Orographic Cirrus Research Articles

Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

AbstractCirrus clouds play an important role in the Earth's radiative energy budget, thereby affecting the climate state and climate change. Orographic gravity wave (OGW)‐induced sub‐grid scale vertical velocity (i.e., cooling rate) is not resolved by large‐scale models and its impact on ice formation in cirrus clouds is not well quantified. In this study, one sub‐grid scale OGW scheme (e.g., McFarlane) is used in the Community Atmosphere Model version 6 (CAM6) to generate vertical velocity variance (σw) for cirrus formation. Results from the default model and simulations with the OGW‐induced σw are evaluated against the DOE ARM Small Particles in Cirrus (SPARTICUS) campaign observations. The OGW based on the McFarlane scheme increases the sub‐grid scale σw over mountains compared to the default model and improves the model agreement with the SPARTICUS observations. Larger σw due to OGWs can trigger more frequent homogeneous nucleation in orographic cirrus and generates a higher number concentration of ice crystals observed during the SPARTICUS campaign. Moreover, our evaluation of the model simulations against satellite observations indicates that the McFarlane scheme generates high in‐cloud ice number concentrations (>200 L−1) in the upper troposphere over mountains and high plateaus at mid‐ and high‐latitudes of the winter hemisphere as shown in the observations. More ice crystals with smaller sizes absorb more infrared radiation (+0.523 ± 0.125 W m−2). The net radiative cloud forcing change at the top of the atmosphere is +0.330 W m−2 due to the orographic cirrus.

Orographic cirrus and radiative forcing in CESM2

Orographic cirrus and radiative forcing in CESM2

Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

Identifying sensitivities for cirrus modelling using a two-moment two-mode bulk microphysics scheme

Cirrus cloud genesis is an inherently multiscale and non-linear problem. The synoptic scale provides the environment, the mesoscale determines the forcing and the actual nucleation events occur on a microscopic scale. This makes the parameterisation in numerical weather prediction models a challenging task. In order to improve the prediction of cirrus clouds and ice supersaturation formation in the German Weather Service (DWD) model chain, the controlling physical processes are investigated and parameterised in a new cloud ice microphysics scheme. The new scheme is an extended version of the ice-microphysical scheme operational in the numerical weather prediction models of DWD. The developed two-moment two-mode cloud ice scheme includes state-of-the-art parameterisations for the two main processes for ice formation, namely homogeneous and heterogeneous nucleation. Homogeneous freezing of supercooled liquid aerosols is triggered in regions with high atmospheric ice supersaturations (145–160%) and high cooling rates. Atmospheric small-scale fluctuations are accounted for by use of the turbulent kinetic energy. Heterogeneous nucleation depends mostly on the existence of ice nuclei in the atmosphere and occurs primarily at lower ice supersaturations. Thus, heterogeneously nucleated ice crystals deplete ice supersaturation via depositional growth and can therefore inhibit subsequent homogeneous freezing. The new cloud ice scheme accounts for pre-existing ice crystals, contains a prognostic budget variable for activated ice nuclei and includes cloud ice sedimentation. Furthermore, a consistent treatment of the depositional growth of the two-ice particle modes and the larger snowflakes is applied by using a relaxation time scale method which ensures a physical representation for depleting ice supersaturation. The new cloud ice scheme is used to identify the relative roles of heterogeneous and homogeneous nucleation in the formation of cirrus clouds and ice supersaturation. A parcel model is used in order to investigate the differences between the operational and new cloud ice scheme. The time scales for the homogeneous nucleation event and for the depositional growth are emphasised. The importance of the new ice nucleation scheme is demonstrated by conducting idealised simulations of orographic cirrus in the COSMO (Consortium for Small-Scale Modeling) model environment.

Influence of heterogeneous freezing on the microphysical and radiative properties of orographic cirrus clouds

Abstract. The influence of heterogeneous freezing on the microphysical and optical properties of orographic cirrus clouds has been simulated with the large eddy simulation model EULAG. Idealised simulations with different concentrations of ice nuclei (IN) in a dynamically dominated regime with high vertical velocities have been performed. Furthermore the temperature at cloud formation as well as the critical supersaturation for initiation of heterogenous freezing have been varied. The shortwave, longwave and net cloud forcing has been calculated under the assumption that the clouds form between 06:00 and 12:00 local time (LT) or between 12:00 and 18:00 LT. In general it can be seen that the onset of homogeneous freezing is shifted in time depending on the IN concentration, as part of the available water vapour is depleted before the critical threshold for homogeneous freezing is reached. Although the high vertical velocities in an orographic gravity wave lead to a strong adiabatic cooling followed by high ice supersaturations, even a small number concentration of IN of the order of 5 L−1 is able to strongly decrease the simulated ice crystal number burden (ICNB), ice water path (IWP) and optical depth of the cloud. In general, the ICNB, IWP and optical depth strongly decrease when the IN concentrations are increased from 0 to 50 L−1. The absolute values of the shortwave, longwave and net cloud forcing are also reduced with increasing IN concentrations. A cloud will produce a net warming or cooling depending on the IN concentration, the temperature and the time of day when the cloud forms. The clouds that form between 06:00 and 12:00 LT are mainly cooling, whereas the clouds with the same microphysical properties can lead to a warming when they form between 12:00 and 18:00 LT. In order to predict the radiative forcing of cirrus clouds it is therefore necessary to take the correct dynamical and thermodynamical processes as well as the possible existence and freezing threshold of heterogeneous IN into account, not only for low vertical velocities but also for dynamically dominated regimes like orographic cirrus.

