scholarly journals Impact of interannual variations in sources of insoluble aerosol species on orographic precipitation over California's central Sierra Nevada

2015 ◽  
Vol 15 (11) ◽  
pp. 6535-6548 ◽  
Author(s):  
J. M. Creamean ◽  
A. P. Ault ◽  
A. B. White ◽  
P. J. Neiman ◽  
F. M. Ralph ◽  
...  
Keyword(s):  
Long Range ◽  
Sierra Nevada ◽  
Biomass Burning ◽  
Water Resource Management ◽  
Cloud Condensation Nuclei ◽  
Convective Cloud ◽  
Orographic Precipitation ◽  
Cloud Systems ◽  
Precipitation Processes ◽  
Aerosol Sources

Abstract. Aerosols that serve as cloud condensation nuclei (CCN) and ice nuclei (IN) have the potential to profoundly influence precipitation processes. Furthermore, changes in orographic precipitation have broad implications for reservoir storage and flood risks. As part of the CalWater field campaign (2009–2011), the variability and associated impacts of different aerosol sources on precipitation were investigated in the California Sierra Nevada using an aerosol time-of-flight mass spectrometer for precipitation chemistry, S-band profiling radar for precipitation classification, remote sensing measurements of cloud properties, and surface meteorological measurements. The composition of insoluble residues in precipitation samples collected at a surface site contained mostly local biomass burning and long-range-transported dust and biological particles (2009), local sources of biomass burning and pollution (2010), and long-range transport (2011). Although differences in the sources of insoluble residues were observed from year to year, the most consistent source of dust and biological residues were associated with storms consisting of deep convective cloud systems with significant quantities of precipitation initiated in the ice phase. Further, biological residues were dominant (up to 40%) during storms with relatively warm cloud temperatures (up to −15 °C), supporting the important role bioparticles can play as ice nucleating particles. On the other hand, lower percentages of residues from local biomass burning and pollution were observed over the three winter seasons (on average 31 and 9%, respectively). When precipitation quantities were relatively low, these insoluble residues most likely served as CCN, forming smaller more numerous cloud droplets at the base of shallow cloud systems, and resulting in less efficient riming processes. Ultimately, the goal is to use such observations to improve the mechanistic linkages between aerosol sources and precipitation processes to produce more accurate predictive weather forecast models and improve water resource management.

scholarly journals Impact of interannual variations in aerosol particle sources on orographic precipitation over California's Central Sierra Nevada

2015 ◽  
Vol 15 (1) ◽  
pp. 931-964 ◽  
Author(s):  
J. M. Creamean ◽  
A. P. Ault ◽  
A. B. White ◽  
P. J. Neiman ◽  
F. M. Ralph ◽  
...  
Keyword(s):  
Long Range ◽  
Sierra Nevada ◽  
Biomass Burning ◽  
Water Resource Management ◽  
Cloud Condensation Nuclei ◽  
Convective Cloud ◽  
Orographic Precipitation ◽  
Cloud Systems ◽  
Precipitation Processes ◽  
Aerosol Sources

Abstract. Aerosols that serve as cloud condensation nuclei (CCN) and ice nuclei (IN) have the potential to profoundly influence precipitation processes. Furthermore, changes in orographic precipitation have broad implications for reservoir storage and flood risks. As part of the CalWater field campaign (2009–2011), the variability and associated impacts of different aerosol sources on precipitation were investigated in the California Sierra Nevada using an aerosol time-of-flight mass spectrometer for precipitation chemistry, S-band profiling radar for precipitation classification, remote sensing measurements of cloud properties, and surface meteorological measurements. The composition of insoluble residues in precipitation samples collected at a surface site contained mostly local biomass burning and long-range transported dust and biological particles (2009), local sources of biomass burning and pollution (2010), and long-range transport from distant sources (2011). Although differences in the sources were observed from year-to-year, the most consistent source of dust and biological residues were associated with storms consisting of deep convective cloud systems with significant quantities of precipitation initiated in the ice phase. Further, biological residues were dominant (up to 40%) during storms with relatively warm cloud temperatures (up to −15 °C), supporting the important role bioparticles can play as ice nucleating particles. On the other hand, lower percentages of residues from local biomass burning and pollution were observed over the three winter seasons (on average 31 and 9%, respectively). When precipitation quantities were relatively low, these residues most likely served as CCN, forming smaller more numerous cloud droplets at the base of shallow cloud systems, and resulting in less efficient riming processes. The correlation between the source of aerosols within clouds and precipitation type and quantity will be further probed in models to understand the mechanisms by which local emissions vs. long-range transported dust and biological aerosols play roles in impacting regional precipitation processes. Ultimately, the goal is to use such observations to improve the mechanistic linkages between aerosol sources and precipitation processes to produce more accurate predictive weather forecast models and improve water resource management.

