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Jørgen B. Jensen and Alison D. Nugent

Abstract

The most basic aspect of cloud formation is condensational growth onto cloud condensation nuclei (CCN). As such, condensational growth of cloud drops is often assumed to be a well-understood process described by the drop growth equation. When this process is represented in models, CCN activate into cloud drops at cloud base, and it is often assumed that drops consist of pure water or that the hygroscopic contribution after drop activation is small because of the inclusion of only small CCN. Drop growth rate in adiabatic ascent in such models is proportional to supersaturation and assumed to be inversely proportional to the drop radius, thereby making the drop spectrum narrow with altitude. However, the present study demonstrates that drop growth on giant sea-salt aerosol particles (GCCN; dry radius 0.5 m) behaves differently. For typical marine stratocumulus updrafts and for drops grown on GCCN with sizes m, these drops typically remain concentrated salt solutions. Because of this, their condensational growth is accelerated, and they rapidly attain precipitation drop sizes through condensation only. Additionally, drops formed on GCCN may also grow by condensation in cloudy downdrafts. The strong effect of condensation on GCCN is important when carried through to calculating rain-rate contribution as a function of aerosol size. GCCN larger than 2 m account for most of the rainfall rate in the modeled precipitating marine stratocumulus.

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Alison D. Nugent and Rosimar Rios-Berrios

Abstract

Tropical cyclones are generally characterized by strong rotating winds, and yet, the associated rainfall can be equally destructive. Tropical Storm Erika (2015) is an example of such a cyclone whose heavy rainfall south of the storm center was responsible for significant loss of life and property. Tropical Storm Erika was a weak tropical storm in a sheared environment that passed through the Lesser Antilles on 27 August 2015. Radar and rain gauges measured at least a half meter of rainfall on the Commonwealth of Dominica in about 5 h. In this study, an analysis of several observational datasets showed that the combination of a sheared environment, dry northern sector, and mesovortex contributed to the significant storm precipitation. The sheared environment affected the storm structure, causing it to weaken, but also organized convection and precipitation in the region that passed over Dominica. Furthermore, a mesovortex embedded within the storm persisted over Dominica, leading to enhanced rainfall totals. Understanding the factors leading to heavy rainfall for this case is important for future prediction of similar weak, sheared tropical storms passing near mountainous islands.

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Tianqi Zuo, Alison D. Nugent, and Gregory Thompson

Abstract

In recent decades, a significant rainfall decline over the island of Hawai‘i has been noted, with many hypothesizing that the drying is associated with the volcanic aerosols emitted from the Kīlauea volcano. While it is clear that volcanic emissions can create hazardous air quality for Hawaiian communities, the impacts on rainfall are less clear. Here we investigate the impact of volcanic aerosol emissions on Hawai‘i Island rainfall. Based on observed daily rainfall and SO2 emissions, it is found that days with high SO2 emissions have on average 8 mm day−1 less rainfall downstream of the Kīlauea volcano. Sensitivity studies with varying volcanic aerosol emission sources from the Kīlauea vent locations have also been conducted by the Weather Research and Forecasting (WRF) Model in order to examine the detailed physical processes. Consistent with SO2 air quality observations, it is found that the diurnal change in aerosol number concentration is strongly dependent on the diurnal variation of local circulations. The added aerosols are lofted into the orographic convection where they modify the microphysical properties of the warm clouds by increasing the cloud droplet number concentration, decreasing the cloud droplet size, increasing cloud water content, and enhancing cloud evaporation. The volcanic aerosols also delay precipitation production and modify the spatial distribution of rainfall on the downstream mountainside. The modification of precipitation on an island has far-reaching consequences. For this reason, we work to quantify the sensitivity of the orographic precipitation to volcanic aerosols and move beyond hypothesized relationships to work toward understanding the underlying problem.

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Alison D. Nugent and Ronald B. Smith

Abstract

Using aircraft data from the recent Dominica Experiment (DOMEX) project in Dominica, the authors evaluate a modified version of Woodcock’s theory of moist convective initiation. Upstream of Dominica, anticorrelated fluctuations in temperature and specific humidity are found in the subcloud layer related to ambient trade wind convection. The associated variances in virtual temperature and air density are surprisingly small due to buoyancy adjustment. When this air is quickly lifted by terrain, the moist patches, having a lower lifting condensation level, become “seeds” for convection. The authors model this process by uniformly lifting an observed layer of air moist adiabatically from 300 to 1300 m. The resulting variations of buoyancy within the layer are converted to vertical accelerations accounting for strong “added mass” effects that include an estimate of layer depth. These estimated fluctuations in vertical acceleration agree with aircraft measurements of updraft speed and length scale over the terrain of Dominica. The authors speculate on the breadth of applicability of this mechanism of convective initiation.

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Campbell D. Watson, Ronald B. Smith, and Alison D. Nugent

Abstract

A sharp reduction in precipitation was observed on the island of Dominica in the Caribbean during a 2011 field campaign when the trade winds weakened and convection transitioned from mechanically to thermally driven. The authors propose four hypotheses for this reduction, which relate to (i) the triggering mechanism, (ii) dry-air entrainment, (iii) giant sea-salt aerosol, and (iv) small-island-derived aerosol. The plausibility of the first three hypotheses is the focus of this study.