Influence of a future climate on the microphysical and optical properties of orographic cirrus clouds in ECHAM5

The European Centre/Hamburg 5 (ECHAM5) general circulation model is used in order to investigate the influence of a warmer climate on the microphysical and optical properties of orographic cirrus clouds. The main goal of this study is to highlight the variety of processes influencing the formation of orographic cirrus and to emphasize the importance of coupling dynamics and cloud microphysics in order to provide realistic predictions of the influence of a future climate on cloud microphysical and radiative properties. Therefore, a coupling of gravity wave dynamics and cloud microphysics is implemented in the model. The influence of additional moisture on the propagation of gravity waves is investigated by using the dry and moist Brunt‐Väisäla frequency in the calculation of the gravity wave‐induced vertical velocity in two different simulations. In both simulations, the vertical velocities increase in the warmer climate. The additional moisture decreases the Brunt‐Väisäla frequencies leading to less flow blocking and thus higher effective mountain heights. As this effect dominates over the decrease of the gravity wave amplitude due to more moisture, higher vertical velocities occur in a future climate. The opposite effect of a decreased vertical velocity in a future climate can be seen over the dry regions. From the present to the future climate, the ice crystal number concentration decreases despite the increased vertical velocities. Higher temperatures lead to a faster growth of the ice crystals, and the supersaturation is depleted faster such that no new crystals can be formed. The ice water content increases as more water vapor is available in a warmer climate. The net effect of a decreased ice crystal number concentration and an increased ice water content is an increased optical depth in a future climate. This result is in good agreement with recent cloud resolving studies. The effect of orographic cirrus clouds on the radiation is given by an increased short‐ and long‐wave cloud forcing, whereas the latter dominates. However, from the present to the future climate, no changes in orographic cloud cover and cloud forcing over mountains can be seen.

Orographic cirrus in a future climate

Abstract. A cloud resolving model (CRM) is used to investigate the formation of orographic cirrus clouds in the current and future climate. The formation of cirrus clouds depends on a variety of dynamical and thermodynamical processes, which act on different scales. First, the capability of the CRM in realistically simulating orographic cirrus clouds has been tested by comparing the simulated results to aircraft measurements of an orographic cirrus cloud. The influence of a warmer climate on the microphysical and optical properties of cirrus clouds has been investigated by initializing the CRM with vertical profiles of horizontal wind, potential temperature and equivalent potential temperature, respectively. The vertical profiles are extracted from IPCC A1B simulations for the current climate and for the period 2090–2099 for two regions representative for North and South America. The influence of additional moisture in a future climate on the propagation of gravity waves and the formation of orographic cirrus could be estimated. In a future climate, the increase in moisture dampens the vertical propagation of gravity waves and the occurring vertical velocities in the moist simulations. Together with higher temperatures fewer ice crystals nucleate homogeneously. Assuming that the relative humidity does not change in a warmer climate the specific humidity in the model is increased. This increase in specific humidity in a warmer climate results in a higher ice water content. The net effect of a reduced ice crystal number concentration and a higher ice water content is an increased optical depth. However, in some moist simulations dynamical changes contribute to changes in the ice water content, ice crystal number concentration and optical depth. For the corresponding dry simulations dynamical changes are more pronounced leading to a decreased optical depth in a future climate in some cases.

Orographic cirrus in the global climate model ECHAM5

A comparison of satellite data with simulations from global circulation models shows that there is a lack of cirrus cloud amount in large‐scale models above and in the lee of mountains. The formation of orographic cirrus clouds due to gravity waves is usually not parameterized in large‐scale models. To improve the simulation of such orographically excited cirrus clouds a coupling of the gravity wave dynamics and the cloud microphysics has been implemented in the climate model European Centre/Hamburg 5 (ECHAM5). As homogeneous freezing of solution droplets strongly depends on the vertical velocity, an increased vertical velocity due to gravity wave activity in the upper troposphere leads to the formation of cirrus clouds with higher ice crystal number densities. A comparison of the new parameterization with measurements shows a better agreement with observations.

