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The Alpha Jet Atmospheric eXperiment (AJAX) flew scientific flights between 2011 and 2018 providing measurements of trace gas species and meteorological parameters over California and Nevada, USA. This paper describes the observations made by the AJAX program over 229 flights and approximately 450 h of flying. AJAX was a multi-year, multi-objective, multi-instrument program with a variety of sampling strategies resulting in an extensive dataset of interest to a wide variety of users. Some of the more common flight objectives include satellite calibration/validation (GOSAT, OCO-2, TROPOMI) at Railroad Valley and other locations and long-term observations of free-tropospheric and boundary layer ozone allowing for studies of stratosphere-to-troposphere transport and long-range transport to the western United States. AJAX also performed topical studies such as sampling wildfire emissions, urban outflow and atmospheric rivers. Airborne measurements of carbon dioxide, methane, ozone, formaldehyde, water vapor, temperature, pressure and 3-D winds made by the AJAX program have been published at NASA's Airborne Science Data Center (https://asdc.larc.nasa.gov/project/AJAXTS9 (last access: 1 November 2022), https://doi.org/10.5067/ASDC/SUBORBITAL/AJAX/DATA001, Iraci et al., 2021a).

The southeastern Atlantic is home to an expansive smoke aerosol plume overlying a large cloud deck for approximately a third of the year. The aerosol plume is mainly attributed to the extensive biomass burning activities that occur in southern Africa. Current Earth system models (ESMs) reveal significant differences in their estimates of regional aerosol radiative effects over this region. Such large differences partially stem from uncertainties in the vertical distribution of aerosols in the troposphere. These uncertainties translate into different aerosol optical depths (AODs) in the planetary boundary layer (PBL) and the free troposphere (FT). This study examines differences of AOD fraction in the FT and AOD differences among ESMs (WRF-CAM5, WRF-FINN, GEOS-Chem, EAM-E3SM, ALADIN, GEOS-FP, and MERRA-2) and aircraft-based measurements from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. Models frequently define the PBL as the well-mixed surface-based layer, but this definition misses the upper parts of decoupled PBLs, in which most low-level clouds occur. To account for the presence of decoupled boundary layers in the models, the height of maximum vertical gradient of specific humidity profiles from each model is used to define PBL heights. Results indicate that the monthly mean contribution of AOD in the FT to the total-column AOD ranges from 44 % to 74 % in September 2016 and from 54 % to 71 % in August 2017 within the region bounded by 25∘ S–0∘ N–S and 15∘ W–15∘ E (excluding land) among the ESMs. ALADIN and GEOS-Chem show similar aerosol plume patterns to a derived above-cloud aerosol product from the Moderate Resolution Imaging Spectroradiometer (MODIS) during September 2016, but none of the models show a similar above-cloud plume pattern to MODIS in August 2017. Using the second-generation High Spectral Resolution Lidar (HSRL-2) to derive an aircraft-based constraint on the AOD and the fractional AOD, we found that WRF-CAM5 produces 40 % less AOD than those from the HSRL-2 measurements, but it performs well at separating AOD fraction between the FT and the PBL. AOD fractions in the FT for GEOS-Chem and EAM-E3SM are, respectively, 10 % and 15 % lower than the AOD fractions from the HSRL-2. Their similar mean AODs reflect a cancellation of high and low AOD biases. Compared with aircraft-based observations, GEOS-FP, MERRA-2, and ALADIN produce 24 %–36 % less AOD and tend to misplace more aerosols in the PBL. The models generally underestimate AODs for measured AODs that are above 0.8, indicating their limitations at reproducing high AODs. The differences in the absolute AOD, FT AOD, and the vertical apportioning of AOD in different models highlight the need to continue improving the accuracy of modeled AOD distributions. These differences affect the sign and magnitude of the net aerosol radiative forcing, especially when aerosols are in contact with clouds.

