Long-term characterisation of the vertical structure of the Saharan Air Layer over the Canary Islands using lidar and radiosonde profiles: implications for radiative and cloud processes over the subtropical Atlantic Ocean

Abstract

Abstract. Every year, large-scale African dust outbreaks frequently pass over the Canary Islands (Spain). Here we describe the seasonal evolution of atmospheric aerosol extinction and meteorological vertical profiles on Tenerife over the period 2007–2018 using long-term micropulse lidar (MPL-3) and radiosonde observations. These measurements are used to categorise the different patterns of dust transport over the subtropical North Atlantic and, for the first time, to robustly describe the dust vertical distribution in the Saharan Air Layer (SAL) over this region. Three atmospheric scenarios dominate the aerosol climatology: dust-free (clean) conditions, the Saharan summer scenario (summer-SAL) and the Saharan winter scenario (winter-SAL). A relatively well-mixed marine boundary layer (MBL) was observed in the case of clean (dust-free) conditions; it was associated with rather constant lidar extinction coefficients (α) below 0.036 km−1 with minimum α (< 0.022 km−1) in the free troposphere (FT). The summer-SAL has been characterised as a dust-laden layer strongly affecting both the MBL (Δα = +48 % relative to clean conditions) and the FT. The summer-SAL appears as a well-stratified layer, relatively dry at lower levels (Δr∼-44 % at the SAL’s base, where r is the water vapour mixing ratio) but more humid at higher levels compared with clean FT conditions (Δr∼+332 % at 5.3 km), with a peak of α> 0.066 km−1 at ∼ 2.5 km. Desert dust is present up to ∼ 6.0 km, the SAL top based on the altitude of SAL's temperature inversion. In the winter-SAL scenario, the dust layer is confined to lower levels below 2 km altitude. This layer is characterised by a dry anomaly at lower levels (Δr∼ −38 % in comparison to the clean scenario) and a dust peak at ∼ 1.3 km height. Clean FT conditions were found above 2.3 km. Our results reveal the important role that both dust and water vapour play in the radiative balance within the summer-SAL and winter-SAL. The dominant dust-induced shortwave (SW) radiative warming in summer (heating rates up to +0.7 K d−1) is found slightly below the dust maximum. However, the dominant contribution of water vapour was observed as a net SW warming observed within the SAL (from 2.1 to 5.7 km) and as a strong cold anomaly near the SAL's top (−0.6 K d−1). The higher water vapour content found to be carried on the summer-SAL, despite being very low, represents a high relative variation in comparison to the very dry clean free troposphere in the subtropics. This relevant aspect should be properly taken into account in atmospheric modelling processes. In the case of the winter-SAL, we observed a dust-induced radiative effect dominated by SW heating (maximum heating of +0.7 K d−1 at 1.5 km, near the dust peak); both dust and atmospheric water vapour impact heating in the atmospheric column. This is the case of the SW heating within the SAL (maximum near the r peak), the dry anomaly at lower levels (Δr∼ −38 % at 1 km) and the thermal cooling (∼ 0.3 K d−1) from the temperature inversion upwards. Finally, we hypothesise that the SAL can impact heterogeneous ice nucleation processes through the frequent occurrence of mid-level clouds observed near the SAL top at relatively warm temperatures. A dust event that affected Tenerife on August 2015 is simulated using the regional DREAM model to assess the role of dust and water vapour carried within SAL in the ice nucleation processes. The modelling results reproduce the arrival of the dust plume and its extension over the island and confirm the observed relationship between the summer-SAL conditions and the formation of mid- and high-level clouds.

The ice-nucleating activity of African mineral dust in the Caribbean boundary layer

“…The SAL is typically confined to ~4 km thickness, with dust concentrations decreasing sharply at its top boundary and a strong inversion at the bottom of the layer separating it from the MBL [Carlson and Prospero, 1972;Gasteiger et al, 2017;Weinzierl et al, 2017]. Mid-level clouds can form in the upper 95 part of the SAL where the dust may promote primary ice formation [Barreto et al, 2022].…”

Aerosol characterisation in the subtropical eastern North Atlantic region using long-term AERONET measurements

“…In this study, we describe the long-term seasonal evolution of atmospheric aerosols by using AERONET observations at four different sites at different altitudes in Tenerife, in the subtropical eastern North Atlantic region. This region can be considered a key location for aerosol monitoring because it is in the path of long-range transports such as mineral dust from the Sahel-Sahara regions (Carlson and Prospero, 1972;Prospero and Carlson, 1972;Tsamalis et al, 2013;Cuevas et al, 2015;Rodríguez et al, 2015Rodríguez et al, , 2020Barreto et al, 2022), dust from North America (García et al, 2017a), or sulfates, biomass burning and other pollutants from North America, Europe or Africa (Viana et al, 2002;Basart et al, 2009;Rodríguez et al, 2011;García et al, 2017b;Rodríguez et al, 2020;Wang et al, 2021). On the northern edge of the dust belt in summer, but still affected by dust transport in winter (Alonso-Pérez et al, 2007, 2011Rodríguez et al, 2011;Cuevas et al, 2015), this region presents a stronger seasonal dependence of dust transport than tropical latitudes, which is representative of the almost pure Saharan dust present in summer and winter (Barreto et al, 2022).…”

“…This region can be considered a key location for aerosol monitoring because it is in the path of long-range transports such as mineral dust from the Sahel-Sahara regions (Carlson and Prospero, 1972;Prospero and Carlson, 1972;Tsamalis et al, 2013;Cuevas et al, 2015;Rodríguez et al, 2015Rodríguez et al, , 2020Barreto et al, 2022), dust from North America (García et al, 2017a), or sulfates, biomass burning and other pollutants from North America, Europe or Africa (Viana et al, 2002;Basart et al, 2009;Rodríguez et al, 2011;García et al, 2017b;Rodríguez et al, 2020;Wang et al, 2021). On the northern edge of the dust belt in summer, but still affected by dust transport in winter (Alonso-Pérez et al, 2007, 2011Rodríguez et al, 2011;Cuevas et al, 2015), this region presents a stronger seasonal dependence of dust transport than tropical latitudes, which is representative of the almost pure Saharan dust present in summer and winter (Barreto et al, 2022). Furthermore, the strong vertical stratification in the lower troposphere that is typical of this eastern side of the subtropical North Atlantic implies the presence of several layers and transition levels with different vertical humidity and temperature gradients, which strongly affect the aerosol layering (Carrillo et al, 2016;Barreto et al, 2022).…”

Volcanic Eruption of Cumbre Vieja, La Palma, Spain: A First Insight to the Particulate Matter Injected in the Troposphere

“…It is worth noting that the gaps in the instantaneous (hourly) values (Figure 2b) were produced by the profile screening applied (Section 2.2.1): Among all available 1 h timeresolution profiles, 62% were classified as not blocked, meaning that 38% of the time, the lowermost volcanic plume was optically so thick that the laser light could not penetrate it. In most cases, h P varied between 0.5 and 2.5 km, i.e., in an interval including the MBL height estimated in Tenerife (where the MBL behavior is comparable to that at Tazacorte in the absence of mineral dust), varying between 1.3 and 1.6 km [19]. Indeed, the average of h P over the whole period was 1.43 ± 0.45 km.…”