UP2.3

How to cite: Hogan, R.: The full-spectrum correlated-k method to accelerate radiative transfer in NWP models, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-444, https://doi.org/10.5194/ems2021-444, 2021.

Forecasting cloud accurately is still a challenge in numerical weather prediction (NWP) models. Detailed qualitative evaluation of such forecasts is needed in order to improve the forecasts. Cloud cover is often used for the evaluation but it is not a good metric due to inconsistency in methods for assessing the cloud cover. Global horizontal irradiance (GHI), also referred to as “global radiation”, provides an objective and quantitative measure for evaluating cloud forecasts during daytime.

Non-dimensional indices for solar energy resource assessment have been developed in recent years and decades that are very useful. One such index
is the clear sky index (CSI), which is the GHI divided by the theoretical GHI during clear sky conditions (e.g. [1,2]). We use the theoretical GHI clear sky model of [3,4], which includes coefficients that account for variable integrated atmospheric water vapour, aerosols and ozone. We have used binned CSI data computed using HARMONIE-AROME NWP forecast data and observations to identify model deficiencies in cloud and to evaluate new model physics options and settings. Sample results include the identification of consistent negative GHI biases under the thickest clouds and positive biases under Stratocumulus clouds. Such results help to pin-point deficiencies in the HARMONIE-AROME NWP model.

[1] Perez, R.; Ineichen, P.; Seals, R.; Zelenka, A. Making full use of the clearness index for parameterizing hourly insolation conditions. Sol. Energy 1990, 45, 111–114.

[2] Skartveit, A.; Olseth, J.A.; Tuft, M.E. An hourly diffuse fraction model with correction for variability and surface albedo. Sol. Energy 1998, 63, 173–183.

[3] Savijärvi, H. Fast radiation parameterization schemes for mesoscale and short-range forecast models. J. Appl. Meteorol. 1990, 437–447.

[4] Gleeson, E.; Nielsen, K.P.; Toll, V.; Rontu, L.; Whelan, E. Shortwave Radiation Experiments in HARMONIE. Tests of the cloud inhomogeneity factor and a new cloud liquid optical property scheme compared to observations. ALADIN-HIRLAM Newsl. 2015, 5, 92–106.

How to cite: Gleeson, E. and Pagh Nielsen, K.: Use of a Clear Sky Index to Evaluate Solar and Cloud NWP Forecasts, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-387, https://doi.org/10.5194/ems2021-387, 2021.

Solar actinic radiation is driving atmospheric photochemistry. Consequently, chemistry-transport models rely on accurate model predictions of actinic flux densities to correctly reproduce the essential impact of photolysis processes. Cloud effects are most challenging in this context because of their potentially large influence and their variability. In this study, the effects of clouds, aerosols and ground albedos on solar actinic radiation are investigated using 1D satellite-aided radiative transfer calculations and in-situ aircraft measurements.

Spectral actinic flux densities in the range 280-650 nm are calculated with the latest version of the libRadtran model utilizing cloud products from geostationary satellites (NASA SatCORPS) as well as aerosol properties (MODIS, MOD08_D3), surface albedos (MODIS, MCD43A3) and total assimilated ozone columns (TEMIS, MSR-2) from polar orbiting satellites as key input parameters. The evaluation of the performance of the model output is made by comparison with data from several campaigns with the research aircraft HALO (High Altitude and Long Range Research Aircraft) where spectral actinic flux densities were measured during a total of around 90 research flights.

As a prerequisite to study cloud influence, clear-sky cases were investigated in detail to quantify the impact of the aerosol optical thickness and surface albedo on spectral actinic flux densities. Over land, radiative transfer calculations show good agreement with the measured data independent of wavelength and altitude within about 10% under clear sky conditions. Over the ocean the situation is complicated, because ocean surface albedos (OSA) are not available from satellite observations. Available OSA parametrizations, that depend on atmospheric conditions, tend to lead to a slight overestimation of upward-directed actinic flux densities in particular in the visible range, but the agreement for total actinic flux densities is still comparable with that over land. With sufficient agreement of modelled and observed actinic flux densities under clear sky conditions, flight paths with clouds will be included comprising above-cloud, in-cloud and below-cloud conditions. In the model, liquid cloud effects can be parametrized using Mie theory, but ice clouds pose a more complex problem, due to the wide range of possible structures of ice crystals. Finally, the intent of this study is to asses the quality of the radiative transfer modelled actinic flux densities based upon the satellite-derived cloud information.

