ALBERT

Description: Abstract Radar‐rain gauge merging techniques have been widely used to improve the applicability of radar and rain gauge rainfall estimates by combining their advantages, while partially overcoming their individual weaknesses. Despite significant research in this area, guidance on the suitability of, and factors affecting merging techniques at the fine spatial‐temporal resolutions required for urban hydrological applications is still insufficient. In this paper, an in‐depth review of radar‐rain gauge merging techniques is conducted, with a focus on their potential for urban hydrological applications. An overview is first given of existing merging techniques and an application‐oriented categorization is proposed: (1) radar bias adjustment methods; (2) rain gauge interpolation methods using radar spatial association as additional information; (3) radar‐rain gauge integration methods. A detailed review is given of studies focusing on the evaluation and inter‐comparison of merging methods, based upon which the most widely used and best performing techniques from each category are identified. These are: Mean Field Bias (MFB) adjustment, Kriging with External Drift (KED), and Bayesian (BAY) merging. Climatological, operational and methodological factors affecting merging performance are then reviewed and their relevance for urban applications discussed. Based on this review, conclusions on merging potential for urban applications are drawn and research gaps are identified which should be addressed to provide further guidance on the use of merging techniques for urban hydrological applications.

Description: The primary goal of this paper is to introduce two new surface reflectivity climatologies. The two databases contain the Lambertian-equivalent reflectivity (LER) of the Earth's surface, and they are meant to support satellite retrieval of trace gases and of cloud and aerosol information. The surface LER databases are derived from the GOME-2 and SCIAMACHY instruments and can be considered as improved and extended descendants of earlier surface LER climatologies based on the TOMS, GOME-1, and OMI instruments. The GOME-2 surface LER database consists of 21 wavelength bands that span the wavelength range from 335 to 772 nm. The SCIAMACHY surface LER database covers the wavelength range between 335 and 1670 nm in 29 wavelength bands. The two databases are made for each month of the year and their spatial resolution is 1 ∘ ×1 ∘ . In this paper we present the methods that are used to derive the surface LER, we analyze the spatial and temporal behavior of the surface LER fields, and study the amount of residual cloud contamination in the databases. For several surface types we analyze the spectral surface albedo and the seasonal variation. When compared to the existing surface LER databases, both databases are found to perform well. As an example of possible application of the databases we study the performance of the FRESCO cloud information retrieval when it is equipped with the new surface albedo databases. We find considerable improvements. The databases introduced here can not only improve retrievals from GOME-2 and SCIAMACHY, but also support those from other instruments, such as TROPOMI, to be launched in 2017.

Description: [1]  This paper discusses Surface Insolation under Clear and Cloudy skies derived from SEVIRI imagery (SICCS), a physics-based, empirically adjusted algorithm developed for estimation of surface solar irradiance from satellite data. Its most important input are a cloud mask product and cloud properties derived from Meteosat/Spinning Enhanced Visible and Infrared Imager (SEVIRI) observations. These observations set the characteristics of the output, namely a temporal resolution of 15 minutes, a nadir spatial resolution of 3 × 3 km 2 , the period from January 2004 until at least November 2012 and the domain equal to most of the Meteosat disc. SICCS computes global, direct and diffuse irradiance separately. Direct irradiance for cloudy skies is estimated with an empirical method. Hourly means retrieved with SICCS were validated with data from eight Baseline Surface Radiation Network (BSRN) stations for the year 2006. We found median values of the station biases of +6 W/m 2 (+5%) for direct irradiance, +1 W/m 2 (+1%) for diffuse irradiance and +7 W/m 2 (+2%) for global irradiance. Replacing the three-hourly aerosol optical thickness input by monthly means introduces considerable additional biases in the clear-sky direct (−6%) and diffuse (+26%) irradiances. The performance of SICCS does not degrade when snow covers the surface. Biases do not vary with cloud optical thickness and cloud particle radius. However, the bias in global transmissivity tends to decrease with increasing cloud heterogeneity and the bias in direct transmissivity is a function of the solar zenith angle. We discuss why satellite retrieval of surface solar irradiance is relatively successful.

Description: The solar radiative absorption by an aerosol layer above clouds is quantified using passive satellite spectrometry from the ultraviolet (UV) to the shortwave infrared (SWIR). UV-absorbing aerosols have a strong signature that can be detected using UV reflectance measurements, even when above clouds. Since the aerosol extinction optical thickness decreases rapidly with increasing wavelength for biomass burning aerosols, the properties of the clouds below the aerosol layer can be retrieved in the SWIR, where aerosol extinction optical thickness is sufficiently small. Using radiative transfer computations, the contribution of the clouds to the reflected radiation can be modeled for the entire solar spectrum. In this way, cloud and aerosol effects can be separated for a scene with aerosols above clouds. Aerosol microphysical assumptions and retrievals are avoided by modeling only the pure (aerosol-free) cloud spectra. An algorithm was developed using the spaceborne spectrometer Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY). The aerosol direct radiative effect (DRE) over clouds over the South Atlantic Ocean west of Africa, averaged through August 2006 was found to be 23 ± 8 Wm−2 with a mean variation over the region in this month of 22 Wm−2. The largest aerosol DRE over clouds found in that month was 132 ± 8 Wm−2. The algorithm can be applied to any instrument, or a combination of instruments, that measures UV, visible and SWIR reflectances at the top of the atmosphere (TOA) simultaneously.

