282 research outputs found
Influence of model resolution on spatial and temporal variability of clouds and precipitation over Germany
Cloud microphysics and aerosol indirect effects in the global climate model ECHAM5-HAM
The double-moment cloud microphysics scheme from ECHAM4 has been coupled to the size-resolved aerosol scheme ECHAM5-HAM. ECHAM5-HAM predicts the aerosol mass and number concentrations and the aerosol mixing state. This results in a much better agreement with observed vertical profiles of the black carbon and aerosol mass mixing ratios than with the previous version ECHAM4, where only the different aerosol mass mixing ratios were predicted. Also, the simulated liquid, ice and total water content and the cloud droplet and ice crystal number concentrations as a function of temperature in stratiform mixed-phase clouds between 0 and –35°C agree much better with aircraft observations in the ECHAM5 simulations. ECHAM5 performs better because more realistic aerosol concentrations are available for cloud droplet nucleation and because the Bergeron-Findeisen process is parameterized as being more efficient.
The total anthropogenic aerosol effect includes the direct, semi-direct and indirect effects and is defined as the difference in the top-of-the-atmosphere net radiation between present-day and pre-industrial times. It amounts to –1.8 W m^−2 in ECHAM5, when a relative humidity dependent cloud cover scheme and present-day aerosol emissions representative for the year 2000 are used. It is larger when either a statistical cloud cover scheme or a different aerosol emission inventory are employed
Seasonal variability of Saharan desert dust and ice nucleating particles over Europe
Dust aerosols are thought to be the main contributor to atmospheric ice nucleation. While there are case studies supporting this, a climatological sense of the importance of dust to atmospheric ice nucleating particle (INP) concentrations, and it\u27s seasonal variability over Europe is lacking. Here, we use a mesoscale model to estimate Saharan dust concentrations over Europe in winter and summer of 2007–2008. There are large differences in median dust concentrations between seasons, with the highest concentrations and highest variability in the lowest 4 km. Laboratory based ice nucleation parameterisations are applied to these dust number concentrations to calculate the potential INP resulting from immersion freezing and deposition nucleation on these dust particles. The potential INP concentrations generally increase with height due to decreasing temperatures in the lower and mid-troposphere and exhibit a maximum in the upper troposphere where INP concentrations decrease again with altitude due to decreasing dust concentrations. The potential INP profiles exhibit similarly large differences between seasons, with the highest concentrations in winter (median potential immersion INP concentrations up to 103 m−3, median potential deposition INP concentrations at 120% relative humidity with respect to ice up to 105 m−3) occurring closer to the ground for both nucleation modes. Using these results, a best-fit function is provided to estimate the potential INPs for use in limited-area models, which is representative of the normal background INP concentrations over Europe. A statistical evaluation of the results against field and laboratory measurements indicates that the INP concentrations are in close agreement with observations
Seasonal variability of Saharan desert dust and ice nucleating particles over Europe
Dust aerosols are thought to be the main contributor to atmospheric ice nucleation. While there are case studies supporting this, a climatological sense of the importance of dust to atmospheric ice nucleating particle (INP) concentrations and its seasonal variability over Europe is lacking. Here, we use a mesoscale model to estimate Saharan dust concentrations over Europe in 2008. There are large differences in median dust concentrations between seasons, with the highest concentrations and highest variability in the lower to mid-troposphere. Laboratory-based ice nucleation parameterisations are applied to these simulated dust number concentrations to calculate the potential INP resulting from immersion freezing and deposition nucleation on these dust particles. The potential INP concentrations increase exponentially with height due to decreasing temperatures in the lower and mid-troposphere. When the ice-activated fraction increases sufficiently, INP concentrations follow the dust particle concentrations. The potential INP profiles exhibit similarly large differences between seasons, with the highest concentrations in spring (median potential immersion INP concentrations nearly 105 m-3, median potential deposition INP concentrations at 120% relative humidity with respect to ice over 105 m-3), about an order of magnitude larger than those in summer. Using these results, a best-fit function is provided to estimate the potential INPs for use in limited-area models, which is representative of the normal background INP concentrations over Europe. A statistical evaluation of the results against field and laboratory measurements indicates that the INP concentrations are in close agreement with observations
Impact of the representation of marine stratocumulus clouds on the anthropogenic aerosol effect
Stratocumulus clouds are important for climate as they reflect large amounts
of solar radiation back into space. However they are difficult to simulate in
global climate models because they form under a sharp inversion and are
thin. A comparison of model simulations with the ECHAM6-HAM2 global aerosol
climate model to observations, reanalysis and literature data revealed too
strong turbulent mixing at the top of stratocumulus clouds and a lack of
vertical resolution. Further reasons for cloud biases in stratocumulus
regions are the too "active" shallow convection scheme, the cloud cover
scheme and possibly too low subsidence rates.