Parameterisation of orographic cloud dynamics in a GCM

A new parameterisation is described that predicts the temperature perturbations due to sub-grid scale orographic gravity waves in the atmosphere of the 19 level HadAM3 version of the United Kingdom Met Office Unified Model. The explicit calculation of the wave phase allows the sign of the temperature perturbation to be predicted. The scheme is used to create orographic clouds, including cirrus, that were previously absent in model simulations. A novel approach to the validation of this parameterisation makes use of both satellite observations of a case study, and a simulation in which the Unified Model is nudged towards ERA-40 assimilated winds, temperatures and humidities. It is demonstrated that this approach offers a feasible way of introducing large scale orographic cirrus clouds into GCMs.

Orographic cloud in a GCM: the missing cirrus

Observations from the International Satellite Cloud Climatalogy Project (ISCCP) are used to demonstrate that the 19-level HadAM3 version of the United Kingdom Met Office Unified Model does not simulate sufficient high cloud over land. By using low-altitude winds, from the European Centre for Medium Range Weather Forecasting (ECMWF) Re-Analysis from 1979 to 1994 (ERA-15) to predict the areas of maximum likelihood of orographic wave generation, it is shown that much of the deficiency is likely to be due to the lack of a representation of the orographic cirrus generated by sub-grid scale orography. It is probable that this is a problem in most GCMs.

Near micron‐sized cirrus cloud particles in high‐resolution infrared spectra: An orographic case study

The high‐resolution spectra of the Atmospheric Infrared Sounder (AIRS) provide a global view of small‐ particle‐dominated cirrus clouds, and they exist over much larger spatial extents than seen in previous aircraft campaigns. As shown by simulations using a plane‐parallel scattering radiative transfer (RT) model and realistic ice particle shapes, the shape of the radiance spectra in the atmospheric windows is uniquely influenced by small ice crystals. Minima in the brightness temperature (BT) spectra between 800 to 850 cm−1 are seen for ice particles smaller than 3 μm in the RT simulations and AIRS spectra. A case study of an orographic cirrus cloud observed on October 2, 2002, over the central Andes of South America is presented with spectral BT differences up to 63K between 998 and 811 cm−1.

Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics

Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics

Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics

AbstractIce particle production by the processes of homogeneous freezing and heterogeneous nucleation in orographic cirrus clouds is studied using a simple adiabatic ascending‐air‐parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observed rates, modified according to the solution effect. Two alternative ice nuclei (IN) activation spectra are applied, one dependent upon temperature and the other upon ice supersaturation. Simulations are performed for air parcels with initial dew‐point temperatures, Td, in the range from −20 to −45°C and constant vertical velocities, w, of 0.1 to 5.0 m s‐1. In modelled clouds with Td ⩾ 30°C, heterogeneous nucleation initiated by IN is found to dominate ice formation, since homogeneous freezing rates are so low. In modelled clouds with Td ⩽ −35°C, pure liquidwater homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with heterogeneous nucleation then remaining dominant even at very low temperatures. the IN number concentrations required for homogeneous freezing suppression, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w‐heterogeneous nucleation at low temperatures is therefore much more viable at the low w values found in non‐orographic compared with orographic cirrus clouds. the common conception of a deficit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol particles, IN measurements at cirrus levels, in addition to CCN measurements, are suggested as vital. the model results suggest that it may be possible to interpret measurements of peak liquid‐water content, or possibly maximum droplet radius, in orographic cirrus clouds in terms of the dominant ice nucleation mode.

Cloud reflectance variations in channel-3

Abstract Using photographic terminology for channel 3 pictures in sunshine, one notes that most ice clouds appear black and that cloud shadows are equally dark, but water droplet clouds appear in all shades. These shades also vary greatly with the direction of sunshine relative to the line of sight because scatter is almost entirely by diffraction. Droplets and ice crystals larger than about 10 fan absorb the incident radiation almost completely and it does not penetrate through clouds unless there exist plenty of unobstructed ray paths through the clouds. The reflection from a water surface is almost metallic in intensity so that glint completely saturates the radiometer. There is no evidence of comparable reflection from ice. All snow-covered surfaces, including sea ice, appear black. Stratus cloud shows large variations in reflectance depending on the state of the convection in it which brings very small droplets to the surface. Small particle size causes some contrails and orographic cirrus to appear ...

Mesoscale and Microscale Structure of Cirrus Clouds: Three Case Studies

Abstract The structure and composition of three basic cirrus cloud types are examined through coordinated aircraft and ground-based polarization lidar and radar measurements. The cloud systems consist of a multilayered orographic cirrus, a 6-km deep cirrostratus, and a group of fibrous cirrus bands at the tropopause. The data reveal the presence of mesoscale generating regions with horizontal dimensions ranging from ∼15 km in narrow cloud bands up to ∼100 km in cirrostratus. These generating regions appear to be composed of complexes of much smaller convective structures, presumably on the ∼1-km scale of cirrus uncinus cells, and so are termed Mesoscale Uncinus Complexes (MUC). Accumulations of ice particles within cirrus, commonly referred to as precipitation trails, are associated with generating regions at or near cloud tops, but are also created by the local production of ice crystals within embedded convective impulses. Supercooled cloud droplets large enough to be detected by aircraft probes (≳5 μm ...