Part 1 (Ryoo et al., 2021) provided a climatological overview of the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) period and assessed the representativeness of the deployment years. In part 2, more detailed meteorological analyses support the interpretation of the airborne measurements for aerosol transport and its interaction with low clouds over the southeastern (SE) Atlantic Ocean during the September 2016, August 2017, and October 2018 deployments at a daily and synoptic scale. The key meteorological characteristics during the September 2016 deployment are (1) the southern African easterly jet (AEJ-S), centered at around 600 hPa (∼4 km), which strengthens throughout the month in concert with a warming continental heat low, with the strongest winds occurring around 23 September. These advect both aerosol and moisture in the free troposphere. (2) Mid-tropospheric black carbon (BC) is entrained at times into the boundary layer, and (3) convection over land is dry south of about 10∘ S and moist north of 10∘ S. The daily mean low-cloud fraction (low CF) is well correlated with the daily mean high low tropospheric stability (LTS, r=0.44–0.73 over the flight domain; 0–10∘ E, 5–25∘ S) and moderately correlated with the daily mean boundary layer height (BLH, r=0.37–0.52), defined as the altitude of the maximum vertical gradient of moisture. For the August 2017 deployment, the primary meteorological characteristics are that (1) the AEJ-S is at a lower altitude (∼3 km; ∼700 hPa) and further north (5–7∘ S) than in September 2016 and only becomes established by 20 August, with a separate easterly jet present aloft above 500 hPa (∼5.5 km) before that, (2) the mid-tropospheric BC–RH coupling strengthens after the AEJ-S develops, at around 3 km, (3) the daily mean low CF is less closely correlated with the daily mean LTS (r=0.16–0.57) and BLH (r=0.11–0.25) than in September 2016, and (4) dry convection reaches 700 hPa over the Namibian–Kalahari dryland (∼15–25∘ S, ∼18–24∘ E), generating the moist plume subsequently advected over the southeastern Atlantic by the AEJ-S. For the October 2018 deployment, the key meteorological characteristics are that (1) the AEJ-S develops around 600 hPa, driven by the Kalahari heat low (∼10–25∘ S, ∼12–24∘ E), but it diminishes over time as moist continental convection moves southward, (2) the offshore advection of mid-tropospheric BC (∼4 km, 600 hPa) and water vapor are strongly modulated by the AEJ-S around 8–10∘ S in early October, but this decreases as mid-latitude frontal systems develop and the AEJ-S weakens around mid-to-late October, (3) the AEJ-S–low-level jet (LLJ) over the coastal Namibian region relationship is the strongest among all deployment months, and the daily mean low CF is largely reduced by the strong daily mean LLJ, especially to the cloud deck's south, and (4) the relationship of the daily mean low CF with the daily mean BLH and LTS are insignificant compared to the other 2 deployment months, partially due to the variability introduced by the passage of the mid-latitude disturbance.

The southeast Atlantic is home to an expansive smoke aerosol plume overlying a large cloud deck for approximately a third of the year. The aerosol plume is mainly attributed to the extensive biomass burning activity that occurs in southern Africa. Current Earth system models (ESMs) reveal significant differences in their estimates of regional aerosol radiative effects over this region. Such large differences partially stem from uncertainties in the vertical distribution of aerosols in the troposphere. These uncertainties translate into different aerosol optical depths (AOD) in the planetary boundary layer (PBL) and the free troposphere (FT). This study examines differences of AOD fraction in the FT and AOD differences among ESMs (WRF-CAM5, WRF-FINN, GEOS-Chem, EAM-E3SM, ALADIN, GEOS-FP, and MERRA-2) and aircraft-based measurements from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. Models frequently define the PBL as the well-mixed surface-based layer, but this definition misses the upper parts of decoupled PBLs, in which most low-level clouds occur. To account for the presence of decoupled boundary layers in the models, the height of maximum vertical gradient of specific humidity profiles from each model is used to define PBL heights. Results indicate that the monthly mean contribution of AOD in the FT to the total-column AOD ranges from 44 % to 74 % in September 2016 and from 54 % to 71 % in August 2017 within the region bounded by 25°S–0° and 15°W–15°E (excluding land) among the ESMs. Using the second-generation High Spectral Resolution Lidar (HSRL-2) to derive an aircraft-based constraint on the AOD and the fractional AOD, we found that WRF-CAM5 produces 40 % less AOD than those from the HSRL-2 measurements, but it performs well at separating AOD fraction between the FT and the PBL. AOD fractions in the FT for GEOS-Chem and EAM-E3SM are, respectively, 10 % and 15 % lower than the AOD fractions from the HSRL-2 and their similarities in the mean AODs are the result of cancellation of high and low AOD biases. GEOS-FP, MERRA-2, and ALADIN produce 24 %–36 % less AOD and tend to misplace more aerosols in the PBL compared to aircraft-based observations. The models generally underestimate AODs for measured AODs that are above 0.8, indicating their limitations at reproducing high AODs. The differences in the absolute AOD, FT AOD, and the vertical apportioning of AOD in different models highlight the need to continue improving the accuracy of modeled AOD distributions. These differences affect the sign and magnitude of the net aerosol radiative forcing, especially when aerosols are in contact with clouds.

