• Climatology of Linear Mesoscale Convective System Morphology in the United States Based on the Random-Forests Method

      Cui, W.; Dong, X.; Xi, B.; Feng, Z.; Department of Hydrology and Atmospheric Sciences, University of Arizona (American Meteorological Society, 2021)
      This study uses machine-learning methods, specifically the random-forests (RF) method, on a radar-based mesoscale convective system (MCS) tracking dataset to classify the five types of linear MCS morphology in the contiguous United States during the period 2004-16. The algorithm is trained using radar- A nd satellite-derived spatial and morphological parameters, along with reanalysis environmental information from a 5-yr manually identified nonlinear mode and five linear MCS modes. The algorithm is then used to automate the classification of linear MCSs over 8 years with high accuracy, providing a systematic, long-term climatology of linear MCSs. Results reveal that nearly 40% of MCSs are classified as linear MCSs, of which one-half of the linear events belong to the type of system having a leading convective line. The occurrence of linear MCSs shows large annual and seasonal variations. On average, 113 linear MCSs occur annually during the warm season (March-October), with most of these events clustered from May through August in the central eastern Great Plains. MCS characteristics, including duration, propagation speed, orientation, and system cloud size, have large variability among the different linear modes. The systems having a trailing convective line and the systems having a back-building area of convection typically move more slowly and have higher precipitation rate, and thus they have higher potential for producing extreme rainfall and flash flooding. Analysis of the environmental conditions associated with linear MCSs show that the storm-relative flow is of most importance in determining the organization mode of linear MCSs. ©2021 American Meteorological Society.
    • Cloud phase and macrophysical properties over the Southern Ocean during the MARCUS field campaign

      Xi, B.; Dong, X.; Zheng, X.; Wu, P.; Department of Hydrology and Atmospheric Sciences, University of Arizona (Copernicus GmbH, 2022)
      To investigate the cloud phase and macrophysical properties over the Southern Ocean (SO), the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF2) was installed on the Australian icebreaker research vessel (R/V) Aurora Australis during the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign (41 to 69°S, 60 to 160°E) from October 2017 to March 2018. To examine cloud properties over the midlatitude and polar regions, the study domain is separated into the northern (NSO) and southern (SSO) parts of the SO, with a demarcation line of 60°S. The total cloud fractions (CFs) were 77.9%, 67.6%, and 90.3% for the entire domain, NSO and SSO, respectively, indicating that higher CFs were observed in the polar region. Low-level clouds and deep convective clouds are the two most common cloud types over the SO. A new method was developed to classify liquid, mixed-phase, and ice clouds in single-layered, low-level clouds (LOW), where mixed-phase clouds dominate with an occurrence frequency (Freq) of 54.5%, while the Freqs of the liquid and ice clouds were 10.1% (most drizzling) and 17.4% (least drizzling). The meridional distributions of low-level cloud boundaries are nearly independent of latitude, whereas the cloud temperatures increased by ∼1/48K, and atmospheric precipitable water vapor increased from ∼1/45mm at 69°S to ∼1/418mm at 43°S. The mean cloud liquid water paths over NSO were much larger than those over SSO. Most liquid clouds occurred over NSO, with very few over SSO, whereas more mixed-phase clouds occurred over SSO than over NSO. There were no significant differences for the ice cloud Freq between NSO and SSO. The ice particle sizes are comparable to cloud droplets and drizzle drops and well mixed in the cloud layer. These results will be valuable for advancing our understanding of the meridional and vertical distributions of clouds and can be used to improve model simulations over the SO. © 2022 Baike Xi et al.
    • Environmental effects on aerosol-cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

