than that for dust aerosols (190 ± 41.6 µmol N m−2 d−1) and around 20 times higher than that for background aerosols (56.8 ± 59.1 µmol N m−2 d−1). Apparently, spring sea fog on the ECSs played an important role in removing atmospheric reactive nitrogen from the Chinese mainland and depositing it into the ECSs, thus effectively preventing its seaward export to the NWPO.
Hygroscopicity of nanoparticles produced from homogeneous nucleation in the CLOUD experiments

Sulfuric acid, amines and oxidized organics have been found to be important compounds in the nucleation and initial growth of atmospheric particles. Because of the challenges involved in determining the chemical composition of objects with very small mass, however, the properties of the freshly nucleated particles and the detailed pathways of their formation processes are still not clear. In this study, we focus on a challenging size range, i.e., particles that have grown to diameters of 10 and 15 nm following nucleation, and measure their water uptake. Water uptake is useful information for indirectly obtaining chemical composition of aerosol particles. We use a nanometer-hygroscopicity tandem differential mobility analyzer (nano-HTDMA) at subsaturated conditions (ca. 90 % relative humidity at 293 K) to measure the hygroscopicity of particles during the seventh Cosmics Leaving OUtdoor Droplets (CLOUD7) campaign performed at CERN in 2012. In CLOUD7, the hygroscopicity of nucleated nanoparticles was measured in the presence of sulfuric acid, sulfuric acid–dimethylamine, and sulfuric acid–organics derived from α-pinene oxidation. The hygroscopicity parameter κ decreased with increasing particle size, indicating decreasing acidity of particles. No clear effect of the sulfuric acid concentration on the hygroscopicity of 10 nm particles produced from sulfuric acid and dimethylamine was observed, whereas the hygroscopicity of 15 nm particles sharply decreased with decreasing sulfuric acid concentrations. In particular, when the concentration of sulfuric acid was 5.1 × 106 molecules cm−3 in the gas phase, and the dimethylamine mixing ratio was 11.8 ppt, the measured κ of 15 nm particles was 0.31 ± 0.01: close to the value reported for dimethylaminium sulfate (DMAS) (κDMAS ∼ 0.28). Furthermore, the difference in κ between sulfuric acid and sulfuric acid–imethylamine experiments increased with increasing particle size. The κ values of particles in the presence of sulfuric acid and organics were much smaller than those of particles in the presence of sulfuric acid and dimethylamine. This suggests that the organics produced from α-pinene ozonolysis play a significant role in particle growth even at 10 nm sizes.
Radiative and climate impacts of a large volcanic eruption during stratospheric sulfur geoengineering

Both explosive volcanic eruptions, which emit sulfur dioxide into the stratosphere, and stratospheric geoengineering via sulfur injections can potentially cool the climate by increasing the amount of scattering particles in the atmosphere. Here we employ a global aerosol-climate model and an Earth system model to study the radiative and climate changes occurring after an erupting volcano during solar radiation management (SRM). According to our simulations the radiative impacts of the eruption and SRM are not additive and the radiative effects and climate changes occurring after the eruption depend strongly on whether SRM is continued or suspended after the eruption. In the former case, the peak burden of the additional stratospheric sulfate as well as changes in global mean precipitation are fairly similar regardless of whether the eruption takes place in a SRM or non-SRM world. However, the maximum increase in the global mean radiative forcing caused by the eruption is approximately 21 % lower compared to a case when the eruption occurs in an unperturbed atmosphere. In addition, the recovery of the stratospheric sulfur burden and radiative forcing is significantly faster after the eruption, because the eruption during the SRM leads to a smaller number and larger sulfate particles compared to the eruption in a non-SRM world. On the other hand, if SRM is suspended immediately after the eruption, the peak increase in global forcing caused by the eruption is about 32 % lower compared to a corresponding eruption into a clean background atmosphere. In this simulation, only about one-third of the global ensemble-mean cooling occurs after the eruption, compared to that occurring after an eruption under unperturbed atmospheric conditions. Furthermore, the global cooling signal is seen only for the 12 months after the eruption in the former scenario compared to over 40 months in the latter. In terms of global precipitation rate, we obtain a 36 % smaller decrease in the first year after the eruption and again a clearly faster recovery in the concurrent eruption and SRM scenario, which is suspended after the eruption. We also found that an explosive eruption could lead to significantly different regional climate responses depending on whether it takes place during geoengineering or into an unperturbed background atmosphere. Our results imply that observations from previous large eruptions, such as Mount Pinatubo in 1991, are not directly applicable when estimating the potential consequences of a volcanic eruption during stratospheric geoengineering.
The sensitivities of emissions reductions for the mitigation of UK PM2.5

The reduction of ambient concentrations of fine particulate matter (PM2.5) is a key objective for air pollution control policies in the UK and elsewhere. Long-term exposure to PM2.5 has been identified as a major contributor to adverse human health effects in epidemiological studies and underpins ambient PM2.5 legislation. As a range of emission sources and atmospheric chemistry transport processes contribute to PM2.5 concentrations, atmospheric chemistry transport models are an essential tool to assess emissions control effectiveness. The EMEP4UK atmospheric chemistry transport model was used to investigate the impact of reductions in UK anthropogenic emissions of primary PM2.5, NH3, NOx, SOx or non-methane VOC on surface concentrations of PM2.5 in the UK for a recent year (2010) and for a future current legislation emission (CLE) scenario (2030). In general, the sensitivity to UK mitigation is rather small. A 30 % reduction in UK emissions of any one of the above components yields (for the 2010 simulation) a maximum reduction in PM2.5 in any given location of ∼  0.6 µg m−3 (equivalent to ∼  6 % of the modelled PM2.5). On average across the UK, the sensitivity of PM2.5 concentrations to a 30 % reduction in UK emissions of individual contributing components, for both the 2010 and 2030 CLE baselines, increases in the order NMVOC, NOx, SOx, NH3 and primary PM2.5; however there are strong spatial differences in the PM2.5 sensitivities across the UK. Consequently, the sensitivity of PM2.5 to individual component emissions reductions varies between area and population weighting. Reductions in NH3 have the greatest effect on area-weighted PM2.5. A full UK population weighting places greater emphasis on reductions of primary PM2.5 emissions, which is simulated to be the most effective single-component control on PM2.5 for the 2030 scenario. An important conclusion is that weighting corresponding to the average exposure indicator metric (using data from the 45 model grids containing a monitor whose measurements are used to calculate the UK AEI) further increases the emphasis on the effectiveness of primary PM2.5 emissions reductions (and of NOx emissions reductions) relative to the effectiveness of NH3 emissions reductions. Reductions in primary PM2.5 have the largest impact on the AEI in both 2010 and the 2030 CLE scenario. The summation of the modelled reductions to the UK PM2.5 AEI from 30 % reductions in UK emissions of primary PM2.5, NH3, SOx, NOx and VOC totals 1.17 and 0.82 µg m−3 for the 2010 and 2030 CLE simulations, respectively (not accounting for non-linearity).