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In dieser Dissertation wird die Parametrisierung von subgitterskaligen (SGS) Prozessen in Atmosphärenmodellen untersucht. Die Arbeit befasst sich mit den stochastisch angetriebenen Flachwassergleichungen, im ersten Teil in einer räumlichen Dimension und im zweiten Teil in zwei Dimensionen. Die Einteilung in aufgelöste und SGS-Variable erfolgt in beiden Fällen über lokale räumliche Mittel der Ursprungsvariable und deren Abweichungen vom lokalen Mittel.
Im eindimensionalen Fall liegt zwischen den Variablen eine deutliche Separation der charakteristischen Zeitskalen vor, wodurch die Anwendung der stochastischen Moden Reduktion (SMR) ermöglicht wird. Die SMR generiert ein reduziertes Modell der aufgelösten Variable mit einer stochastischen SGS-Parametrisierung, im Folgenden auch Schließung genannt. Die SMR-Schließung basiert auf den Grundgleichungen des Flachwassermodells und ist numerisch effizient einsetzbar, da sie nur eine geringe Anzahl von benachbarten Zellen koppelt. Sie verbessert die Ergebnisse des reduzierten Modells und übertrifft die Ergebnisse zweier zum Vergleich untersuchter empirischer stochastischer Schließungen. Den größten Zugewinn liefert sie im Energiespektrum, insbesondere für kleine Skalen. Das Ergebnis der SMR-Schließung kann verbessert werden, indem die Amplitude der stochastischen Schließungskomponente gedämpft wird. Die SMR-Schließung ist skalenabhängig im Sinne der räumlichen Modellauflösung. Untersucht wird die Schließung bei Halbierung und Viertelung der räumlichen Auflösung, wo sie ihre Überlegenheit gegenüber den empirischen Schließungen wiederholt bestätigt.
Im Unterschied zum eindimensionalen Fall ist in zwei Dimensionen auch die Corioliskraft enthalten und eine räumliche Divergenz der Schwerewellen möglich. Zwischen der aufgelösten und der SGS-Variable kommt es erneut zu einer Separation der charakteristischen Zeitskalen. Die Separation ist allerdings weniger stark ausgeprägt als im eindimensionalen Fall. Grund hierfür ist das Auftreten einer lang korrelierten geostrophisch balancierten Mode, welche auch auf die SGS-Variable projiziert. Das Vorgehen zur Bestimmung der SMR-Schließung für das zweidimensionale Modell verläuft analog zum eindimensionalen Fall. Es werden die Ergebnisse des hoch aufgelösten Referenzmodells und zweier Modelle ohne SGS-Schließung verglichen.
The weather of the atmospheric boundary layer significantly affects our life on Earth. Thus, a realistic modelling of the atmospheric boundary layer is crucial. Hereby, the processes of the atmospheric boundary layer depend on an accurate representation of the land-atmosphere coupling in the model. In this context the land surface temperature (LST) plays an important role. In this thesis, it is examined if the assimilation of LST can lead to improved estimates of the boundary layer and its processes.
To properly assimilate the LST retrievals, a suitable model equivalent in the weather prediction model is necessary. In the weather forecast model of the German Weather Service used here, the LST is modelled without a vegetation temperature. To compensate for this deficit, two different vegetation parameterizations were investigated and the better one, a conductivity scheme, was implemented. In order to make optimal use of the influence of the assimilation of the LST observation on the model system, it is useful to pass on the information of the observation to land and atmosphere already in the assimilation step. For that reason, a fully coupled land-atmosphere prediction model was used. Therefore, the existing control vector of the assimilation system, a local ensemble transform Kalman filter, was extended by the soil temperature and moisture. In two-day case studies in March and August 2017, different configurations of the augmented assimilation system were evaluated based on observing system simulation experiments (OSSE).
LST was assimilated hourly over two days in the weakly and strongly coupled assimilation system. In addition, every six hours a free 24-hour forecast was simulated. The experiments were validated with the simulated truth (a high-resolution model run) and compared against an experiment without assimilation. It was shown that the prediction of the boundary layer temperature, especially during the day, and the prediction of the soil temperature, during the whole day and night, could be improved.
