Besides frequently occurring shallow cumulus clouds in the trade wind region, thin clouds of stratiform extent are often accompanying shallow convection and create, in symbiosis, various cloud patterns.
Using geostationary satellite data, the high-resolution measurements of shallow convection gathered at the Barbados Cloud Observatory are categorized to study the macrophysical properties of clouds depending on their mesoscale pattern, in particular the ones of stratiform clouds. Within such an environment stratiform clouds express a different behaviour than within other environments. This flower pattern is one of few organizational patterns of shallow convection.
It has its appearance largely from stratiform cloud cover, which can become very thin, but still long lasting. To gain a better understanding of the lifecycle of these clouds and their environmental conditions, differences between the stratiform clouds in an organized environment to a less organized one are discussed. It is well recognized that ice clouds have a significant influence on the global radiation budget and climate system. Previous studies indicate that improved assessment of the cloud influence requires a better understanding of their microphysical properties, such as ice water content and hydrometeor size, shape, and hydrometeor type.
However, estimates of ice water content IWC and ice particle size have been severely limited in both temporal and spatial coverage because of the lack of observational data , which results in a large uncertainty in numerical weather forecasting. Furthermore, important microphysical processes such as the conversion between different hydrometeor classes and ice particle growth are difficult to observe in-situ as well as from single remote sensing observations. The synergy of radars operating at different wavelengths offer a unique way of providing the necessary insight into ice microphysics at a high vertical resolution.
For this study, two radars operating at different wavelengths are used for coordinated measurements. The two radars are separated by a distance of about 23 km and have a large overlapping region. The Ka-band radar is vertically probing the atmosphere continuously as well as running a synchronous RHI scan with the C-band radar. Based on the well-established IWC-Ze relationship, we retrieved the IWC from the reflectivity observations with the Ka-band radar and further their statistical distribution.
Furthermore, dual-wavelength reflectivity ratio DWR measurements are used to determine the size distribution of ice crystals. To this aim, series of measurements have been performed during rain, snow and fog events of past few months in Warsaw, Poland.
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Aforementioned method involves illuminating the region of interest from behind by using incoherent, expanded and collimated laser light beam and collecting shadow images of droplets as well as particles at up to 30 frames or pairs of frames per second with a high resolution digital camera. Lastly, size distributions are built by analyzing series of images. The tests described here demonstrate the technique and establish the potential for further more quantitative studies of size distributions of cloud and rain droplets as well as other precipitation particles.
In most of the models used for operational weather forecasts and climate studies, shallow cumulus clouds and their convective transport are simulated using a mass flux approach, which predicts the vertical structure of cumulus up- and downdrafts. This makes the shallow convective mass flux a key parameter for model simulations.
For this reason, measurements of the shallow convective mass flux are as important as simulations. In this study, we use measurements to derive the shallow convective mass flux by using a cloud radar and a Doppler lidar instrument from the Barbados Cloud Observatory BCO.
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Vertical profiles of the shallow convective mass flux will be shown and compared with prior studies from Ghate et al. Furthermore, we will present its diurnal variability and the relationship to the diurnal cloud cover.
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Ghate, V. Miller, and L. DiPretore , Vertical velocity structure of marine boundary layer trade wind cumulus clouds, J. With the objective to better understand the impact of convective organization on climate, this study characterizes the organizational state of convective clouds in observations and evaluates its representation in ICON-LEM simulations.
First, signatures of convective clouds objects are identified in fields of radar reflectivities and satellite brightness temperatures observed over Germany for the summer seasons of the years and using a threshold-based segmentation algorithm. To assess the organizational state and the dominant shape of the objects a series of organization indices are applied and compared: the nearest-neighbor distance-based index I.
The difference may be caused by the much larger area of the cirrus anvil shields observed by satellite in comparison to precipitation cores detected by radar. Overall organization shows an increase in the rainfall amounts compared to non-organized objects, which, however, seems to be associated with larger number of objects and sizes and not with the organizational state itself.
In the second part of the study outputs of ICON-LEM simulations for four selected days are translated into observation space and evaluated their ability to represent convective organizations.
In order to investigate also the impact of the model resolution, outputs of the model runs with grid resolutions of m, m and m are considered. Based on the case studies, the most robust indices I. SCAI has been identified as sensitive to model deficiencies like underestimation of heavy precipitation and the overestimation of cold cloud coverage, small-scale clouds and precipitation cores and thus less suited for model evaluation studies.
The coarsest simulations with m spatial resolution generate objects with shapes close to the observed ones, while finer resolutions lead to more pronounced deviations. The presented study i suggests the combined use of I. The continued development of model parameterizations to represent clouds, precipitation and turbulence in weather and climate models requires measurements that can provide information on vertical structure and sub-grid scale variability of key quantities. To support development of globally-applicable parameterizations, such measurements must be provided over a wide variety of conditions and locations.