scholarly journals Long range transport and mixing of aerosol sources during the 2013 North American biomass burning episode: analysis of multiple lidar observations in the Western Mediterranean basin

2015 ◽  
Vol 15 (22) ◽  
pp. 32323-32365 ◽  
Author(s):  
G. Ancellet ◽  
J. Pelon ◽  
J. Totems ◽  
P. Chazette ◽  
A. Bazureau ◽  
...  
Keyword(s):  
Long Range ◽  
Biomass Burning ◽  
North American ◽  
Western Mediterranean ◽  
Saharan Dust ◽  
Trade Wind ◽  
Long Range Transport ◽  
Range Transport ◽  
Aerosol Sources ◽  
Aerosol Source

Abstract. Long range transport of biomass burning (BB) aerosols between North America and the Mediterranean region took place in June 2013. A large number of ground based and airborne lidar measurements were deployed in the Western Mediterranean during the Chemistry-AeRosol Mediterranean EXperiment (ChArMEx) intensive observation period. A detailed analysis of the potential North American aerosol sources is conducted including the assessment of their transport to Europe using forward simulations of the FLEXPART Lagrangian particle dispersion model initialized using satellite observations by MODIS and CALIOP. The three dimensional structure of the aerosol distribution in the ChArMEx domain observed by the ground-based lidars (Menorca, Barcelona and Lampedusa), a Falcon-20 aircraft flight and three CALIOP tracks, agree very well with the model simulation of the three major sources considered in this work: Canadian and Colorado fires, a dust storm from Western US and the contribution of Saharan dust streamers advected from the North Atlantic trade wind region into the Westerlies region. Four aerosol types were identified using the optical properties of the observed aerosol layers (aerosol depolarization ratio, lidar ratio) and the transport model analysis of the contribution of each aerosol source: (I) pure BB layer, (II) weakly dusty BB, (III) significant mixture of BB and dust transported from the trade wind region (IV) the outflow of Saharan dust by the subtropical jet and not mixed with BB aerosol. The contribution of the Canadian fires is the major aerosol source during this episode while mixing of dust and BB is only significant at altitude above 5 km. The mixing corresponds to a 20–30 % dust contribution in the total aerosol backscatter. The comparison with the MODIS AOD horizontal distribution during this episode over the Western Mediterranean sea shows that the Canadian fires contribution were as large as the direct northward dust outflow from Sahara.

scholarly journals Long-range transport and mixing of aerosol sources during the 2013 North American biomass burning episode: analysis of multiple lidar observations in the western Mediterranean basin

2016 ◽  
Vol 16 (7) ◽  
pp. 4725-4742 ◽  
Author(s):  
Gerard Ancellet ◽  
Jacques Pelon ◽  
Julien Totems ◽  
Patrick Chazette ◽  
Ariane Bazureau ◽  
...  
Keyword(s):  
Long Range ◽  
Biomass Burning ◽  
North American ◽  
Western Mediterranean ◽  
Saharan Dust ◽  
Trade Wind ◽  
Long Range Transport ◽  
Range Transport ◽  
Aerosol Sources ◽  
Aerosol Source