Atmospheric rivers (ARs) are narrow bands of enhanced integrated water vapor transport, modulated by large-scale and synoptic-scale variability. Here, we investigate how ARs and snowpack are shaped by large-scale variability such as arctic oscillation (AO) by examining the synoptic conditions and characteristics of ARs and snowpack in the different phases of AO. Using Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG) data, Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA2) reanalysis data, and in-situ observation data over the eastern Pacific and western United States. we found that more precipitation is observed in lower latitudes (35° N–45° N) during negative AO months and farther north (north of 45° N) in latitude during positive AO months. These are associated with wavelike synoptic patterns in negative AO months and more straight-line type synoptic patterns in positive AO months. The different phases of AO also modulate the AR characteristics: 2.6% less intense (5.3% more intense) integrated water vapor transport and total precipitation, and 16.0% shorter (21.1% longer) duration of ARs than the climatological mean (1980–2019) for positive AO (negative AO) phase. AR frequency is also higher (~50.4%) than the climatological mean for negative AO phase, but there is no statistically significant difference between either negative AO or positive AO phase, especially in southern California. In addition, the snow water equivalent (SWE) tends to be reduced in the positive AO phase and under high-temperature conditions, especially in recent years (2010s). The similar relationships are found in the early 1990s and 2000s, but their statistical significances are low. Considering that lower atmospheric temperature keeps increasing over the eastern Pacific and the western U.S., and SWE tends to be reduced in the positive AO phase in recent years, SWE may decrease over northern California if the warming condition persists. These findings highlight how the characteristics of local extreme weather can be shaped by large-scale climate variability.

Part 1 provided a climatological overview of the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) period and assessed the representativeness of the deployment years. In this Part 2, more detailed meteorological analyses support the interpretation of the airborne measurements for aerosol transport and its interaction with low clouds over the Southeastern (SE) Atlantic Ocean during the September 2016, August 2017, and October 2018 deployments at a daily and synoptic scale. The key meteorological characteristics during the September 2016 deployment are: 1) the southern African easterly jet (AEJ-S), centered at around 600 hPa (~ 4 km), strengthens throughout the month in concert with a warming continental heat low, with the strongest winds occurring around 23 September advecting both aerosol and moisture; 2) The mid-tropospheric black carbon mixing ratio (BC) mixes and is entrained into the boundary layer over the SE Atlantic Ocean; 3) Dry convection is found over land south of about 10° S, with moist convection north of 10° S. The daily-mean low-cloud fraction (low-CF), is well correlated with the daily-mean high low-tropospheric stability (LTS, r = 0.44 – 0.73 over flight regions (0-10° E, 5-25° S)), is also moderately correlated with a low daily-mean boundary layer height (BLH, r = 0.37 – 0.52), which is defined as the altitude of the maximum vertical gradient of moisture. For the August 2017 deployment, the primary meteorological characteristics are: 1) the AEJ-S at a lower altitude (~ 3 km, ~700 hPa) further north (5–7° S) are not established until August 20 with another upper-level easterly jet present aloft above 500 hPa (~ 5.5. km) before that; 2) the mid-tropospheric BC–RH coupling strengthens after the AEJ-S develops, at around 3 km; 3) the enhanced daily-mean low-CF tends to be associated with high daily-mean LTS (r = 0.16 – 0.57) and shallow to the moderate daily-mean BLH (r = 0.11 – 0.25), but its correlation is looser than in September 2016; 4) a layer of dry convection to 700 hPa is found over the Namibian-Kalahari dryland, with the moist plume originating from the dry convection over the land advected over the southeast Atlantic by the AEJ-S. For the October 2018 deployment, the key meteorological characteristics are: 1) the AEJ-S develops around 600 hPa, driven by the Kalahari heat low, but it diminishes as continental convection and precipitation march southward at the end of the month; 2) the advection of mid-tropospheric BC (~ 4 km, 600 hPa), RH, and water vapor are strongly modulated by the AEJ-S around 8–10° S at the early October, but this decreases as mid-latitude frontal systems develop and the AEJ-S weakens around mid- to late October as the season changes; 3) the AEJ-S – low-level jet (LLJ) over the coastal Namibian region relationship is strongest among all deployment months and the daily-mean low-CF is largely reduced by the strong daily-mean LLJ, especially to its south; 4) The relationship of the daily-mean low-CF with the daily-mean BLH and LTS are insignificant compared to the other two deployment months, partially due to the variability associated with the passage of the mid-latitude disturbance.