To address some of these issues and improve the representation of
stratocumulus clouds, we made three distinct changes to ECHAM6-HAM2. With a
"sharp" stability function in the turbulent mixing scheme we have observed,
similar to previous studies, increases in stratocumulus cloud cover and
liquid water path. With an increased vertical resolution in the lower
troposphere in ECHAM6-HAM2 the stratocumulus clouds form higher up in the
atmosphere and their vertical extent agrees better with reanalysis data. The
recently implemented in-cloud aerosol processing in stratiform clouds is
used to improve the aerosol representation in the model.
Including the improvements also affects the anthropogenic aerosol effect.
In-cloud aerosol processing in ECHAM6-HAM2 leads to a decrease in the anthropogenic aerosol effect in the global annual mean from −1.19 Wm−2 in
the reference simulation to −1.08 Wm−2, while using a "sharp" stability
function leads to an increase to −1.34 Wm−2. The results from the
simulations with increased vertical resolution are diverse but increase the
anthropogenic aerosol effect to −2.08 Wm−2 at 47 levels and
−2.30 Wm−2 at 95 levels
Different contact angle distributions for heterogeneous ice nucleation in the community atmospheric model version 5
In order to investigate the impact of different
treatments for the contact angle (α) in heterogeneous ice nucleating
properties of natural dust and black carbon (BC) particles, we implement the
classical-nucleation-theory-based parameterization of heterogeneous ice
nucleation (Hoose et al., 2010) in the Community Atmospheric Model version 5
(CAM5) and then improve it by replacing the original single-contact-angle
model with the probability-density-function-of-α (α-PDF)
model to better represent the ice nucleation behavior of natural dust found
in observations. We refit the classical nucleation theory (CNT) to
constrain the uncertain parameters (i.e., onset α and activation
energy in the single-α model; mean contact angle and standard
deviation in the α-PDF model) using recent observation data sets for
Saharan natural dust and BC (soot). We investigate the impact of the time dependence of droplet freezing on mixed-phase clouds and climate in
CAM5 as well as the roles of natural dust and soot in different nucleation
mechanisms. Our results show that, when compared with observations, the
potential ice nuclei (IN) calculated by the α-PDF model show better
agreement than those calculated by the single-α model at warm
temperatures (T; T > −20 °C). More ice crystals can form at low
altitudes (with warm temperatures) simulated by the α-PDF model than compared to the single-α model in CAM5. All of these can be
attributed to different ice nucleation efficiencies among aerosol particles,
with some particles having smaller contact angles (higher efficiencies) in
the α-PDF model. In the sensitivity tests with the α-PDF
model, we find that the change in mean contact angle has a larger impact on
the active fraction at a given temperature than a change in standard deviation,
even though the change in standard deviation can lead to a change in freezing behavior. Both the single-α and the α-PDF model
indicate that the immersion freezing of natural dust plays a more important
role in the heterogeneous nucleation than that of soot in mixed-phase
clouds. The new parameterizations implemented in CAM5 induce more
significant aerosol indirect effects than the default parameterization
Spatial and temporal variability of clouds and precipitation over Germany: multiscale simulations across the "gray zone"