Abstract. Long-range transport of biomass burning (BB) aerosols between North America and the Mediterranean region took place in June 2013. A large number of ground-based and airborne lidar measurements were deployed in the western Mediterranean during the Chemistry-AeRosol Mediterranean EXperiment (ChArMEx) intensive observation period. A detailed analysis of the potential North American aerosol sources is conducted including the assessment of their transport to Europe using forward simulations of the FLEXPART Lagrangian particle dispersion model initialized using satellite observations by MODIS and CALIOP. The three-dimensional structure of the aerosol distribution in the ChArMEx domain observed by the ground-based lidars (Minorca, Barcelona and Lampedusa), a Falcon-20 aircraft flight and three CALIOP tracks, agrees very well with the model simulation of the three major sources considered in this work: Canadian and Colorado fires, a dust storm from western US and the contribution of Saharan dust streamers advected from the North Atlantic trade wind region into the westerlies region. Four aerosol types were identified using the optical properties of the observed aerosol layers (aerosol depolarization ratio, lidar ratio) and the transport model analysis of the contribution of each aerosol source: (i) pure BB layer, (ii) weakly dusty BB, (iii) significant mixture of BB and dust transported from the trade wind region, and (iv) the outflow of Saharan dust by the subtropical jet and not mixed with BB aerosol. The contribution of the Canadian fires is the major aerosol source during this episode while mixing of dust and BB is only significant at an altitude above 5 km. The mixing corresponds to a 20–30 % dust contribution in the total aerosol backscatter. The comparison with the MODIS aerosol optical depth horizontal distribution during this episode over the western Mediterranean Sea shows that the Canadian fire contributions were as large as the direct northward dust outflow from Sahara.

scholarly journals Long-term observations of cloud condensation nuclei over the Amazon rain forest – Part 2: Variability and characteristics of biomass burning, long-range transport, and pristine rain forest aerosols

2018 ◽  
Vol 18 (14) ◽  
pp. 10289-10331 ◽  
Author(s):  
Mira L. Pöhlker ◽  
Florian Ditas ◽  
Jorge Saturno ◽  
Thomas Klimach ◽  
Isabella Hrabě de Angelis ◽  
...  
Keyword(s):  
Water Vapor ◽  
Long Range ◽  
Biomass Burning ◽  
Rain Forest ◽  
Cloud Condensation Nuclei ◽  
Long Range Transport ◽  
Accumulation Mode ◽  
Range Transport ◽  
Mass Fractions ◽  
Vapor Supersaturation