We examine thermodynamic and kinematic structures of terrain trapped airflows (TTAs) during an atmospheric river (AR) event impacting Northern California 10–11 March 2016 using Alpha Jet Atmospheric eXperiment (AJAX) aircraft data, in situ observations, and Weather and Research Forecasting (WRF) Model simulations. TTAs are identified by locally intensified low-level winds flowing parallel to the coastal ranges and having maxima over the near-coastal waters. Multiple mechanisms can produce TTAs, including terrain blocking and gap flows. The changes in winds can significantly alter the distribution, timing, and intensity of precipitation. We show here how different mechanisms producing TTAs evolve during this event and influence local precipitation variations. Three different periods are identified from the time-varying wind fields. During period 1 (P1), a TTA develops during synoptic-scale onshore flow that backs to southerly flow near the coast. This TTA occurs when the Froude number (Fr) is less than 1, suggesting low-level terrain blocking is the primary mechanism. During period 2 (P2), a Petaluma offshore gap flow develops, with flows turning parallel to the coast offshore and with Fr > 1. Periods P1 and P2 are associated with slightly more coastal than mountain precipitation. In period 3 (P3), the gap flow initiated during P2 merges with a pre-cold-frontal low-level jet (LLJ) and enhanced precipitation shifts to higher mountain regions. Dynamical mixing also becomes more important as the TTA becomes confluent with the approaching LLJ. The different mechanisms producing TTAs and their effects on precipitation pose challenges to observational and modeling systems needed to improve forecasts and early warnings of AR events.

In 2016–2018, the ObsErvation of Aerosols above CLouds and their intEractionS (ORACLES) project undertook three major field campaigns in the Southeast (SE) Atlantic Ocean using research aircraft to better understand the impact of biomass burning (BB) aerosol transport to the SE Atlantic Ocean on climate. In particular, ORACLES was designed to investigate how BB aerosols interact with oceanic stratocumulus clouds, and how that interaction affects the radiation budget. Here, a meteorological analysis has been performed to support the interpretation of the airborne measurements for aerosol transport and its interaction with clouds during the deployments of September 2016, August 2017, and October 2018.The southern African easterly jet (AEJ-S), represented by the zonal wind at 600–700 hPa < −6 m s−1 with entrance over land and exit over the ocean around 5–15° S, is a dominant feature of the mid-level circulation over western Africa during austral winter and spring (August, September, and October). AEJ-S develops at lower altitudes (~3 km, 700 hPa) in the north (5–10° S) in August, while it develops at around 4 km (~600 hPa) in the south (5–15° S) in September and October, largely driven by the strong sensible heating over the African plateau. AEJ-S advects air plumes that are both aerosol-laden and moist, implying a clear potential impact by both on SE Atlantic stratocumulus. Benguela low-level jet (LLJ) also develops off the Namibian coast in the SE Atlantic for all three months. Low-level cloud fraction (low-CF) is positively associated with low-level tropospheric stability (LTS) and negatively associated with boundary layer height (BLH). This relationship is especially strong in September and weaker in August and October. Correlation analysis indicates that simple relationships among low-CF, LTS, and BLH break up when rapidly varying large-scale flow and mid-latitude frontal systems intrude.There are some notable meteorological anomalous characteristics for the three deployment months compared to the climatology: 1) During August 2017, the AEJ-S is slightly weaker than the climatological mean with an anomalous upper-level jet aloft (~6 km) around 10° S. The AEJ-S strength and moisture transport offshore increase at the end of August 2017. August 2017 is also drier over the SE Atlantic than climatology, with a stronger (by ~2 m s−1) LLJ. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is stronger and closer to the coast than the climatological mean. 2) During September 2016, the AEJ-S intensity is slightly weaker than the climatological mean and the mid-level RH is slightly higher than the climatological mean, although differences are small. The LLJ is stronger (~1 m s−1) than the climatological mean. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is stronger than the climatological mean. 3) During October 2018, the AEJ-S is slightly weaker than the climatological mean and slightly wetter compared to the climatological mean. LLJ is also weaker compared to the climatological mean. The large-scale anticyclone associated with St. Helena high in the SE Atlantic is slightly weaker and further southeast than the climatological mean. Precipitation regions migrate southward from August through October due to seasonal change. During all the deployment years, the sea surface temperatures (SST) over the SE Atlantic are warmer than the climatological means, although its impact on low-CF over the deployment region remains unclear at the daily to synoptic time scale.