      Zheng, X.; Xi, B.; Dong, X.; Wu, P.; Logan, T.; Wang, Y.; Department of Hydrology and Atmospheric Sciences, University of Arizona (Copernicus GmbH, 2022)
      Over the eastern North Atlantic (ENA) ocean, a total of 20 non-precipitating single-layer marine boundary layer (MBL) stratus and stratocumulus cloud cases are selected to investigate the impacts of the environmental variables on the aerosol-cloud interaction (ACIr) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016-2018. The ACIr represents the relative change in cloud droplet effective radius re with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2ĝ€¯% supersaturation (NCCN,0.2%) in the stratified water vapor environment. The ACIr values vary from -0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that re is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher Nc and PWVBL. The principal component analysis shows that the most pronounced pattern during the selected cases is the co-variations in the MBL conditions characterized by the vertical component of turbulence kinetic energy (TKEw), the decoupling index (Di), and PWVBL. The environmental effects on ACIr emerge after the data are stratified into different TKEw regimes. The ACIr values, under both lower and higher PWVBL conditions, more than double from the low-TKEw to high-TKEw regime. This can be explained by the fact that stronger boundary layer turbulence maintains a well-mixed MBL, strengthening the connection between cloud microphysical properties and the below-cloud CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size spectra and consequently results in an enlargement of re. The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease re, which jointly presents as the increased ACIr. This study examines the importance of environmental effects on the ACIr assessments and provides observational constraints to future model evaluations of aerosol-cloud interactions. © Copyright:
    • Maritime Aerosol and CCN Profiles Derived From Ship-Based Measurements Over Eastern North Pacific During MAGIC

      Brendecke, J.; Dong, X.; Xi, B.; Zheng, X.; Department of Hydrology and Atmospheric Sciences, University of Arizona (John Wiley and Sons Inc, 2022)
      Atmospheric aerosols are widely recognized to give rise to a substantial radiative forcing to the climate by scattering and absorbing radiation and by modifying the microphysical, lifetime, and radiative properties of clouds. During the Marine ARM GPCI Investigation of Clouds (MAGIC) over the Eastern North Pacific (ENP), the ship-based measured cloud condensation nuclei (CCN) concentration at 0.2% supersaturation (NCCN,0.2) and condensation nuclei concentration (NCN) had mean values of 116.7 and 219.4 cm−3, with the highest concentrations found closest to LA due to an increase in aerosol sources. Moving westward, both NCCN,0.2 and NCN gradually decreased until stabilizing near 100 cm−3 and 200 cm−3, respectively. Using the methods proposed by Ghan and Collins (2004), https://doi.org/10.1175/1520-0426(2004)021<0387:uoisdt>2.0.co;2 and Ghan et al. (2006), https://doi.org/10.1029/2004jd005752, NCCN,0.2 profiles are retrieved using the surface measured NCCN,0.2 as a constraint. For coupled conditions, correlations between the retrieved NCCN,0.2 profiles and cloud-droplet number concentration (NC) increase from 0.26 at the surface to 0.38 near cloud base, particularly true for non-drizzling clouds. Although the correlations are lower than expected, the percentage increase (46.2%) is encouraging. Finally, the relationships between cloud breakup (CB) and the stratocumulus to cumulus transition (SCT) with environmental conditions and associated aerosols are also studied. The decreased NCN trend east of CB is mainly caused by precipitation scavenging, while the increased NCN trend west of CB is strongly associated with the increased surface wind speed and fewer drizzle events. A further study is needed using high-resolution models to simulate these events. © 2022 The Authors.
    • Maritime Cloud and Drizzle Microphysical Properties Retrieved From Ship-Based Observations During MAGIC