Abstract. Size-resolved measurements of atmospheric aerosol and cloud condensation nuclei (CCN) concentrations and hygroscopicity were conducted over a full seasonal cycle at the remote Amazon Tall Tower Observatory (ATTO, March 2014–February 2015). In a preceding companion paper, we presented annually and seasonally averaged data and parametrizations (Part 1; Pöhlker et al., 2016a). In the present study (Part 2), we analyze key features and implications of aerosol and CCN properties for the following characteristic atmospheric conditions: Empirically pristine rain forest (PR) conditions, where no influence of pollution was detectable, as observed during parts of the wet season from March to May. The PR episodes are characterized by a bimodal aerosol size distribution (strong Aitken mode with DAit ≈ 70 nm and NAit ≈ 160 cm−3, weak accumulation mode with Dacc ≈ 160 nm and Nacc≈ 90 cm−3), a chemical composition dominated by organic compounds, and relatively low particle hygroscopicity (κAit≈ 0.12, κacc ≈ 0.18). Long-range-transport (LRT) events, which frequently bring Saharan dust, African biomass smoke, and sea spray aerosols into the Amazon Basin, mostly during February to April. The LRT episodes are characterized by a dominant accumulation mode (DAit ≈ 80 nm, NAit ≈ 120 cm−3 vs. Dacc ≈ 180 nm, Nacc ≈ 310 cm−3), an increased abundance of dust and salt, and relatively high hygroscopicity (κAit≈ 0.18, κacc ≈ 0.35). The coarse mode is also significantly enhanced during these events. Biomass burning (BB) conditions characteristic for the Amazonian dry season from August to November. The BB episodes show a very strong accumulation mode (DAit ≈ 70 nm, NAit ≈ 140 cm−3 vs. Dacc ≈ 170 nm, Nacc ≈ 3400 cm−3), very high organic mass fractions (∼ 90 %), and correspondingly low hygroscopicity (κAit≈ 0.14, κacc ≈ 0.17). Mixed-pollution (MPOL) conditions with a superposition of African and Amazonian aerosol emissions during the dry season. During the MPOL episode presented here as a case study, we observed African aerosols with a broad monomodal distribution (D ≈ 130 nm, NCN,10 ≈ 1300 cm−3), with high sulfate mass fractions (∼ 20 %) from volcanic sources and correspondingly high hygroscopicity (κ< 100 nm ≈ 0.14, κ>100nm≈ 0.22), which were periodically mixed with fresh smoke from nearby fires (D ≈ 110 nm, NCN,10 ≈ 2800 cm−3) with an organic-dominated composition and sharply decreased hygroscopicity (κ<150nm≈ 0.10, κ>150nm≈ 0.20). Insights into the aerosol mixing state are provided by particle hygroscopicity (κ) distribution plots, which indicate largely internal mixing for the PR aerosols (narrow κ distribution) and more external mixing for the BB, LRT, and MPOL aerosols (broad κ distributions). The CCN spectra (CCN concentration plotted against water vapor supersaturation) obtained for the different case studies indicate distinctly different regimes of cloud formation and microphysics depending on aerosol properties and meteorological conditions. The measurement results suggest that CCN activation and droplet formation in convective clouds are mostly aerosol-limited under PR and LRT conditions and updraft-limited under BB and MPOL conditions. Normalized CCN efficiency spectra (CCN divided by aerosol number concentration plotted against water vapor supersaturation) and corresponding parameterizations (Gaussian error function fits) provide a basis for further analysis and model studies of aerosol–cloud interactions in the Amazon.

scholarly journals Influx of African biomass burning aerosol during the Amazonian dry season through layered transatlantic transport of black carbon-rich smoke

2020 ◽  
Vol 20 (8) ◽  
pp. 4757-4785 ◽  
Author(s):  
Bruna A. Holanda ◽  
Mira L. Pöhlker ◽  
David Walter ◽  
Jorge Saturno ◽  
Matthias Sörgel ◽  
...  
Keyword(s):  
Black Carbon ◽  
Long Range ◽  
Biomass Burning ◽  
Amazon Basin ◽  
Hydrological Cycle ◽  
Atmospheric Transport ◽  
Cloud Condensation Nuclei ◽  
Number Concentration ◽  
Brazilian Coast ◽  
Strong Impact

