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Impacts of Saharan Dust As CCN on the Evolution of an Idealized Tropical Cyclone Henian Zhang,1 Greg M

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Impacts of Saharan Dust As CCN on the Evolution of an Idealized Tropical Cyclone Henian Zhang,1 Greg M GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L14812, doi:10.1029/2007GL029876, 2007 Click Here for Full Article Impacts of Saharan dust as CCN on the evolution of an idealized tropical cyclone Henian Zhang,1 Greg M. McFarquhar,1 Stephen M. Saleeby,2 and William R. Cotton2 Received 2 March 2007; revised 30 May 2007; accepted 20 June 2007; published 28 July 2007. [1] The impact of dust in the Saharan Air Layer (SAL) the temperature inversion at lower level. However, the acting as cloud condensation nuclei (CCN) on the evolution additional role of dust in the SAL acting as cloud-nucleating of a tropical cyclone (TC) was examined by conducting aerosol on the storm development has yet to be determined. simulations initialized with an idealized pre-TC mesoscale [3] It is estimated that 240 ± 80 million tons of Saharan convective vortex (MCV) using the Regional Atmospheric dust are transported from Africa to the Atlantic Ocean every Modeling System (RAMS). Increasing the background CCN year [Kaufman et al., 2005]. Dust can directly impact the concentration from 100 to 1000 and 2000 cmÀ3 in a layer radiative balance of the earth by absorbing and scattering between 1 and 5 km led to increases in averaged cloud sunlight. Dust, especially those coated with sulfur and other droplet number concentration, and decreases in cloud soluble materials [Levin et al., 1996], can also affect the droplet mean mass diameter through the entire simulation cloud development by acting as cloud condensation nuclei except during the initial spin-up. Dust in the SAL as CCN (CCN), giant CCN (GCCN) and ice nuclei (IN) [DeMott et influenced the TC development by inducing changes in the al., 2003; van den Heever et al., 2006]. Analysis of visible hydrometeor properties, modifying the storm diabatic satellite imagery over the east Atlantic shows that the dust heating distribution and thermodynamic structure, and in the SAL can be drawn into nearby mesoscale convective ultimately influencing the TC intensity through complex systems, which can later develop into TCs. Hurricane Erin dynamical responses. The simulated storm intensities (2001) was one example. Although previous studies have differed by up to 22 hPa depending on CCN concentration. shown that large numbers of dust particles can impact the The impact of CCN on storm intensity was sensitive to the development of shallow convective clouds, the role of CCN background giant CCN (GCCN) vertical profile and on the development of highly organized, strong convective presumably other environmental factors. The physical systems such as TCs has not been thoroughly investigated. processes responsible for the impact of dust as nucleating This study will examine the impact of dust acting as CCN aerosols on TC development need to be examined in the on the initiation and evolution of a TC using numerical future under a wide range of environmental conditions. simulations. Citation: Zhang, H., G. M. McFarquhar, S. M. Saleeby, and W. R. Cotton (2007), Impacts of Saharan dust as CCN on the evolution of 2. Model Configuration an idealized tropical cyclone, Geophys. Res. Lett., 34, L14812, doi:10.1029/2007GL029876. [4] To highlight the effect of dust as CCN, numerical simulations of a TC developing from an axisymmetric mesoscale convective vortex (MCV) in an idealized envi- 1. Introduction ronment were conducted using the Regional Atmospheric [2] During summer and early fall, intense heating over Modeling System (RAMS) version 4.3 [Cotton et al., 2003]. the Sahara desert forms a deep mixed atmospheric layer The simulations were initialized with the pressure, temper- containing hot, dry air and mineral dust. As the air mass ature and wind fields of an axisymmetric MCV consistent propagates westward over the Atlantic, it is undercut by with observations obtained from several pre-TC MCVs cool, moist marine air. This elevated air mass is often [Montgomery et al.,2006].Threenesteddomainswith referred to as the Saharan Air Layer (SAL). The SAL horizontal resolutions of 24, 6 and 2 km were used. The advances westward in association with the African Easterly numbers of horizontal grid points for Domain 1, Domain 2 Waves (AEWs), and it has frequently been observed to and Domain 3 were 80 Â 80, 102 Â 102 and 152 Â 152, interact with tropical cyclones (TCs) within its vicinity respectively. There were 40 unevenly spaced vertical levels [Dunion and Velden, 2004]. Hurricane Cindy (1999), Floyd extending from the surface to 30 km with a minimum grid (1999), Debby (2000), Joyce (2000), Felix (2001) and Erin spacing near the surface of 300 m and a maximum vertical (2001) are such examples. Dunion and Velden [2004] grid spacing of 1 km at higher altitudes. The Rayleigh hypothesized that the SAL tends