In southern Africa, widespread agricultural fires produce substantial biomass burning (BB) emissions over the region. The seasonal smoke plumes associated with these emissions are then advected westward over the persistent stratocumulus cloud deck in the Southeast Atlantic (SEA) Ocean, resulting in aerosol effects which vary with time and location. Much work has focused on the effects of these aerosol plumes, but previous studies have also described an elevated free-tropospheric water vapor signal over the SEA. Water vapor influences climate in its own right, and it is especially important to consider atmospheric water vapor when quantifying aerosol-cloud interactions and aerosol radiative effects. Here we present airborne observations made during the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign over the SEA Ocean. In observations collected from multiple independent instruments on the NASA P-3 aircraft (from near-surface to 6–7 km), we observe a strongly linear correlation between pollution indicators (carbon monoxide (CO) and aerosol loading) and atmospheric water vapor content, seen at all altitudes above the boundary layer. The focus of the current study is on the especially strong correlation observed during the ORACLES-2016 deployment (out of Walvis Bay, Namibia), but a similar relationship is also observed in the August 2017 and October 2018 ORACLES deployments. Using ECMWF and MERRA-2 reanalyses and specialized WRF-Chem simulations, we trace the plume-vapor relationship to an initial humid, smoky continental source region, where it is subjected to conditions of strong westward advection, namely the South African Easterly Jet (AEJ-S). Our analysis indicates that airmasses likely left the continent with the same relationship between water vapor and carbon monoxide as was observed by aircraft. This linear relationship developed over the continent due to daytime convection within a deep continental boundary layer (up to 5–6 km) which produced fairly consistent vertical gradients in CO and water vapor, decreasing with altitude and varying in time, but does not originate as a product of the BB combustion itself. Due to a combination of conditions and mixing between the smoky, moist continental boundary layer and the dry and fairly clean upper-troposphere air above (~6 km), the smoky, humid air is transported by strong zonal winds and then advected over the SEA (to the ORACLES flight region) following largely isentropic trajectories. HYSPLIT back trajectories support this interpretation. Better understanding of the conditions and processes which cause the water vapor to covary with plume strength is important to accurately quantify direct, semi-direct, and indirect aerosol effects in this region.

International audience; The Soberanes Fire burned 53,470 ha (132,127 acres) along the central California coast between 22 July and 12 October 2016, generating dense smoke and a variety of gaseous compounds that drifted eastward into the San Joaquin Valley Air Basin (SJVAB), an “extreme” nonattainment area for ozone (O3). These gases included nitrogen oxides (NOx) and volatile organic compounds, the photochemical precursors of O3. The fire started during the California Baseline Ozone Transport Study, a field campaign that brought aircraft, surface, and remote sensing measurements of O3 and related species to central California. In this paper, we use the California Baseline Ozone Transport Study measurements to assess the impact of the Soberanes Fire on ozone and particulate air quality in the SJVAB. We focus our analysis on 27 July to 2 August when the smoke haze was heaviest and the highest O3 concentrations in the SJVAB during 2016 were recorded. Our analyses suggest that while 40 to 60 ppbv of fire‐generated O3 was transported to the eastern SJVAB in the 1‐ to 3‐km‐altitude range, relatively little smoke or fire‐generated O3 reached the surface in the Visalia area.

High ozone (O3) concentrations at low altitudes (1.5e4 km) were detected from airborne Alpha Jet Atmospheric eXperiment (AJAX) measurements on 30 May 2012 off the coast of California (CA). We investigate the causes of those elevated O3 concentrations using airborne measurements and various models. GEOS-Chem simulation shows that the contribution from local sources is likely small. A back trajectory model was used to determine the air mass origins and how much they contributed to the O3 over CA. Low-level potential vorticity (PV) from Modern Era Retrospective analysis for Research and Applications 2 (MERRA-2) reanalysis data appears to be a result of the diabatic heating and mixing of airs in the lower altitudes, rather than be a result of direct transport from stratospheric intrusion. The Q diagnostic, which is a measure of the mixing of the air masses, indicates that there is sufficient mixing along the trajectory to indicate that O3 from the different origins is mixed and transported to the western U.S.The back-trajectory model simulation demonstrates the air masses of interest came mostly from the mid troposphere (MT, 76), but the contribution of the lower troposphere (LT, 19) is also significant compared to those from the upper troposphere/lower stratosphere (UTLS, 5). Air coming from the LT appears to be mostly originating over Asia. The possible surface impact of the high O3 transported aloft on the surface O3 concentration through vertical and horizontal transport within a few days is substantiated by the influence maps determined from the Weather Research and Forecasting Stochastic Time Inverted Lagrangian Transport (WRF-STILT) model and the observed increases in surface ozone mixing ratios. Contrasting this complex case with a stratospheric-dominant event emphasizes the contribution of each source to the high O3 concentration in the lower altitudes over CA. Integrated analyses using models, reanalysis, and diagnostic tools, allows high ozone values detected by in-situ measurements to be attributed to multiple source processes.