Abstract. Black carbon (BC) aerosols influence the Earth's atmosphere and climate, but their microphysical properties, spatiotemporal distribution, and long-range transport are not well constrained. This study presents airborne observations of the transatlantic transport of BC-rich African biomass burning (BB) smoke into the Amazon Basin using a Single Particle Soot Photometer (SP2) as well as several complementary techniques. We base our results on observations of aerosols and trace gases off the Brazilian coast onboard the HALO (High Altitude and LOng range) research aircraft during the ACRIDICON-CHUVA campaign in September 2014. During flight AC19 over land and ocean at the northeastern coastline of the Amazon Basin, we observed a BC-rich layer at ∼3.5 km altitude with a vertical extension of ∼0.3 km. Backward trajectories suggest that fires in African grasslands, savannas, and shrublands were the main source of this pollution layer and that the observed BB smoke had undergone more than 10 d of atmospheric transport and aging over the South Atlantic before reaching the Amazon Basin. The aged smoke is characterized by a dominant accumulation mode, centered at about 130 nm, with a particle concentration of Nacc=850±330 cm−3. The rBC particles account for ∼15 % of the submicrometer aerosol mass and ∼40 % of the total aerosol number concentration. This corresponds to a mass concentration range from 0.5 to 2 µg m−3 (1st to 99th percentiles) and a number concentration range from 90 to 530 cm−3. Along with rBC, high cCO (150±30 ppb) and cO3 (56±9 ppb) mixing ratios support the biomass burning origin and pronounced photochemical aging of this layer. Upon reaching the Amazon Basin, it started to broaden and to subside, due to convective mixing and entrainment of the BB aerosol into the boundary layer. Satellite observations show that the transatlantic transport of pollution layers is a frequently occurring process, seasonally peaking in August/September. By analyzing the aircraft observations together with the long-term data from the Amazon Tall Tower Observatory (ATTO), we found that the transatlantic transport of African BB smoke layers has a strong impact on the northern and central Amazonian aerosol population during the BB-influenced season (July to December). In fact, the early BB season (July to September) in this part of the Amazon appears to be dominated by African smoke, whereas the later BB season (October to December) appears to be dominated by South American fires. This dichotomy is reflected in pronounced changes in aerosol optical properties such as the single scattering albedo (increasing from 0.85 in August to 0.90 in November) and the BC-to-CO enhancement ratio (decreasing from 11 to 6 ng m−3 ppb−1). Our results suggest that, despite the high fraction of BC particles, the African BB aerosol acts as efficient cloud condensation nuclei (CCN), with potentially important implications for aerosol–cloud interactions and the hydrological cycle in the Amazon.

Impact of Long-Range Transport Biomass Burning (BB) Emissions on Cloud Condensation Nuclei (CCN) Activation in Continental Polluted Air of Delhi, India

2020 ◽  
Author(s):  
Subha S. Raj ◽  
Mira L. Pӧhlker ◽  
Thomas Klimach ◽  
Jan-David Förster ◽  
David Walter ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Diurnal Variation ◽  
Long Range ◽  
Biomass Burning ◽  
Mass Fraction ◽  
Cloud Condensation Nuclei ◽  
Average Concentration ◽  
Long Range Transport ◽  
Organic Mass ◽  
Range Transport