      Brendecke, J.; Dong, X.; Xi, B.; Wu, P.; Department of Hydrology and Atmospheric Sciences, University of Arizona (Blackwell Publishing Ltd, 2021)
      The Marine ARM GPCI Investigation of Clouds (MAGIC) field campaign provided a wealth of information looking at the stratocumulus to cumulus transition (SCT) over the Eastern-North Pacific (ENP), however, the lack of cloud in situ measurements gave limited information. Using the observations of Marine W-band ARM cloud radar, ceilometer, and three-channel microwave radiometer onboard the ship, we retrieve the single-layer, low-level cloud-droplet effective radius and drizzle median radius (rc and rm,d), number concentration (Nc and Nd), and liquid water content (LWCc and LWCd) using the methods in Wu et al. (2020, https://doi.org/10.1029/2019JD032205). Based on the results during MAGIC, we found that both cloud base and top heights increase approximately 0.75 km from Los Angeles (LA) until cloud breakup (CB) before leveling off. Low cloud fractions (CFs) ranged from ∼85% halfway between LA and CB to ∼20% near Hawaii. Retrieved rc values decreased approximately 2 μm from peak CF to Hawaii while rm,d increased more than 20 μm over the same path. Mean values of rc, rm,d, Nc, Nd, LWCc, and LWCd during MAGIC are 12.1 μm, 55.8 μm, 97.9 cm−3, 0.09 cm−3, 0.40 g m−3, and 0.05 g m−3, respectively. Compared to the mean values over the Azores in Wu et al. (2020, https://doi.org/10.1029/2019JD032205), the mean cloud and drizzle microphysical properties during MAGIC, except LWCd which is roughly equal, are greater due to higher liquid water path and warmer sea surface temperature. This information allows for a better understanding of the SCT over the ENP and can be used to better improve model simulations. © 2021. The Authors.
    • New Observational Constraints on Warm Rain Processes and Their Climate Implications

      Dong, X.; Wu, P.; Wang, Y.; Xi, B.; Huang, Y.; Department of Hydrology and Atmospheric Sciences, University of Arizona (Blackwell Publishing Ltd, 2021)
      Low stratiform clouds have profound impacts on the hydrological cycle and the Earth’s radiation budget. However, realistic simulation of low clouds in climate models presents a major challenge. Here we employ the newly retrieved cloud and drizzle microphysical properties to improve the autoconversion and accretion parameterizations in a microphysical scheme. We find that the new autoconversion (accretion) rate contributes 14% lower (greater) to total drizzle water content than the original scheme near the cloud top. Compared to satellite results, the simulated cloud liquid water path (LWP) and shortwave cloud radiative effect using the original scheme in a climate model agree well on global average but with large regional differences. Simulations using the updated scheme show a 7.3% decrease in the light rain frequency, and a 10% increase in LWP. The updated microphysics scheme alleviates the long-lasting problem in most climate models, that is “too frequent and too light precipitation.”. © 2021. American Geophysical Union. All Rights Reserved.
    • Quantifying Long-Term Seasonal and Regional Impacts of North American Fire Activity on Continental Boundary Layer Aerosols and Cloud Condensation Nuclei

      Logan, T.; Dong, X.; Xi, B.; Zheng, X.; Wang, Y.; Wu, P.; Marlow, E.; Maddux, J.; Department of Hydrology and Atmospheric Sciences, University of Arizona; Department of Hydrology and Atmospheric Sciences, University of Arizona (Blackwell Publishing Ltd, 2020)
      An intimate knowledge of aerosol transport is essential in reducing the uncertainty of the impacts of aerosols on cloud development. Data sets from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement platform in the Southern Great Plains region (ARM-SGP) and the National Aeronautics and Space Administration (NASA) Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), showed seasonal increases in aerosol loading and total carbon concentration during the spring and summer months (2008–2016) which was attributed to fire activity and smoke transport within North America. The monthly mean MERRA-2 surface carbonaceous aerosol mass concentration and ARM-SGP total carbon products were strongly correlated (R = 0.82, p < 0.01) along with a moderate correlation with the ARM-SGP cloud condensation nuclei (NCCN) product (0.5, p ~ 0.1). The monthly mean ARM-SGP total carbon and NCCN products were strongly correlated (0.7, p ~ 0.01). An additional product denoting fire number and coverage taken from the National Interagency Fire Center (NIFC) showed a moderate correlation with the MERRA-2 carbonaceous product (0.45, p < 0.01) during the 1981–2016 warm season months (March–September). With respect to meteorological conditions, the correlation between the NIFC fire product and MERRA-2 850-hPa isobaric height anomalies was lower (0.26, p ~ 0.13) due to the variability in the frequency, intensity, and number of fires in North America. An observed increase in the isobaric height anomaly during the past decade may lead to frequent synoptic ridging and drier conditions with more fires, thereby potentially impacting cloud/precipitation processes and decreasing air quality. ©2020. The Authors.