The climate, topography, high population and associated emissions of California pose unique challenges in attaining the ozone (O3) National Ambient Air Quality Standard. California's Sierra Nevada Mountains are situated east, and downwind of, the San Joaquin Valley (SJV), which has some of the worst air quality in the US. This work reports surface and vertical O3 observations over the Sierra Nevada Mountains and the SJV from 1998 to 2018. Over this period, the O3 concentrations observed on highly polluted days (95th percentile) decreased, primarily because of summertime emission reductions. Analysis of surface site diurnal data highlights the impacts that the SJV has on the Sierra Nevada Mountains, particularly at lower elevations. O3 typically remains higher overnight at mountain sites, compared to SJV sites, and as a result mountain sites can have high cumulative exposure to O3. Evidence of local and transport effects on O3 trends across the Sierra Nevada Mountains is presented through examining of the timing of O3 exceedances and correlations with different surface sites and with O3 aloft. The data indicate a strong regional influence of emissions from the SJV over the Sierra Nevada Mountains and aloft up to 3 km.

Emission estimates of carbon dioxide (CO2) and methane (CH4) and the meteorological factors affecting them are investigated over Sacramento, California, using an aircraft equipped with a cavity ring–down greenhouse gas sensor as part of the Alpha Jet Atmospheric eXperiment (AJAX) project. To better constrain the emissions fluxes, we designed flights in a cylindrical pattern and computed the emission fluxes from three flights using a kriging method and Gauss's divergence theorem. The CO2 and CH4 mixing ratios at the downwind side of Sacramento show relatively consistent patterns across the three flights, but the fluxes vary – as a function of different wind patterns on a given flight day. The wind variability, seasonality, and assumptions about background concentrations affect the emissions estimates, by a factor of 1.5 to 8. The uncertainty is also impacted by meteorological conditions and distance from the emissions sources. The largest CH4 mixing ratio was found over a local landfill. The importance of vertical mass transfer for flux estimates is examined, but the difference in the total emission estimate with and without vertical mass transfer is found to be small, especially at the local scale. The total flux estimates accounting for the entire circumference are larger than those based solely on the downwind region. This indicates that a closed-shape flight profile can better contain total emissions relative to one-sided curtain flight because most cities have more than one point source and wind direction can change with time and altitude. To reduce the uncertainty of the emissions estimate, it is important that the sampling and modeling strategy account not only for known source locations but also possible unidentified sources around the city. Our results highlight that aircraft-based measurements using a closed shape flight pattern are an efficient and useful strategy for identifying emission sources and estimating local and city-scale greenhouse gas emission fluxes.

Methane (CH4) inventory uncertainties are large, requiring robust emission derivation approaches. We report on a fused airborne–surface data collection approach to derive emissions from an active oil field near Bakersfield, central California. The approach characterizes the atmosphere from the surface to above the planetary boundary layer (PBL) and combines downwind trace gas concentration anomaly (plume) above background with normal winds to derive flux. This approach does not require a well-mixed PBL; allows explicit, data-based, uncertainty evaluation; and was applied to complex topography and wind flows. In situ airborne (collected by AJAX – the Alpha Jet Atmospheric eXperiment) and mobile surface (collected by AMOG – the AutoMObile trace Gas – Surveyor) data were collected on 19 August 2015 to assess source strength. Data included an AMOG and AJAX intercomparison transect profiling from the San Joaquin Valley (SJV) floor into the Sierra Nevada (0.1–2.2 km altitude), validating a novel surface approach for atmospheric profiling by leveraging topography. The profile intercomparison found good agreement in multiple parameters for the overlapping altitude range from 500 to 1500 m for the upper 5 % of surface winds, which accounts for wind-impeding structures, i.e., terrain, trees, buildings, etc. Annualized emissions from the active oil fields were 31.3 ± 16 Gg methane and 2.4 ± 1.2 Tg carbon dioxide. Data showed the PBL was not well mixed at distances of 10–20 km downwind, highlighting the importance of the experimental design.

