Pollen grains emitted from vegetation can rupture, releasing subpollen particles (SPPs) as fine atmospheric particulates. Previous laboratory research demonstrates potential for SPPs as efficient ...cloud condensation nuclei (CCN). We develop the first model of atmospheric pollen grain rupture and implement the mechanism in regional climate model simulations over spring pollen season in the United States with a CCN‐dependent moisture scheme. The source of SPPs (surface or in‐atmosphere) depends on region and sometimes season, due to the distribution of relative humidity and rain. Simulated concentrations of SPPs are approximately 1–10 or 1–1,000 cm−3, depending on the number of SPPs produced per pollen grain (nspg). Lower nspg (103) produces a negligible effect on precipitation, but high nspg (106) in clean continental CCN background concentrations (100 CCN per cubic centimeter) shows that SPPs suppress average seasonal precipitation by 32% and shift rates from heavy to light while increasing dry days. This effect is smaller (2% reduction) for polluted air.
Plain Language Summary
Pollen grains emitted by wind from a variety of plants can swell from exposure to high levels of humidity, creating internal pressure that may cause the grains to rupture. Particles that are 10 to a thousand times smaller than pollen grains are released in the process. These subpollen particles (SPPs) have been found in laboratory studies to efficiently collect water on their surfaces, making them potential cloud condensation nuclei (i.e., particles that may grow into cloud droplets). We have developed a numerical model of pollen rupture that interfaces with an atmosphere model to determine (1) how many SPPs are produced during the pollen season from two different sources: rupture of pollen at the surface and rupture of airborne pollen grains; (2) the geographic and vertical distribution of SPPs seasonally; and (3) the impact of SPPs on regional precipitation. We find that the strength of either source in any region or phase of season depends on rain and relative humidity. We also find that SPPs have the potential to suppress seasonal precipitation in clean conditions when anthropogenic pollution is not present depending on how many are released for each pollen grain that ruptures. The magnitude of suppression regionally is dependent on source magnitude of SPPs, as well as the availability of water vapor.
Key Points
The first model of moisture‐induced pollen rupture and release of subpollen particles (SPPs) is coupled to a regional climate model
During peak pollen season in the United States, simulated SPPs range from 1 to 1,000 cm−3, depending on the number produced per pollen grain ruptured
SPP may have the ability to suppress precipitation regionally in clean continental CCN conditions and induce a negative feedback to SPP production
We examine the sensitivity of dust radiative feedback to the dust absorption property during the Indian summer monsoon (June–September) season from 2005 to 2010 using the Regional Climate Model ...version 4.1. The dust direct radiative forcing at top of atmosphere switches from cooling to warming for absorbing dust over the Indian subcontinent. The dust‐induced low pressure anomaly plays a crucial role in building up large‐scale convergence particularly over the Arabian Peninsula, which strengthens the monsoon circulation leading to enhanced precipitation over India. The intensity of precipitation and wind field at 850 hPa increases over India by considering more absorbing dust. Air temperature and cloud fraction change by 20–50% in the lower to middle troposphere as a result of dynamic response due to modification in dust absorptive characteristics. Our results demonstrate that the response of monsoon circulation to dust radiative feedback is highly sensitive to dust absorption; and hence, it should be accurately represented in the models for improved simulation of monsoon precipitation. The results have important implications in case of break‐to‐active transition within the Indian summer monsoon season.
Key Points
Sensitivity of Indian summer monsoon intensity to dust SSA is examined
Absorbing dust over Arabian Peninsula enhances rainfall over India
Dust feedback leads to a vertical temperature dipole over the Gangetic Plain
A new version of the RegCM regional climate modeling system, RegCM4, has been recently developed and made available for public use. Compared to previous versions, RegCM4 includes new land surface, ...planetary boundary layer, and air–sea flux schemes, a mixed convection and tropical band configuration, modifications to the pre-existing radiative transfer and boundary layer schemes, and a full upgrade of the model code towards improved flexibility, portability, and user friendliness. The model can be interactively coupled to a 1D lake model, a simplified aerosol scheme (including organic carbon, black carbon, SO₄, dust, and sea spray), and a gas phase chemistry module (CBM-Z). After a general description of the model, a series of test experiments are presented over 4 domains prescribed under the CORDEX framework (Africa, South America, East Asia, and Europe) to provide illustrative examples of the model behavior and sensitivities under different climatic regimes. These experiments indicate that, overall, RegCM4 shows an improved performance in several respects compared to previous versions, although further testing by the user community is needed to fully explore its sensitivities and range of applications.