How to cite: Kremer, A. and Bohn, B.: Evaluation of Radiative Transfer Model Calculations of Solar Actinic Flux Densities with HALO Aircraft Measurements, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-280, https://doi.org/10.5194/ems2021-280, 2021.

Parameters of latent heat release were analyzed using LES model data. The system of trade wind cumulus clouds observed during the RICO field project was simulated over a domain size of a mesoscale model grid. The initialization of simulations were described in detail in the LES model intercomparison study by van Zanten et al 2011. Over 2000 clouds were analyzed focusing on relationship between parameters of latent heat release (phase transition rates) and dynamical/microphysical cloud characteristics.

Thephase transition rates, which in warm tropical clouds are represented by processes of condensation/evaporation, were analyzed by stratifying the clouds by their size/stage of maturity. The analyzed parameters included, among others, integral mass and buoyancy fluxes, cloud and rain water parameters, supersaturation. In addition to phase transition processes, we also analyzed the formation of precipitation and its dependence on cloud dynamical parameters. Of particular interest was the ratio of precipitation to condensation rate, which can be considered as an indicator of cloud “precipitation efficiency” (PE=PR/CR). We found that a critical vertical cloud depth separates clouds where PE is predominantly  < 1, from clouds where precipitation efficiency is mostly larger than one

The investigation of the relationships between phase transion rates and  cloud thermodynamical parameters revealed a remarkably strong correlation  between integral latent heat released in a cloud and its integral mass flux. The anticipated dependence on buoyancy flux was significanly weaker.

The identified latent heat-mass flux dependency and, based upon it, derived simple functional formulation can be important for the development of parameterization of subgrid latent heat release in meso- and large-scale forecast models.

How to cite: Kogan, Y.: Relating Integral Latent Heat Release to Integral Mass Flux in Cumulus Convective Clouds., EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-38, https://doi.org/10.5194/ems2021-38, 2021.

Cloud condensation nuclei (CCN) are the subset of aerosol particles able to form cloud droplets. CNN activation is influenced by the size distribution, chemical composition and number of particles. They consequently impact the cloud microstructure, which affects the radiative properties of clouds, atmospheric circulation and thermodynamics, as well as radiative budgets. By influencing the single scattering albedos of clouds, some particles lead to an increase of the solar irradiation absorption and solar heating in the cloud layers. A good example of these absorbing particles is those made of black carbon (BC), which is emitted during the combustion of various types of fuel and non-exhaust traffic-related processes. The present study deals with the role of BC in a solar radiative scheme and its interaction with clouds during a well-documented case of a fog that evolves into a low stratus cloud. To do so, the solar scheme of the computational fluid dynamic model Code Saturne is used for the estimation of fluxes and heating rates in the atmosphere. It is based on the two-stream parameterization with calculations done in the ultraviolet-visible (UV-Vis) and solar infrared (SIR) bands.

A special attention is given on the impact of BC on the dissipation of the fog. As expected, the introduction of BC in cloud droplets accentuates the heating in the layers at the top of the cloud where water liquid content is maximum. In the SIR band, there is an increase of approximatively 80 %. In the UV-Vis band, where absorption of solar irradiance by ozone is minor, the heating rate is now 10 times higher. The contribution of the UV-Vis band becomes more important. The augmentation of solar heating leads to a reduction of the liquid water content and, consecutively, to a faster dissipation of the fog and the stratus. Therefore, direct surface fluxes are also increased.

When increasing the volume fraction of black carbon in cloud droplets, the water liquid content is furthermore reduced leading to a faster dissipation of the fog. However, this impact is small, because the fog is formed in the morning. At this time, the cooling rate due to thermal radiation is higher than the solar heating at the top of the cloud. We expect the impact of black carbon in cloud droplets to be higher for more persistent clouds or for a fog in the boundary layer of the urban atmosphere, where the fraction of BC in particles is higher.