Description: In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small-scale turbulent eddies (i.e. not the energy-containing eddies) alone are responsible for broadening the droplet size spectrum during the initial stage of droplet growth due to condensation. It is likely, though, that small-scale turbulence plays a significant role in the growth of droplets through collisions and coalescence. Moreover, it has been possible through detailed numerical simulations to assess the relative importance of different processes to the turbulent collision kernel and how this varies in the parameter space that is important to clouds. The focus of research on the role of turbulence in condensational and collisional growth has tended to ignore the effect of entrainment and mixing and it is arguable that they play at least as important a role in the evolution of the droplet spectrum. We consider the role of turbulence in the mixing of dry and cloudy air, methods of quantifying this mixing and the effect that it has on the droplet spectrum. Copyright © 2012 Royal Meteorological Society and British Crown Copyright, the Met Office

Description: A climate model, coupled to a sophisticated land model, is used to explore the impact of nitrogen and phosphorous limitations on carbon uptake under increasing atmospheric carbon dioxide concentration, or [CO2], from 1870 to 2009. Adding nitrogen limitation strongly reduces the capacity of land CO2 uptake under increasing [CO2]. The further limitation by phosphorous has a smaller impact on the global uptake of CO2. However, phosphorous limitation has a strong impact on regional carbon uptake: increasing CO2 sinks over North America and Eurasia and decreasing sinks over China and Australia. Thus, while the global carbon balance can be resolved with just nitrogen limitation, simulations of continental-scale carbon sinks will need to include the additional limitation of phosphorous through the 20th century.

Description: Using an estimated bicarbonate concentration ([HCO3−]) in water and discharge rates of surface water and underground water from the Houzhai Basin, southwest China, from 1986 to 2007, we estimate that the mean carbon uptake rate was 20.7 g C m−2 yr−1. The surface water and underground water contribute about equally to the total carbon uptake from 1986 to 2007. About 97% of the interannual variation of annual carbon uptake can be explained by the discharge rate. Within a year, the net carbon uptake rate by karst during the wet season (May–October) was found to be about 2.4 times that during the dry season (November–April). If the seasonal variations of discharge rate and bicarbonate concentrations are not accounted for, estimates of annual net carbon uptake by karst can be biased by 〉25%, but that bias becomes very small (

Description: Abstract One of the most important sources of magnetospheric plasma is particle entry through the distant magnetotail boundary, the nightside magnetopause. This entry mechanism depends on the magnetopause configuration. Off the equator, the strong lobe magnetic field renders the magnetopause a tangential or a rotational discontinuity, and thus the magnetosheath field orientation predominantly controls particle entry through magnetic reconnection. At the equatorial, distant tail magnetopause, however, the magnetic field's control of particle entry is diminished because the plasma beta there is large on both sides of the boundary. Thus, transport there can be significantly different from that at the dayside and off‐equatorial magnetopauses. Using observations from two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes, we investigate plasma transport mechanisms around the distant equatorial magnetopause. We find that transport occurs as a series of abrupt transitions in density, ion and electron temperatures, and ion kinetic energy of spatial scales as small as a typical plasma sheet ion gyroradius. Analysis of the particle phase space density reveals that an energy‐selection mechanism controls electron transport across magnetopause, whereas ion transport is likely controlled by spatial diffusion driven by low‐frequency magnetic field fluctuations. We discuss the importance of these fluctuations for the magnetopause structure (e.g., the thickness of the transitions in plasma density, ion and electron temperatures, and ion kinetic energy).

Description: Three-dimensional simulations of bubble formation in Newtonian and non-Newtonian fluids through a microchannel T-junction are conducted by the volume-of-fluid method. For Newtonian fluids, the critical capillary number Ca for the transition of the bubble breakup mechanism is dependent on the velocity ratio between the two phases and the microchannel dimension. For the power law fluid, the bubble diameter decreases and the generation frequency increases with higher viscosity parameter K and power law index n . For a Bingham fluid, the viscous force plays a more important role in microbubble formation. Due to the yield stress τ y , a high-viscous region is developed in the central area of the channel and bubbles deform to a flat ellipsoid shape in this region. The bubble diameter and generation frequency are almost independent of K . Microbubbles are an important part of chemical and biomedical engineering. Three-dimensional simulations of bubble generation in both Newtonian and non-Newtonian fluids were performed in a microchannel T-junction by the volume-of-fluid model. The influences of viscosity parameter K , power law index n , and yield stress τ y on the bubble breakup mechanism, shape, size, and generation frequency were investigated.