&lt;p&gt;Cloud Condensation Nuclei (CCN) and other aerosol properties were investigated in Delhi, India, from Feb. to Mar. 2018. The high anthropogenic influence on aerosol was studied with size-resolved CCN measurements (supersaturation (&lt;em&gt;S&lt;/em&gt;) between 0.13 to 0.66% and selected diameters from 10 to 300 nm). Furthermore the chemical composition (Aerosol Chemical Speciation Monitor and Aethalometer AE33) of the particles was measured. The aerosol number size distribution was derived by size data inversion of Differential Mobility Particle Sizer (DMPS) from size-resolved CCN measurements. Based on multi-year back trajectory (BT) data, a spatial clustering analysis was done for the actual campaign period and two distinct clusters were identified: northwest- west northwest-long range transport (NW-LRT) and south-southeast-east southeast (SE).&lt;/p&gt;&lt;p&gt;There was preponderant organic mass fraction (&lt;em&gt;f&lt;/em&gt;&lt;sub&gt;org)&lt;/sub&gt; in the aerosols throughout the campaign, with prominent diurnal variation except during the SE period. Pronounced diurnal variation was observed also in black carbon (BC) with an average concentration of 16 &lt;em&gt;&amp;#181;&lt;/em&gt;g/m&lt;sup&gt;3&lt;/sup&gt; during NW-LRT, in contrast to a weak diurnal cycle with lower average concentration of 8 &lt;em&gt;&amp;#181;&lt;/em&gt;g/m&lt;sup&gt;3&lt;/sup&gt; during SE. During the NW-LRT cluster the air masses traversed over agriculture fields with biomass burning (BB) activities identified using the fire radiative power (FRP) observations of Copernicus Atmosphere Monitoring Service (CAMS) Global Fire Assimilation System (GFAS). So it can be speculated that the BB emissions from the fields have contributed to enhanced BC concentrations during this period over Delhi. The remaining period, showing a mixture of local and long-range transported emissions also had a BC concentration higher than SE period when only local/regional emissions were observed. This is an important insight into the air pollution apocalypse in Delhi.&lt;/p&gt;&lt;p&gt;The overall average values of critical dry diameter (&lt;em&gt;D&lt;/em&gt;&lt;sub&gt;c&lt;/sub&gt;) for CCN activation varied from 54 &amp;#177; 8 nm at &lt;em&gt;S&lt;/em&gt; = 0.66% to 139 &amp;#177; 12 nm at &lt;em&gt;S&lt;/em&gt; = 0.13%.The hygroscopicity parameter derived from CCN data (&lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;CCN&lt;/sub&gt;) was in the range from 0.1 to 0.9 with an arithmetic mean of 0.27 &amp;#177; 0.10, which is close to that of Beijing, another polluted continental region (0.31 &amp;#177; 0.08, Gunthe et al., 2011). &lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;CCN&lt;/sub&gt; also shows good agreement with the hygroscopicity parameter derived from the chemical composition measurements. A linear fit (Gunthe et al., 2009) applied to the relationship between refractory/non-refractory organic mass fraction and &lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;CCN &lt;/sub&gt;at &lt;em&gt;S&lt;/em&gt; = 0.13%, gives an effective hygroscopicity parameter &lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;org&lt;/sub&gt; = 0.17 &amp;#177; 0.09 and &lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;inorg&lt;/sub&gt; = 0.80 &amp;#177; 0.09, when extrapolated to &lt;em&gt;f&lt;/em&gt;&lt;sub&gt;org&lt;/sub&gt; = 1 and &lt;em&gt;f&lt;/em&gt;&lt;sub&gt;org&lt;/sub&gt; = 0, respectively. The presence of externally mixed inactive CCN particles is indicated by an average maximum activated fraction (&lt;em&gt;MAF&lt;/em&gt;) of 0.82 &amp;#177; 0.17 at &lt;em&gt;S&lt;/em&gt; = 0.13%. The overall average &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;c&lt;/sub&gt;, &lt;em&gt;&amp;#1082;&lt;/em&gt;&lt;sub&gt;CCN&lt;/sub&gt;, and &lt;em&gt;MAF &lt;/em&gt;did not vary much between NW-LRT and SE periods, although the particle number concentration was higher during NW-LRT. Moreover, high CCN efficiency was observed during NW-LRT, in spite of its enhanced BC concentration, indicating the presence of aged internally mixed aerosols. Further details will be presented.&lt;/p&gt;

Impact of field biomass burning on local pollution and long-range transport of PM2.5 in Northeast Asia

2019 ◽  
Vol 244 ◽  
pp. 414-422 ◽  
Author(s):  
Katsushige Uranishi ◽  
Fumikazu Ikemori ◽  
Hikari Shimadera ◽  
Akira Kondo ◽  
Seiji Sugata
Keyword(s):  
Long Range ◽  
Biomass Burning ◽  
Northeast Asia ◽  
Long Range Transport ◽  
Local Pollution ◽  
Range Transport

scholarly journals Colorado air quality impacted by long-range-transported aerosol: a set of case studies during the 2015 Pacific Northwest fires

2016 ◽  
Vol 16 (18) ◽  
pp. 12329-12345 ◽  
Author(s):  
Jessie M. Creamean ◽  
Paul J. Neiman ◽  
Timothy Coleman ◽  
Christoph J. Senff ◽  
Guillaume Kirgis ◽  
...  
Keyword(s):  
Air Quality ◽  
Case Studies ◽  
Pacific Northwest ◽  
Long Range ◽  
Biomass Burning ◽  
Forest Fires ◽  
Surface Radiation ◽  
Cloud Formation ◽  
Synoptic Scale ◽  
Transport Pathways