Author(s): Ryoo, JM; Waliser, DE; Waugh, DW; Wong, S; Fetzer, EJ; Fung, I | Abstract: We investigate transport pathways of water vapor associated with landfalling atmospheric river (AR) events that result in precipitation along the West Coast of the U.S. for winters of 1997-2010. The water vapor transport pathways are determined by computing back trajectories with a trajectory model using the Modern Era Retrospective analysis for Research and Applications reanalysis data set. The majority of AR events (86%) over the West Coast of the U.S. are grouped into three trajectory types, and two of them are closely associated with the AR events. We designate the first type as Ascending near landfall and of Tropical Origin (AT), the second type as Ascending near landfall and of Extratropical Origin (AE), and the third type as Descending or parallel near landfall and of Extratropical Origin (DE), which is accompanied but not directly associated with the AR events. The magnitude and spatial distribution of precipitation of a given AR event are found to be strongly determined by the type of trajectories. In general, AR events composed of both AT and AE trajectories have more frequent precipitation over a broad region of the western U.S. and AR events composed of both AT and DE trajectories have intense precipitation over the southwestern U.S. due to AT trajectories. AR events of AT-only trajectories have intense precipitation, especially over the northwestern U.S., but are less frequent compared to those of AT + AE trajectories. In addition, different patterns of trajectory types among AR events are closely linked to upper level potential vorticity (PV) anomalies; 66% of AR events are associated with anticyclonic Rossby wave breaking events.

Deserts are fragile and highly sensitive ecosystems that increasingly are affected by upwind urban areas and industrial activities. The Los Angeles Basin (LAB) contributes to poor air quality in downwind deserts including the Mojave Desert. Few studies have investigated potential air pollution inputs to the Mojave, whose fragile ecosystem includes endangered plant and animal species. Data were collected on 19 August 2015 by a mobile air quality laboratory, AMOG (AutoMObile trace Gas) Surveyor, that observed inputs can arise from the LAB as well as the San Joaquin Valley (SJV), California. The campaign used a strong methane (CH4) plume as a tracer for the downwind fate of emissions from Bakersfield area petroleum production and also measured ozone (O3). Additional in situ concurrent airborne GHG and O3 data were collected by AJAX - Alpha Jet Atmospheric eXperiment. Both AMOG and AJAX measure winds. Mojave Desert air quality was very poor (visibility ~4 km). Based on the winds, an additional source was inferred beyond the LAB and SJV Basins. Numerical transport modeling and analysis of aerosol lidar data collected the same day by the Cloud Profiling LiDAR onboard the Earth Research-2 stratospheric airplane demonstrated that fires in Northern California were responsible, with prevailing winds transporting air southwards along the eastern Sierra Nevada Range (Bishop Valley) to the Mojave. Whereas the southern and eastern Mojave are impacted by SJV and LAB outflow, the north Mojave generally avoids these inputs. This study shows it can be affected by even distant wildfires, which likely will increase in occurrence and intensity from climate change. Thus, regulatory efforts to reduce air quality impacts on the endangered Mojave ecosystem must include wildfires and also account for the significant differences between different regions of the Mojave. Currently, there is a paucity of studies, highlighting the critical need for field research.

This study demonstrates that water vapor transport and precipitation are largely modulated by the intensity of the subtropical jet, transient eddies, and the location of wave breaking events during the different phases of ENSO. Clear differences are found in the potential vorticity (PV), meteorological fields, and trajectory pathways between the two different phases. Rossby wave breaking events have cyclonic and anticyclonic regimes, with associated differences in the frequency of occurrence and the dynamic response. During La Niña, there is a relatively weak subtropical jet allowing PV to intrude into lower latitudes over the western United States. This induces a large amount of moisture transport inland ahead of the PV intrusions, as well as northward transport to the west of a surface anticyclone. During El Niño, the subtropical jet is relatively strong and is associated with an enhanced cyclonic wave breaking. This is accompanied by a time-mean surface cyclone, which brings zonal moisture transport to the western United States. In both (El Niño and La Niña) phases, there is a high correlation (>0.3–0.7) between upper-level PV at 250 hPa and precipitation over the west coast of the United States with a time lag of 0–1 days. Vertically integrated water vapor fluxes during El Niño are up to 70 kg m−1 s−1 larger than those during La Niña along the west coast of the United States. The zonal and meridional moist static energy flux resembles wave vapor transport patterns, suggesting that they are closely controlled by the large-scale flows and location of wave breaking events during the different phase of ENSO.