The best impact of LST assimilation was achieved with the fully coupled system. The humidity variables of the model benefited only partially from the LST assimilation. For this reason, covariances in the model ensemble were investigated in more detail. To check their compatibility with the high-resolution model run the ensemble consistency score was introduced. It was found that the covariances between the LST and the temperatures of the high-resolution model run were better represented in the ensemble than those between the LST and the humidity variables.
Climatology of morphology and cloud-radiative properties of marine low-level mixed-phase clouds
(2023)
Marine stratocumuli cover about 40 - 60% of the ocean surface. They self-organize into different morphological regimes. The two organized cellular regimes are called open and closed mesoscale-cellular convective (MCC) clouds. In mid-to-high latitudes, open and closed cells are the two most frequent types of MCC clouds. In particular, many MCC clouds consist of a mixture of vapor, liquid droplets, and ice particles, referred to as mixed-phase clouds (MPCs). Even for the same cloud fraction, the albedo of open cells is, on average, lower than that of closed MCC clouds. Cloud phase and morphology individually influence the cloud radiative effect. Thus, this thesis investigates the relationships between the cloud phase, MCC organization, cell size, and differences regarding the cloud-radiative effect.
This thesis focuses on space-borne retrievals to achieve extensive temporal and spatial coverage. The liDAR-raDAR (DARDAR) version 2 product collocates two active and one passive satellite: CloudSat, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and Moderate Resolution Imaging Spectroradiometer (MODIS). The cloud phase of DARDAR is vertically integrated to establish a single cloud phase at each data point. The MCC classification data set based on the liquid water path (LWP) of MODIS scenes is collocated with the DARDAR product to determine the MCC organization. Cell-size statistics of both MCC clouds are obtained using a marker-based image segmentation method on MODIS reflectance scenes. In addition, based on MODIS reflectance scenes, a convolutional neural network (CNN) is developed to classify open and closed MCC scenes to avoid missing mature MPCs with a low LWP.
The first part of this thesis explores the relationships between cloud phase, morphology, and cloud albedo in the Southern Ocean (SO). At a given cloud-top temperature (CTT), seasonal changes in the mixed-phase fraction, defined as the number of MPCs divided by the sum of MPC and supercooled liquid cloud (SLC) pixels, are stronger than the morphological changes. Therefore, external factors seem to influence these changes instead of morphology. The dependence of cloud phase on cloud-top height (CTH) is more substantial than on CTT in clouds with CTHs below 2.5 km. The previously observed acceleration of closed-to-open transition in MPCs, known as preconditioning, is not the primary driver of climatological cloud morphology statistics in the SO. The morphological differences in cloud albedo are more pronounced in SLCs than in MPCs. This change in albedo alters the cloud radiative effect in the SO by 21Wm−2 to 39Wm−2 depending onseason and cloud phase.
Open and closed MCC clouds exhibit larger equivalent cell diameters in the MPCs than in SLCs in austral summer, whereas, in austral winter, the SLCs are larger. The cell’s aspect ratio accounts for varying CTHs. Closed cells have smaller aspect ratios than open cells, so their cell diameter is smaller, independent of CTH. While the seasonal differences in closed cells are due to changes in CTH, the seasonal aspect ratio differences in open cells are mainly caused by MPCs. With increasing aspect ratios, the cloud albedo decreases in both open and closed MCC clouds, with the most substantial decrease in open MPCs clouds. This leads to cloud-radiative changes of 60 - 75Wm−2 in the SO, depending on cloud phase and aspect ratio.
The established CNN exhibits a good accuracy of 80.6%, with even higher accuracies in the Open (85.5%) and Closed (87.3%) categories. The global MCC climatology based on the CNN generally agrees well with previous MCC distributions. The most notable difference occurs in the Northern Hemisphere (NH) in boreal winter, with a higher occurrence frequency of closed and open MCC clouds. This might indicate missing MPCs in previous studies based on the LWP and some restricted to warm cloud scenes. Thus, the developed CNN seems to better represent the different morphologies in MPCs than in previous classifications.
