Investigations Of Fire Activities Over North America And Their Short-term Effective Radiative Effects

Fires form an integral component of the Earth-system and have great ecological,social and economic impacts.Understanding fires' role in the Earth system,including the causes,effects and variability is of great importance to effective fire management.Previous studies have pointed out that fires aerosols have significant radiative effects on both long and short time scales.However,current model evaluations of fire aerosol effects have large uncertainties.This is partly due to the uncertainties of fire emission datasets and model abilities.Besides,extracting signals of fire aerosols from model results can also result in uncertainties.This is because life cycles of aerosols and clouds as well as aerosol-cloud interations are affected by natural variabilities.The noise resulting from simulating these natural variabilities will affect the extraction of fire aerosol induced change.Therefore,long-term integrations are often used to help reduce noise and help to extract signals when investigating cloud radiative effects(CRE)of fire aerosols.Investigations of short-term fire aerosol CRE are mainly based on observations,which results in difficulties to understand the detailed processes of aerosol-cloud interation.This paper investigated the fire activities over North America with a focus on their spatial and temporal distributions and short-term radiative effects.6-hourly horizontal wind nudging was applied to constrain large-scale circulations.We proposed a new method which combines nuding with ensemble to investigate the short-term fire aerosol effects.Daily mean emissions from three fire inventories were used to consider the uncertainty in emission strength and injection heights.The main results are as follows:(1)Fire emissions during the spring-peak burning over southern Mexico shows a biennial variability during 2003-2014 and is induced by the biennial variability of precipitation.On short time scales,interactions between precipitation and fire emitted aerosols form a positive feedback and strengthen the anomalous fire activities.Here we report that the spring-peak biomass burning in the southern Mexico region has a distinct biennial variability as shown in both fire emissions and optical depth of fire aerosols over the study period(2003-2014).The biennial variability is predominantly caused by the variability in precipitation.Weaker precipitation(compared to adjacent years)tends to enhance fire occurrence and results in more fire emission.On short time scales,analysis shows that the influence of precipitation could last for about 20 days.On the other hand,our analysis also indicates that more fires that result from weaker precipitation often lead to further precipitation decrease at short time scales(3-5 days).This interaction between precipitation and fires amplifies the fire anomalies.(2)Model shows reasonably good skills in simulating column-integrated aerosol properties(e.g.aerosol optical depth).However,model performance on simulating vertical distributions of fire aerosols is affected by injection height.Model evaluation results show that among the three emission datasets,QFEDv2.4 has the most reasonable emission rates.The simulated AOD agrees well with AERONET data in both magnitude and temporal variation with model is driven by QFEDv2.4.Besides,model forced with QFEDv2.4 data also shows good performance when compared with IMPROVE datasets in the downwind regions.However,in the source region,model forced with QFEDv2.4 overestimates aerosol concentrations.This is due to the fire inject height used in QFEDv2.4 data,which uses the standard CAM5 emission dataset and has the largest emission in the lowest model level.When sampling is done in the lowest model level to compare with IMPROVE data,this would lead to an overestimation.(3)Although nudging helps to constrain distributions of aerosols,cloud properties are still sensitive to small perturbations in atmospheric state.It is important to combine nudging with ensemble simulations in order to obtain a robust estimate of the effective fire aerosol effect during a short period.When investigating fire aerosol effects on short time scales,variation in the simulated radiative forcing can be large among the ensemble members despite indistinguishable AOD.The large ensemble spread of total aerosol and cloud forcing indicates large uncertainties in the estimation.This suggests that although nudging helps to constrain large-scale features,the simulated cloud properties and their response to aerosol changes can still be sensitive to small perturbations in the atmospheric state.Therefore for investigations of the short-term aerosol effect,it is important to conduct ensemble simulations with sufficient ensemble members to obtain a robust estimate.The number of members required is case dependent.For the studied case in this paper,futher investigations show that the simulated ensemble mean and spread of SCRE in simulations with fewer than nine members differs considerably from those with more members.(4)On short time scales,the shortwave cloud radiative effects of fire aerosols are much stronger than the direct effective radiative effects over the central US and southern Mexico regions.Fire-emitted aerosols substantially affect both warm cloud and ice cloud properties through different mechanisms and lead to significant shortwave cloud radiative effect(SCRE)with opposite signs.Besides,the relative vertical positons between fire aerosols and clouds would affect the shortwave direct radiative effect(SDRE)of fire aerosols.The short-term radiative effects of fire aerosols were investigated for an individual day and ten-day mean.On April 7,fire aerosol SDRE over the southern Mexico region shows a contrast of a warming effect in the land region and a cooling effect in the adjacent ocean despite their similar aerosol loading.This is due to the difference in low-level cloud distributions between the two areas.Over land,clouds appear under elevated aerosol layers,thus more solar radiation is reflected back to space.This leads to amplified BC absorption and more positive direct radiative forcing.In contrast,neither absorption nor scattering changes significantly from clear-sky to all-sky conditions over adjacent ocean,since the cloud fraction is small.For the 10-day mean results,decomposition of total radiative forcing shows that fire aerosol effects in the two selected regions are dominated by the SCRE.Negative SCRE covers almost the entirety of southern Mexico with a regional mean value of-3.02Wm-2.Over the central US,the SCRE is positive in the north and negative in the south,and the regional mean is small(-0.56 Wm-2).The negative SCRE in both regions is associated with the increased liquid water path and cloud droplet number concentrations induced by fire aerosols,while the positive SCRE results from decrease in ice water path and ice number concentrations.Fire aerosols produce a negative SDRE of-0.1 Wm-2 in the central US and-0.86 Wm-2 in the southern Mexico.