Aircraft observations show the dynamics of the orographic cumulus clouds at flight level are surprisingly similar, irrespective of how they are triggered. However, the orographic cumulus clouds are consistently shallower when the trade winds are weak, which the authors attribute to a drier and shallower cloud layer compared to days with stronger trade winds. The strong negative influence of dry-air entrainment in a drier environment on cumulus depth and liquid water content is qualitatively demonstrated using an entraining plume model and the WRF Model. Although the models appear more sensitive than observations to entrainment and cloud size, the sensitivity tests have some resemblance to observations. The authors also find evidence of sea-salt aerosol entering the base of marine cumulus on strong wind days using an aircraft-mounted lidar and other instruments. Although each hypothesis is plausible, the complex interplay of these processes makes determining the controlling mechanisms difficult. Ultimately, the authors’ analysis rejects the hypothesis (i) triggering, while supporting (ii) entrainment and (iii) sea-salt aerosol.

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Alison D. Nugent, Ronald B. Smith, and Justin R. Minder

Abstract

This study compares observations from the Dominica Experiment (DOMEX) field campaign with 3D and 2D Weather Research and Forecasting Model (WRF) simulations to understand how ambient upstream wind speed controls the transition from thermally to mechanically forced moist orographic convection. The environment is a conditionally unstable, tropical atmosphere with shallow trade wind cumulus clouds. Three flow indices are defined to quantify the convective transition: horizontal divergence aloft, cloud location, and island surface temperature. As wind speed increases, horizontal airflow divergence from plume detrainment above the mountain changes to convergence associated with plunging flow, convective clouds relocate from the leeward to the windward side of the mountain as mechanically triggered convection takes over, and the daytime mountaintop temperature decreases because of increased ventilation and cloud shading. Possible mechanisms by which wind speed controls island precipitation are also discussed. The result is a clearer understanding of orographic convection in the tropics.

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Justin R. Minder, Ronald B. Smith, and Alison D. Nugent

Abstract

The mountainous Caribbean island of Dominica was chosen as a natural laboratory for studying orographic convection in the tropics. Here, the authors focus on a prototypical case study, taken from the Dominica Experiment (DOMEX) field campaign in the spring of 2011. Airborne measurements and high-resolution numerical experiments are used to examine the mesoscale dynamics of moist airflow over Dominica and its relationship to convection, turbulence, and rainfall.

Upwind of the island, there is minimal lateral deflection or lifting of the flow, largely because of latent heat release in the overisland convection. Over the terrain, forced ascent leads to rapid development of moist convection, buoyancy-generated turbulence, and rainfall. Although this convection produces sporadic bursts of heavy rainfall, it does not appear to enhance the time-mean rainfall. Over the lee slopes, a dry plunging flow produces anisotropic shear-generated turbulence and strong low-level winds while quickly dissipating convection and rainfall. In the wake, low-level air is decelerated, both by turbulence in the plunging flow and by frictional drag over the island. Low-level wake air is also dried and warmed, primarily by turbulent vertical mixing and regional descent, both associated with the downslope flow. Rainfall and latent heating play only a secondary role in warming and drying the wake.

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Alison D. Nugent, Campbell D. Watson, Gregory Thompson, and Ronald B. Smith

Abstract

Observations from the Dominica Experiment (DOMEX) field campaign clearly show aerosols having an impact on cloud microphysical properties in thermally driven orographic clouds. It is hypothesized that when convection is forced by island surface heating, aerosols from the mostly forested island surface are lofted into the clouds, resulting in the observed high concentration of aerosols and the high concentration of small cloud droplets. When trying to understand the impact of these surface-based aerosols on precipitation, however, observed differences in cloud-layer moisture add to the complexity. The WRF Model with the aerosol-aware Thompson microphysics scheme is used to study six idealized scenarios of thermally driven island convection: with and without a surface aerosol source, with a relatively dry cloud layer and with a moist cloud layer, and with no wind and with a weak background wind. It is found that at least a weak background wind is needed to ensure Dominica-relevant results and that the effect of cloud-layer moisture on cloud and precipitation formation dominates over the effect of aerosol. The aerosol impact is limited by the dominance of precipitation formation through accretion. Nevertheless, in order to match observed cloud microphysical properties and precipitation, both a relatively dry cloud layer and a surface aerosol source are needed. The impact of a surface aerosol source on precipitation is strongest when the environment is not conducive to cloud growth.

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Malte F. Stuecker, Christina Karamperidou, Alison D. Nugent, Giuseppe Torri, Sloan Coats, and Steven Businger
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Chung Taing, Katherine L. Ackerman, Alison D. Nugent, and Jorgen B. Jensen

Abstract

Sea salt aerosol (SSA) plays a significant role in the atmosphere through aerosol direct and indirect effects, and in atmospheric chemistry as a source of tropospheric bromine. In situ measurements of coarse-mode SSA particles are limited because of their low concentration and relatively large sizes (dry radius r d > 0.5 μm). With this in mind, a new, low-cost, easily usable method for sampling coarse-mode SSA particles in the marine boundary layer was developed. An SSA particle sampler that uses an impaction method was designed and built using 3D printing and Arduino microcontrollers and sensors. It exposes polycarbonate slides to ambient airflow remotely on a kite-based platform to capture coarse-mode SSA particles. Because it is a smaller version of the Giant Nucleus Impactor (GNI), designed for use on aircraft, it is named the miniature Giant Nucleus Impactor (miniGNI). After sample collection, the same optical microscope methodology utilized by the GNI was used to analyze the wetted salt particles that impacted onto the slides. In this proof-of-concept study, multiple miniGNIs were attached serially to a kite string, allowing for sampling at multiple altitudes simultaneously. The robustness of the results from this new instrument and methodology for sampling at ambient RH (~75%) the SSA particle size distribution with r d > 3.3 μm are compared with a similar study. We find that the SSA particle number concentration decreases weakly with altitude and shows no correlation to instantaneous U 10 wind speed along the windward coastline of Oʻahu in the Hawaiian Islands.

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