Effects of atmospheric river (AR) landfalls in the California coast on the cold-season precipitation in California are examined for the cold seasons of 10 water years (WYs) 2001–2010 using observed data and regional modeling in conjunction with AR-landfall inventory based on visual inspections of precipitable water vapor (PWV) from remote sensing and reanalysis. The PWV in the SSM/I and SSMIS retrievals and the ERA-Interim reanalysis shows 95 AR-landfall days in the California coast that are almost evenly split between the northern and southern coasts across 37.5N. The CPC/NCEP gridded daily precipitation analysis shows that 10–30% of the cold-season precipitation totals in California have occurred during these AR landfalls. The analysis also reveals that the percentage of precipitation and the precipitation intensity during AR landfalls in California are characterized by strong north-to-south gradient. This north–south contrast in the AR precipitation is reversed for the non-AR precipitation in the coastal range. The frequency of AR landfalls and the cold-season precipitation totals in the Sierra Nevada region are only marginally correlated. Instead, AR landfalls are closely related with the occurrence of heavy precipitation events. The freezing-level altitudes are systematically higher for AR wet days than non-AR wet days indicating warmer low-troposphere during AR storms. Cold season simulations for the 10 WYs 2001–2010 show that the Weather Research and Forecast (WRF) model can reasonably simulate important features in both the seasonal and AR precipitation totals. The daily pattern correlation coefficients between the simulated and ERA-Interim upper-air fields exceed 0.9 for most of the period. This suggests that the simulated temporal variations in the atmospheric circulation agree reasonably with the reanalysis over seasonal time scales, characteristics critical for reliable simulations of regional scale hydrologic cycle. The simulated seasonal and AR precipitation totals also agree reasonably with the CPC/NCEP precipitation analysis. The most notable model errors include the overestimation (underestimation) of the season-total and AR precipitation in the northern (southern) California region. The differences in the freezing-level altitudes during the AR- and non-AR wet days in the simulation agree with those from the ERA-Interim reanalysis. The freezing level altitudes are systematically overestimated in the simulations, suggesting warm biases in the low troposphere. Overall, WRF appears to perform reasonably in simulating the key features in the cold season precipitation related with AR landfalls, an important capability for assessing the impact of global climate variations and change on future hydrology in California.

Increasing free tropospheric ozone (O3), combined with the high elevation and often deep boundary layers at western US surface stations, poses challenges in attaining the more stringent 70 ppb O3 National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O3 is increasingly important. This work analyzes surface and vertical O3 observations over California and Nevada from 1995 to 2015. Over this period, the number of high O3 events (95th percentile) at US EPA CASTNET sites has decreased during summer, as a result of decreasing US emissions. In contrast, an increase in springtime 5th percentile O3 indicates a general increase of baseline O3. During 2012 there was a peak in exceedances and in the average spring-summer O3 mixing ratios at CASTNET sites. GEOS-Chem results show that the surface O3 attributable to transport from the upper troposphere and stratosphere were increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O3 measurements from aircraft, ozonesondes and lidar show distinct seasonal trends, with a high percentage of elevated O3 laminae (O3 >70 ppb, 3-8 km) during spring and summer. Analysis of the timing of high O3 surface events and correlation between surface and vertical O3 data is used to discuss varying sources of western US surface O3.

The spatial variations in the probability density functions (PDFs) of relative humidity (RH) in the tropical and subtropical troposphere are examined using observations from the Atmospheric Infrared Sounder (AIRS) and the Microwave Limb Sounder (MLS) instruments together with a simple statistical model. The model, a generalization of that proposed by Sherwood et al., assumes the RH is determined by a combination of drying by uniform subsidence and random moistening events and has two parameters: r, the ratio of the drying time by subsidence to the time between moistening events, and k, a measure of the variability of the moistening events. The observations show that the characteristics of the PDFs vary between the tropics and subtropics, within the tropics or subtropics, and with altitude. The model fits the observed PDFs well, and the model parameters concisely characterize variations in the PDFs and provide information on the processes controlling the RH distributions. In tropical convective regions, the model PDFs that match the observations have large r and small k, indicating rapid random remoistening, which is consistent with direct remoistening in convection. In contrast, in the nonconvective regions there are small r and large k, indicating slower, less random remoistening, consistent with remoistening by slower, quasi-horizontal transport. The statistical model derived will be useful for quantifying differences between, or temporal changes in, RH distributions from different datasets or models, and for examining how changes in physical processes could alter the RH distribution.