Over the past several years unmanned aircraft and tethered balloons have been deployed to make measurements of thermodynamic structure, winds, turbulent fluxes of heat and momentum, turbulence, aerosol properties, and cloud microphysics. These systems can provide high resolution perspectives on the vertical structure of the atmosphere and its evolution, horizontal variability and co-variability of key quantities such as vertical velocity and humidity, and the atmospheric state over difficult-to-sample regions. In this presentation, we will provide an overview of unmanned aerial systems recently deployed under projects lead by the University of Colorado and the NOAA Physical Sciences Division.
The presentation will focus on scientific questions that these systems can help to address, touching upon topics such as lower atmospheric evolution and structure under different cloud regimes, the turbulent structure of the subcloud environment, the influence of clouds on the vertical distribution of aerosol particles, and the use of these systems in advancing our understanding of ice crystal shape. Wind shear is an important factor in the upscale transport of energy in deep convective cloud fields, contributing to the formation organized features such as squall lines and MCSs.
Cloud Resolving Models CRMs can be used to diagnose the effects of wind shear on the cloud field, investigating e. Here, we use two sets of wind shear profiles to drive our experiments. The first is a set of scaled profiles known to produced a squall line like feature, which are used to systematically investigate the effect of wind shear on organization. The second is a representative set of wind profiles derived from a GCM, which represent different shear regimes found in a GCM.
We present results showing how the different shear profiles affect the cloud field, with particular attention paid to how these results could be used to modify a GCM's convective parametrization scheme. Shallow maritime clouds have been identified as a major uncertainty factor in climate predictions. Advanced observations of key quantities such as the liquid water path LWP are a key to better understanding of changing climate.
Different satellite LWP records differ by a factor up to two in the tropics. Microwave satellites provide the longest LWP record over ice-free oceans for several decades. However, they suffer from large footprints and superposing signals of clouds and precipitation. Therefore we use spatially highly resolved airborne microwave observations complemented by simultaneous radar and lidar measurements to better understand the LWP of warm clouds over the tropical North Atlantic. The measurements were taken with the HALO research aircraft which was equipped with several remote sensing instruments, all mounted in nadir orientation.
A combination of passive microwave and active cloud radar observations allows to extend the application of LWP retrieval to drizzle and light precipitation. As to be expected the IWV clearly shows that the wet season is more humid, but also reveals that the frequency distribution is strongly affected by the choice of the flight pattern. Therefore, the airborne measurements need to be combined with long-term ground and space-borne measurements to draw statistically sound conclusions. While current weather prediction models are able to explicitly model convection, they still struggle with adequately representing the initiation of convection.
One such process is secondary convection initiation driven by cold pools and their gust fronts: precipitating downdrafts generate density currents lead by gust fronts which can provoke the triggering of new convective cells nearby via mechanical and thermodynamic lifting. Misrepresenting such secondary triggering mechanisms can potentially cause a lack of convection organization and skew the diurnal cycle of precipitation.
Both a shortage of organization and a deficit in late afternoon precipitation have been observed in current km-scale models. Consequently, a better understanding of these effects and their representation in km-scale models is crucial to enable advancements in forecasting convective precipitation. We address this challenge by identifying cold pools and initiated convective cells in high-resolution ICON model simulations over Germany with model grid sizes from m to m. We compare cold pool characteristics and probabilities of convection initiation between different model resolutions. We confirm that cold pools are indeed related to missing late afternoon convection.
We investigate how cold-pool-driven convection initiation is misrepresented in km-scale models. First results show that vertical velocity amplitudes in cold pool gust fronts, which strongly influence the probability of convection initiation, are significantly lower in coarser resolution runs. This leads to a lower probability of triggering new convection at gust fronts at coarser resolution. This improved understanding of cold-pool-driven convection initiation and the corresponding deficits in km-scale models will be used to develop a perturbation scheme for use in kilometer-scale models that will increase convective initiation, leading to better precipitation forecasts.
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Understanding important factors determining the spatial distribution of precipitation over land is frustrated by a lack of consensus as to the basic effect of soil moisture on precipitation. Such effects are often blurred by the complexity of the land-atmosphere system or biased by the use of convective parameterizations.
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Here we take a simpler approach and apply the radiative convective equilibrium RCE framework to an idealized land planet that only retains the basic control of soil moisture on precipitation. The goal is to understand whether soil moisture acts to support or to impede the organization of convection. Results of our convection-permitting simulations show that, initially, shallow circulations driven by differential radiative cooling induce a self-aggregation of the convection into a single band, as has become familiar from simulations over idealized sea-surfaces.
With time, however, the drying out of the non-precipitating region induces a reversal of the shallow circulation, drawing the flow at low-levels from the precipitating to the non-precipitating region.