How to cite: Al Asmar, L., Musson-Genon, L., Dupont, E., and Sartelet, K.: Study on the role of Black Carbon aerosols in cloud droplets during the dissipation of a fog., EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-72, https://doi.org/10.5194/ems2021-72, 2021.

Fennoscandian boreal forest is a region with commonly occurring particle formation, which benefits from the abundance of biogenic volatile organic compounds emitted by the vegetation. The same vegetation also regulates the exchange of water vapour between the ecosystem and the atmosphere. Thus, as the forest has the potential to provide the two components needed in cloud formation, i.e. condensation nuclei and humidity, there is reason to suspect consequent changes in air masses that are influenced by the forest below.

We investigated the link between boreal forest air mass transport and cloud related properties in air masses that arrived to the SMEAR II station (61°10’N, 24°17’E, 170m a.s.l.), Finland, from between western and norther directions. These selected air masses were originally marine and travelled only across a land area with relatively minor anthropogenic emissions sources, allowing us to focus on biogenic influences. The source region and the time each air mass spent above land before arrival, were determined from 96-hour long air mass back trajectories. We used a long-term comprehensive data sets, spanning up to 11 growing seasons (April-September, 2006-2016).

Air masses with short transport times over the forest, often coincided with measurements of particles in smaller size ranges. Higher numbers of larger cloud condensation nuclei sized particles became more common in air masses with longer transport times over the forest. Similarly, air masses that spent little time over land, were often relatively cool and carried less water vapour. Whereas, higher specific humidities were more likely in air masses with longer times spent over land, as associated warming had most likely facilitated an increased uptake of water vapour from plant evapotranspiration. We also observed corresponding moderate increases in satellite observed cloud optical thickness and in-situ measured precipitation. Air masses with very short transport times over land were an exception, as these fast-moving air masses are likely to be connected to weather fronts and therefore also have a high probability for clouds and precipitation. The reported differences between air masses more or less disappeared when the transport time over land reached approximately 60 hours, and any further increase in land transport time no longer caused a substantial change. This appears to be the time scale in which most of the forest environment’s influence on these cloud related properties is realised and a balance is reached.

How to cite: Räty, M., Sogacheva, L., Keskinen, H.-M., Kerminen, V.-M., Petäjä, T., Ezhova, E., and Kulmala, M.: Transformation of marine air masses in the Fennoscandian Boreal forest – changes in aerosol, humidity, and clouds, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-175, https://doi.org/10.5194/ems2021-175, 2021.

A benchmarking study to compare various cloud optical property schemes is proposed. The transmittance, reflectance and absorptance of clouds of cloud optical property schemes that are widely used in weather and climate have in recent years been shown to give different results. Not much attention has been paid to this, probably due to the fact that errors in cloud cover have larger impacts on the shortwave and longwave radiative fluxes than the cloud optical properties. Here the optical properties are the mass extinction coefficient, the single scattering albedo and the asymmetry factor. Cloud optical property schemes are typically based on detailed theoretical calculations, which are then parametrized. Furthermore, spectral band averaging is performed to get optical properties that match the spectral bands of the radiative transfer scheme. Here the 14 shortwave and 16 longwave spectral bands of the RRTM scheme are mostly used presently. Both the equations used in the parametrizations and the spectral averaging matter. For the spectral averaging it must be considered that both the shortwave (solar) and longwave (thermal) spectra change following the circumstances in the atmospheric environment. Thus, a dry atmosphere has different spectral characteristics from a moist atmosphere. In the ultraviolet part of the spectrum ozone is also important to account for, as is the impact of carbon dioxide, methane, nitrous oxide and lesser greenhose gasses in the longwave part of the spectrum. This also affects the spectral distribution of the available radiative flux within individual spectral bands. Thus, the impact of the atmospheric environment must also be accounted for in the benchmarking study. The setup and an online framework for sharing benchmarking results will be presented.

How to cite: Nielsen, K. P.: Cloud optical property benchmarking study, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-401, https://doi.org/10.5194/ems2021-401, 2021.