This paper assesses the resolution dependance of clouds and precipitation over Germany by numerical simulations with the COnsortium for Small-scale MOdeling (COSMO) model. Six intensive observation periods of the HOPE (HD(CP)2 Observational Prototype Experiment) measurement campaign conducted in spring 2013 and one summer day of the same year are simulated. By means of a series of grid-refinement resolution tests (horizontal grid spacing 2.8, 1 km, 500 and 250 m), the applicability of the COSMO model to real weather events in the terra incognita, i. e. the scale ranging between the mesoscale limit (no turbulence resolved) and the large-eddy simulation limit (energy-containing turbulence resolved), is tested. It is found that although the representation of a number of processes is enhanced with resolution (e. g. boundary-layer thermals, low-level convergence zones, gravity waves), their influence on the temporal evolution of precipitation is rather weak. However, rain intensities may vary with resolution, leading to differences in the total rain amount of up to +48 %. Furthermore, the location of rain is similar for the springtime cases with moderate and strong synoptic forcing, whereas significant differences are obtained for the summertime case with air mass convection. Probability density functions of convection-related parameters are analyzed to investigate their dependance on model resolution and their impact on cloud formation and subsequent precipitation
A new temperature and humidity dependent surface site density approach for deposition ice nucleation
Deposition nucleation experiments with Arizona Test Dust (ATD) as
a surrogate for mineral dusts were conducted at the AIDA cloud
chamber at temperatures between 220 and 250 K. The influence
of the aerosol size distribution and the cooling rate on the ice
nucleation efficiencies was investigated. Ice nucleation active
surface site (INAS) densities were calculated to quantify the ice
nucleation efficiency as a function of temperature, humidity and the
aerosol surface area concentration. Additionally, a contact angle
parameterization according to classical nucleation theory was fitted
to the experimental data in order to relate the ice nucleation
efficiencies to contact angle distributions. From this study it can
be concluded that the INAS density formulation is a very useful tool
to describe the temperature- and humidity-dependent ice nucleation
efficiency of ATD particles.
Deposition nucleation on ATD particles can be described by
a temperature- and relative-humidity-dependent INAS density function
ns(T, Sice) with
ns(xtherm) = 1.88 ×105 · exp(0.2659 · xtherm) [m−2] , (1)
where the temperature- and saturation-dependent function xtherm is defined as
xtherm = −(T−273.2)+(Sice−1) ×100, (2)
with the saturation ratio with respect to ice Sice >1 and within a temperature range between 226 and
250 K. For lower temperatures, xtherm deviates
from a linear behavior with temperature and relative humidity over
ice.
Also, two different approaches for describing the time dependence of
deposition nucleation initiated by ATD particles are proposed. Box
model estimates suggest that the time-dependent contribution is only
relevant for small cooling rates and low number fractions of
ice-active particles
Global simulations of aerosol processing in clouds
An explicit and detailed representation of in-droplet and in-crystal aerosol particles in stratiform clouds has been introduced in the global aerosol-climate model ECHAM5-HAM. The new scheme allows an evaluation of the cloud cycling of aerosols and an estimation of the relative contributions of nucleation and collision scavenging, as opposed to evaporation of hydrometeors in the global aerosol processing by clouds. On average an aerosol particle is cycled through stratiform clouds 0.5 times. The new scheme leads to important changes in the simulated fraction of aerosol scavenged in clouds, and consequently in the aerosol wet deposition. In general, less aerosol is scavenged into clouds with the new prognostic treatment than what is prescribed in standard ECHAM5-HAM. Aerosol concentrations, size distributions, scavenged fractions and cloud droplet concentrations are evaluated and compared to different observations. While the scavenged fraction and the aerosol number concentrations in the marine boundary layer are well represented in the new model, aerosol optical thickness, cloud droplet number concentrations in the marine boundary layer and the aerosol volume in the accumulation and coarse modes over the oceans are overestimated. Sensitivity studies suggest that a better representation of below-cloud scavenging, higher in-cloud collision coefficients, or a reduced water uptake by seasalt aerosols could reduce these biases
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