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Emissions of methane (CH₄) and nitrous oxide (N₂O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH₄and N₂O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought‐induced saltwater intrusion on CH₄and N₂O emissions remain unclear. In this study, a process‐driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification‐DeComposition (TFW‐DNDC), was applied to examine the responses of CH₄and N₂O emissions to episodic drought‐induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH₄and N₂O emission responsiveness to coastal droughts and drought‐induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH₄and N₂O emissions and suggests that simple linkages to salinity may not always be relevant, as non‐linear relationships dominated our simulations. Along the Savannah River, N₂O emissions in the moderate‐oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH₄emission decreased. For the Waccamaw River, emissions of both CH₄and N₂O in the moderate‐oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate‐oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH₄and N₂O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought‐induced seawater intrusion.

 

Abstract A new method that automatically determines the modality of an observed particle size distribution (PSD) and the representation of each mode as a gamma function was used to characterize data obtained during the High Altitude Ice Crystals and High Ice Water Content (HAIC-HIWC) project based out of Cayenne, French Guiana, in 2015. PSDs measured by a 2D stereo probe and a precipitation imaging probe for particles with maximum dimension ( D max ) > 55 μ m were used to show how the gamma parameters varied with environmental conditions, including temperature ( T ) and convective properties such as cloud type, mesoscale convective system (MCS) age, distance away from the nearest convective peak, and underlying surface characteristics. Four kinds of modality PSDs were observed: unimodal PSDs and three types of multimodal PSDs (Bimodal1 with breakpoints 100 ± 20 μ m between modes, Bimodal2 with breakpoints 1000 ± 300 μ m, and Trimodal PSDs with two breakpoints). The T and ice water content (IWC) are the most important factors influencing the modality of PSDs, with the frequency of multimodal PSDs increasing with increasing T and IWC. An ellipsoid of equally plausible solutions in ( N o – λ–μ ) phase space is defined for each mode of the observed PSDs for different environmental conditions. The percentage overlap between ellipsoids was used to quantify the differences between overlapping ellipsoids for varying conditions. The volumes of the ellipsoid decrease with increasing IWC for most cases, and ( N o – λ–μ ) vary with environmental conditions related to distribution of IWC. HIWC regions are dominated by small irregular ice crystals and columns. The parameters ( N o – λ–μ ) in each mode exhibit mutual dependence. 

This dataset contains the result of simulated daily emissions of methane (CH4) and nitrous oxide (N2O) from the soils in Tidal Freshwater Forested Wetlands (TFFW) along the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA) under drought-induced saltwater intrusion using a process-driven biogeochemistry model. 

Abstract High Ice Water Content (HIWC) regions above tropical mesoscale convective systems are investigated using data from the second collaboration of the High Altitude Ice Crystals and High Ice Water Content projects (HAIC-HIWC) based in Cayenne, French Guiana in 2015. Observations from in-situ cloud probes on the French Falcon 20 determine the microphysical and thermodynamic properties of such regions. Data from a 2-D stereo probe and precipitation imaging probe show how statistical distributions of ice crystal mass median diameter ( MMD ), ice water content ( IWC ), and total number concentration ( N t ) for particles with maximum dimension ( D max ) > 55 μm vary with environmental conditions, temperature ( T ), and convective properties such as vertical velocity ( w ), MCS age, distance away from convective peak ( L ), and surface characteristics. IWC is significantly correlated with w , whereas MMD decreases and N t increases with decreasing T consistent with aggregation, sedimentation and vapor deposition processes at lower altitudes. MMD typically increases with IWC when IWC < 0.5 g m -3 , but decreases with IWC when IWC > 0.5 g m -3 for -15 °C ≤ T ≤ -5 °C. Trends also depend on environmental conditions, such as presence of convective updrafts that are the ice crystal source, MMD being larger in older MCSs consistent with aggregation and less injection of small crystals into anvils, and IWC s decrease with increasing L at lower T . The relationship between IWC and MMD depends on environmental conditions, with correlations decreasing with decreasing T . The strength of correlation between IWC and N t increases as T decreases.