In conclusion, this thesis shows that understanding the dependencies of cloud phase, cloud morphology, and cell size is important to enhance predictions of the cloud-radiative effect and thus, it is important to evaluate how cloud phase, cloud morphology, and cellsize change in a warming climate.
In this dissertation, different aspects of turbulent transport and thermally driven flows over complex terrain are investigated. Two publications concentrate on the vertical heat and moisture exchange in the convective boundary layer over mountainous terrain. To study this, Large-Eddy Simulation (LES) is used. Both turbulent and advective transport mechanisms are evaluated over the simple orography of a quasi-two-dimensional, periodic valley with prescribed surface fluxes. Here, terrain elevation varies along only one of the horizontal coordinate axes. Even a relatively shallow orography, possibly unresolved in existing numerical weather prediction models, modifies the domain-averaged moisture and temperature profiles. For the analysis, the flow is decomposed into a local turbulent part, a local mean circulation, and a large-scale part. An analysis of the turbulent kinetic energy and turbulent heat and moisture flux budgets shows that the thermal circulation significantly contributes to the vertical transport. It is found that thermal upslope winds are important for the moisture transport from the valley to the mountain tops. In total, moisture export out of the valley is mostly accomplished by the mean circulation. On the temperature distribution, which is horizontally relatively homogeneous, the thermal circulation has a weaker impact. If an upper-level wind is present, it interacts with the thermal circulation. This weakens the vertical transport of moisture and thus reduces its export out of the valley. The heat transport is less affected by the upper-level wind because of its weaker dependence on the thermal circulation. These findings were corroborated in a more realistic experiment simulating the full diurnal cycle using radiation forcing and an interactive land surface model.
Based on these results, coherent turbulent structures in the convective boundary layer over non-flat terrain are studied in further detail. A conditional sampling method based on the concentration of a decaying passive tracer is implemented in order to identify the boundary-layer plumes objectively. Conditional sampling allows to quantify the contribution of plume structures to the vertical transport of heat and moisture. In case of the idealized valley, vertical transport by coherent structures is the dominant contribution to the turbulent components of both heat and moisture flux. It is comparable in magnitude to the advective transport by the mean slope-wind circulation, although it is more important for heat than for moisture transport. A set of less idealized simulations considers the flow over three-dimensional terrain. In this case, conditional sampling is carried out by using a simple domain-decomposition approach. We demonstrate that thermal updrafts are generally more frequent on hill tops than over the surroundings, but they are less persistent on the windward sides when large-scale winds are present in the free atmosphere.
The tools for flow decomposition and budget analysis are also applied in another idealized case with a quasi-two-dimensional valley featuring the stable boundary layer. Here, the formation of a low stratus cloud is investigated. The main driver for the cloud formation is radiative cooling due to outgoing longwave radiation. Despite a purely horizontal flow, the advection terms in the prognostic equations for heat and moisture produce vertical mixing across the upper cloud edge leading to a loss of cloud water content. However, this behavior is not due to any kind of thermally-driven circulation. Instead, this spurious mixing is caused by the diffusive error of the advection scheme in regions where the sloping surfaces of the terrain-following vertical coordinate intersect the cloud top. It is shown that the intensity of the (spurious) numerical diffusion strongly depends on the horizontal resolution, the order of advection, and the choice of the scalar advection scheme. A LES with 4 m horizontal resolution serves as a reference. For horizontal resolutions of a few hundred meters, carried out with a model setup as it is used in Numerical Weather Prediction, a strong reduction of the simulated liquid-water path is observed. In order to keep the (spurious) numerical diffusion at coarser resolutions small, at least a fifth-order advection scheme should be used. In the present case, a WENO scalar advection scheme turns out to increase the numerical diffusion along a sharp cloud edge compared to an upwind scheme. Furthermore, the choice of the vertical coordinate has a strong impact on the simulated liquid-water path over orography. With a modified definition of the terrain-following sigma coordinate, it is possible to produce cloud water where the classical sigma coordinate does not allow any cloud formation.