Abstract. Biomass burning plumes containing aerosols from forest fires can be transported long distances, which can ultimately impact climate and air quality in regions far from the source. Interestingly, these fires can inject aerosols other than smoke into the atmosphere, which very few studies have evidenced. Here, we demonstrate a set of case studies of long-range transport of mineral dust aerosols in addition to smoke from numerous fires (including predominantly forest fires and a few grass/shrub fires) in the Pacific Northwest to Colorado, US. These aerosols were detected in Boulder, Colorado, along the Front Range using beta-ray attenuation and energy-dispersive X-ray fluorescence spectroscopy, and corroborated with satellite-borne lidar observations of smoke and dust. Further, we examined the transport pathways of these aerosols using air mass trajectory analysis and regional- and synoptic-scale meteorological dynamics. Three separate events with poor air quality and increased mass concentrations of metals from biomass burning (S and K) and minerals (Al, Si, Ca, Fe, and Ti) occurred due to the introduction of smoke and dust from regional- and synoptic-scale winds. Cleaner time periods with good air quality and lesser concentrations of biomass burning and mineral metals between the haze events were due to the advection of smoke and dust away from the region. Dust and smoke present in biomass burning haze can have diverse impacts on visibility, health, cloud formation, and surface radiation. Thus, it is important to understand how aerosol populations can be influenced by long-range-transported aerosols, particularly those emitted from large source contributors such as wildfires.

scholarly journals Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

2010 ◽  
Vol 67 (4) ◽  
pp. 999-1025 ◽  
Author(s):  
Scott A. Braun ◽  
Michael T. Montgomery ◽  
Kevin J. Mallen ◽  
Paul D. Reasor
Keyword(s):  
Sea Level ◽  
Tropical Storm ◽  
Sea Level Pressure ◽  
Cloud Systems ◽  
Low Level ◽  
Level Pressure ◽  
Stratiform Region ◽  
Stratiform Precipitation ◽  
Precipitation Processes ◽  
The Mean

Abstract Several hypotheses have been put forward for the mechanisms of generation of surface circulation associated with tropical cyclones. This paper examines high-resolution simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during NASA’s Tropical Cloud Systems and Processes Experiment, to investigate the development of low-level circulation and its relationship to the precipitation evolution. Two simulations are examined: one that better matches available observations but underpredicts the storm’s minimum sea level pressure and a second one that somewhat overintensifies the storm but provides a set of simulations that encapsulates the overall genesis and development characteristics of the observed storm. The roles of convective and stratiform precipitation processes within the mesoscale precipitation systems that formed Gert are discussed. During 21–25 July, two episodes of convective system development occurred. In each, precipitation system evolution was characterized by intense and deep convective upward motions followed by increasing stratiform-type vertical motions (upper-level ascent, low-level descent). Potential vorticity (PV) in convective regions was strongest at low levels while stratiform-region PV was strongest at midlevels, suggesting that convective processes acted to spin up lower levels prior to the spinup of middle levels by stratiform processes. Intense vortical hot towers (VHTs) were prominent features of the low-level cyclonic vorticity field. The most prominent PV anomalies persisted more than 6 h and were often associated with localized minima in the sea level pressure field. A gradual aggregation of the cyclonic PV occurred as existing VHTs near the center continually merged with new VHTs, gradually increasing the mean vorticity near the center. Nearly concurrently with this VHT-induced development, stratiform precipitation processes strongly enhanced the mean inflow and convergence at middle levels, rapidly increasing the midlevel vorticity. However, the stratiform vertical motion profile is such that while it increases midlevel vorticity, it decreases vorticity near the surface as a result of low-level divergence. Consequently, the results suggest that while stratiform precipitation regions may significantly increase cyclonic circulation at midlevels, convective vortex enhancement at low to midlevels is likely necessary for genesis.

Sign in / Sign up

Export Citation Format

Share Document