The regional climate model RegCM 4.4 at 50 km resolution is used to conduct a sensitivity study over South Asia Coordinated Regional climate Downscaling Experiment domain during the period 1998–2002, ...in order to investigate the best cumulus convective precipitation scheme, planetary boundary layer (PBL) and land-surface scheme. The inferences obtained from 11 sensitivity experiments include the better performance of community land model version 4.5 (CLM 4.5) over biosphere–atmosphere transfer scheme, Tiedtke as cumulus convective precipitation scheme and University of Washington (UW) as PBL scheme. The simulation with these parameterization schemes well captures the monsoon precipitation pattern over India ~ 7 mm/day and North Eastern Region of India (NER) ~ 12 mm/day, which are comparable to observations with a significant correlation of R
2
> 0.93. The observed temperatures are also well simulated by the model. Therefore, RegCM 4.4 with these parameterization schemes is further used to simulate the aerosol fields (aerosol optical depth, AOD and black carbon, BC) and aerosol direct radiative forcing (DRF) for the period 2011–2014 over the same domain with special emphasis on NER. The model captures the seasonality in AOD and BC over the Indian Subcontinent and NER. BC hotspots in the Indo-Gangetic Plain and China are well captured by the model. The observed to simulated BC ratio over Dibrugarh (located in NER) is found to be improved. The model underestimation is significant in the dry season when burning over the region is predominant, which has not been considered by the emission inventories properly. Simulated DRF is found to exhibit seasonality qualitatively as well as a North–South latitudinal gradient.
We used the Regional Circulation Model (RegCM) to investigate the direct effect of dust aerosol on climate over West Africa, with a specific focus on the Sahel region. First, we characterized the ...mechanisms linking dust radiative forcing and convective activity over Sahel and the net impact of dust on precipitation: The mean effect of dust over 11 summer seasons is to reduce precipitation over most of the Sahel region as a result of strong surface cooling and elevated diabatic warming inhibiting convection. However, on the very northern Sahel and in the vicinity of dust sources, a relative increase of precipitation is obtained as a result of enhanced diabatic warming in the lower atmosphere associated with high dust concentrations at low altitude. In the second part of the paper, we investigated the robustness of this signal with regards to different modeling conditions that are thought to be sensitive, namely the extension of the domain, the effect of dust on sea surface temperature, the land surface scheme, the convective scheme and the dust single scattering albedo. The simulated dust induced precipitation anomaly over West Africa is consistent and robust in these tests, but significant variations over the northern Sahel region are nevertheless pointed out. Among different factors, single scattering and surface albedo, as well as the nature of the convective scheme, have the greatest influence on the simulated response of West African climate to dust forcing.
The regional climate model RegCM3 was used to simulate the direct and semidirect radiative effects of biomass burning and dust aerosol over southern Africa during the austral winter season. Simulated ...aerosols were found to induce changes in the regional surface fluxes and atmospheric dynamics. Clear‐sky surface radiative forcing decreased by up to −60 W/m2 in the main biomass burning region, resulting in decreased surface turbulent fluxes and PBL height as well as reduced surface temperatures. The positive temperature bias over the western half of the subcontinent was thus reduced. Radiative absorption by biomass burning aerosols resulted in diabatic warming of the atmosphere, peaking near 700 hPa at a rate of up to 1°C/d. Simulated surface cooling and heating at altitude stabilized the lower troposphere below 700 hPa. Above 700 hPa, stability was reduced in the equatorial region between 5°N and 5°S through an elevated heat pump mechanism, enhancing deep convection and precipitation. The southern branch of the African Easterly Jet was enhanced and shifted southward, likely as a result of the changes in the surface temperature gradient induced by both the reduction in solar radiation reaching the surface and through precipitation‐induced surface cooling in the equatorial region. Daily‐scale aerosol outflow events to the southwest Indian Ocean were also investigated, these events occurring with the passage of a westerly wave. It was found that the aerosol loading enhanced baroclinicity along the leading edge of the frontal system, thus intensifying and narrowing the band of precipitation in this zone.
This study compares recent CO, NOx, NMVOC, SO2, BC, and OC anthropogenic emissions from several state‐of‐the‐art top‐down estimates to global and regional bottom‐up inventories and projections from ...five Shared Socioeconomic Pathways (SSPs) in several regions. Results show that top‐down emissions derived in several recent studies exhibit similar uncertainty as bottom‐up inventories in some regions for certain species and even less in the case of Chinese CO emissions. In general, the largest discrepancies are found outside of regions such as the United States, Europe, and Japan where the most accurate and detailed information on emissions is available. In some regions such as China, which has recently undergone dynamical economic growth and changes in air quality regulations, the top‐down estimates better capture recent emission trends than global bottom‐up inventories. These results show the potential of top‐down estimates to complement bottom‐up inventories and to aide in the development of emission scenarios, particularly in regions where global inventories lack the necessary up‐to‐date and accurate information regarding regional activity data and emission factors such as Africa and India. Areas of future work aimed at quantifying and reducing uncertainty are also highlighted. A regional comparison of recent CO and NOx trends in the five SSPs indicate that SSP126, a strong pollution control scenario, best represents the trends from the top‐down and regional bottom‐up inventories in the United States, Europe, and China, while SSP460, a low‐pollution control scenario, lies closest to actual trends in West Africa. This analysis can be useful for air quality forecasting and near‐future pollution control/mitigation policy studies.
Key Points
Top‐down emissions from several recent studies are within the range of bottom‐up inventories and exhibit a similar level of uncertainty for some regions and species
In China, the United States, and Europe emission trends in the last decade from SSP126 match most closely actual trends from bottom‐up and top‐down estimates
In Western Africa and India recent emission trends from low pollution control scenarios (SSP460 and SSP370, respectively) match most closely actual trends
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