4 research outputs found
UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation
Challenges in understanding the aerosol–cloud interactions and their impacts
on global climate highlight the need for improved knowledge of the underlying
physical processes and feedbacks as well as their interactions with cloud and
boundary layer dynamics. To pursue this goal, increasingly sophisticated
cloud-scale models are needed to complement the limited supply of
observations of the interactions between aerosols and clouds. For this
purpose, a new large-eddy simulation (LES) model, coupled with an interactive
sectional description for aerosols and clouds, is introduced. The new model
builds and extends upon the well-characterized UCLA Large-Eddy Simulation
Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications
(SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the
aerosol, cloud and precipitation bin discretisation are presented. These
enable tracking the effects of cloud processing and wet scavenging on the
aerosol size distribution as accurately as possible, while keeping the
computational cost of the model as low as possible. The model is tested with
two different simulation set-ups: a marine stratocumulus case in the
DYCOMS-II campaign and another case focusing on the formation and evolution
of a nocturnal radiation fog. It is shown that, in both cases, the
size-resolved interactions between aerosols and clouds have a critical
influence on the dynamics of the boundary layer. The results demonstrate the
importance of accurately representing the wet scavenging of aerosol in the
model. Specifically, in a case with marine stratocumulus, precipitation and
the subsequent removal of cloud activating particles lead to thinning of the
cloud deck and the formation of a decoupled boundary layer structure. In
radiation fog, the growth and sedimentation of droplets strongly affect their
radiative properties, which in turn drive new droplet formation. The
size-resolved diagnostics provided by the model enable investigations of
these issues with high detail. It is also shown that the results remain
consistent with UCLALES (without SALSA) in cases where the dominating
physical processes remain well represented by both models
Aerosol-landscape-cloud interaction : Signatures of topography effect on cloud droplet formation
Long-term in situ measurements of aerosol–cloud interactions are usually performed in measurement stations residing on hills, mountains, or high towers. In such conditions, the surface topography of the surrounding area can affect the measured cloud droplet distributions by increasing turbulence or causing orographic flows and thus the observations might not be representative for a larger scale. The objective of this work is to analyse, how the local topography affects the observations at Puijo measurement station, which is located in the 75 m high Puijo tower, which itself stands on a 150 m high hill. The analysis of the measurement data shows that the observed cloud droplet number concentration mainly depends on the cloud condensation nuclei (CCN) concentration. However, when the wind direction aligns with the direction of the steepest slope of the hill, a clear topography effect is observed. This finding was further analysed by simulating 3-D flow fields around the station and by performing trajectory ensemble modelling of aerosol- and wind-dependent cloud droplet formation. The results showed that in typical conditions, with geostrophic winds of about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in the air parcels arriving at the station. This is enough to produce in-cloud supersaturations (SSs) higher than typically found at the cloud base of  ∼  0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bimodal cloud droplet size distribution. The effect is strongest with high winds across the steepest slope of the hill and with low liquid water contents, and its relative importance quickly decreases as these conditions are relaxed. We therefore conclude that, after careful screening for wind speed and liquid water content, the observations at Puijo measurement station can be considered representative for clouds in a boreal environment
Aerosol-landscape-cloud interaction : Signatures of topography effect on cloud droplet formation
Long-term in situ measurements of aerosol–cloud interactions are usually performed in measurement stations residing on hills, mountains, or high towers. In such conditions, the surface topography of the surrounding area can affect the measured cloud droplet distributions by increasing turbulence or causing orographic flows and thus the observations might not be representative for a larger scale. The objective of this work is to analyse, how the local topography affects the observations at Puijo measurement station, which is located in the 75 m high Puijo tower, which itself stands on a 150 m high hill. The analysis of the measurement data shows that the observed cloud droplet number concentration mainly depends on the cloud condensation nuclei (CCN) concentration. However, when the wind direction aligns with the direction of the steepest slope of the hill, a clear topography effect is observed. This finding was further analysed by simulating 3-D flow fields around the station and by performing trajectory ensemble modelling of aerosol- and wind-dependent cloud droplet formation. The results showed that in typical conditions, with geostrophic winds of about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in the air parcels arriving at the station. This is enough to produce in-cloud supersaturations (SSs) higher than typically found at the cloud base of  ∼  0.2 %), and thus additional cloud droplets may form inside the cloud. In the observations, this is seen in the form of a bimodal cloud droplet size distribution. The effect is strongest with high winds across the steepest slope of the hill and with low liquid water contents, and its relative importance quickly decreases as these conditions are relaxed. We therefore conclude that, after careful screening for wind speed and liquid water content, the observations at Puijo measurement station can be considered representative for clouds in a boreal environment
Aerosol–landscape–cloud interaction: signatures of topography effect on cloud droplet formation
Long-term in situ measurements of aerosol–cloud interactions are usually
performed in measurement stations residing on hills, mountains, or high
towers. In such conditions, the surface topography of the surrounding area
can affect the measured cloud droplet distributions by increasing turbulence
or causing orographic flows and thus the observations might not be
representative for a larger scale. The objective of this work is to analyse,
how the local topography affects the observations at Puijo measurement
station, which is located in the 75 m high Puijo tower, which itself stands
on a 150 m high hill. The analysis of the measurement data shows that the
observed cloud droplet number concentration mainly depends on the cloud
condensation nuclei (CCN) concentration. However, when the wind direction
aligns with the direction of the steepest slope of the hill, a clear
topography effect is observed. This finding was further analysed by
simulating 3-D flow fields around the station and by performing trajectory
ensemble modelling of aerosol- and wind-dependent cloud droplet formation.
The results showed that in typical conditions, with geostrophic winds of
about 10 m s−1, the hill can cause updrafts of up to 1 m s−1 in
the air parcels arriving at the station. This is enough to produce in-cloud
supersaturations (SSs) higher than typically found at the cloud base of
 ∼  0.2 %), and thus additional cloud droplets may form inside the
cloud. In the observations, this is seen in the form of a bimodal cloud
droplet size distribution. The effect is strongest with high winds across the
steepest slope of the hill and with low liquid water contents, and its
relative importance quickly decreases as these conditions are relaxed. We
therefore conclude that, after careful screening for wind speed and liquid
water content, the observations at Puijo measurement station can be
considered representative for clouds in a boreal environment