to suppress Atlantic friction absorbing layer extended from 20 km to 30 km. A tropical cyclone activity by introducing dry, stable air into sufficient depth of the Rayleigh friction absorbing layer and the storm, increasing the vertical wind shear and enhancing refined vertical resolution especially at higher altitudes are necessary to damp gravity waves so that they are not reflected downward as they can significantly affect the 1Department of Atmospheric Sciences, University of Illinois at Urbana- simulated storm intensity. The vortex was allowed to grow Champaign, Urbana, Illinois, USA. for 3 days in a zero wind environment over the ocean with a 2Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA. constant sea surface temperature (SST) of 29°C. Pre-TC MCVs are usually found on the southern edge of the SAL, Copyright 2007 by the American Geophysical Union. which is a transient zone with hot and dry SAL air to the 0094-8276/07/2007GL029876$05.00 L14812 1of6 L14812 ZHANG ET AL.: IMPACTS OF SAHARAN DUST L14812 represent elevated dust particles in the SAL, the polluted simulation (hereafter ‘‘Polluted’’) was initialized with a CCN concentration of 1000 cmÀ3 between 1 km and 5 km. The double polluted simulation (hereafter ‘‘Double’’) was initial- ized with a CCN concentration of 2000 cmÀ3 in the same layer. Since no direct measurement of a vertical profile of CCN over the ocean in the SAL was available, the CCN concentrations in ‘‘Polluted’’ were chosen based upon pre- vious measurements in the dust plume [Levin et al., 2005; van den Heever et al., 2006]. The CCN concentrations at the surface and above 5 km in ‘‘Polluted’’ and ‘‘Double’’ were 100 cmÀ3, by which a clean marine air layer was assumed to undercut the SAL. The CCN were further assumed to be mostly hydrophobic with a water solubility of 10% to represent desert dust particles coated with sulfate. The percentage of CCN activated during a time step depends upon temperature, vertical velocity, CCN median radius and number concentration [Saleeby and Cotton, 2004]. During the course of the simulation, CCN are advected, diffused, depleted through nucleation scavenging, and replenished through droplet evaporation. [6] The initial background IN and GCCN concentrations were horizontally homogeneous. The IN concentration was 0.1 cmÀ3 from the surface to 20 km [van den Heever et al., 2006]. The background GCCN concentration decreased from a maximum of 0.5 cmÀ3 at the surface to 0.08 cmÀ3 at 5 km and then reduced to zero at higher altitudes. To examine if the trends observed in the ‘‘Clean’’, ‘‘Polluted’’ and ‘‘Double’’ are valid in general, an additional set of three simulations was performed using CCN concentrations that were identical to those described in ‘‘Clean’’, ‘‘Polluted’’ and ‘‘Double’’, but initialized with a different background Figure 1. Temporal evolutions of (a) V ,(b)N ,(c)N, GCCN profile. The GCCN profile had the same maximum ccn ccn c value of 0.5 cmÀ3 at surface but was reduced to 0.01 cmÀ3 (d) Dm, and (e) qc for ‘‘Clean’’ (dotted line), ‘‘Polluted’’ À3 (thin solid line) and ‘‘Double’’ (thick solid line). (instead of 0.08 cm )at5km[van den Heever et al., 2006]. Similarities and differences between both series of simulations are presented. north and warm moist marine air to the south. Intrusion of dry air from the SAL into the convective systems could 3. Results limit the impact of dust as nucleating aerosols. However, introducing this horizontal temperature gradient would [7] Figure 1 shows the time evolution of the total air require constructing an environmental wind field that sat- volume having CCN activation (Vccn), mean number con- isfies the thermal wind balance and hence bring the impact centration of activated CCN (Nccn), mean cloud droplet of large scale wind shear into the simulations. Thus, to focus number concentration (Nc), mean cloud droplet diameter on the impacts of dust in the SAL acting as CCN, a (Dm), and mean cloud mixing ratio (qc)for‘‘Clean’’, horizontally uniform Jordan sounding [Jordan, 1958], ‘‘Polluted’’ and ‘‘Double’’. The cloud Nc, Dm, and qc were which is likely a composite of soundings from both the averaged over the cloudy area in Domain 3. Figure 1a SAL and non-SAL environments [Dunion and Velden, shows that, except for the first 18 hours when the model 2004], was used. was initializing, CCN activation occurred in a greater [5] A two-moment microphysics scheme described by volume of air if more ambient CCN were available even Cotton et al. [2003], which emulates bin microphysics for though the CCN activation maximum and minimum for the collection and sedimentation, was used in the simulations. three simulations were not reached at the same time. The Saleeby and Cotton [2004] described the extension of that average Nccn shown in Figure 1b was calculated by dividing scheme to include two modes of droplets and explicit the total number of CCN activated in Domain 3 by the corresponding Vccn.TheNccn averaged over the last activation of CCN and GCCN.
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