Function Of Surface Albedo Research Articles

Effects of variable ice–ocean surface properties and air mass transformation on the Arctic radiative energy budget

Abstract. Low-level airborne observations of the Arctic surface radiative energy budget are discussed. We focus on the terrestrial part of the budget, quantified by the thermal-infrared net irradiance (TNI). The data were collected in cloudy and cloud-free conditions over and in the vicinity of the marginal sea ice zone (MIZ) close to Svalbard during two aircraft campaigns conducted in the spring of 2019 and in the early summer of 2017. The measurements, complemented by ground-based observations available from the literature and radiative transfer simulations, are used to evaluate the influence of surface type (sea ice, open ocean, MIZ), seasonal characteristics, and synoptically driven meridional air mass transports into and out of the Arctic on the near-surface TNI. The analysis reveals a typical four-mode structure of the frequency distribution of the TNI as a function of surface albedo, the sea ice fraction, and surface brightness temperature. Two modes prevail over sea ice and another two over open ocean, each representing cloud-free and cloudy radiative states. Characteristic shifts and modifications of the TNI modes during the transition from winter to spring and early summer conditions are discussed. Furthermore, the influence of warm air intrusions (WAIs) and marine cold-air outbreaks (MCAOs) on the near-surface downward thermal-infrared irradiances and the TNI is highlighted for several case studies. It is concluded that during WAIs the surface warming depends on cloud properties and evolution. Lifted clouds embedded in warmer air masses over a colder sea ice surface, decoupled from the ground by a surface-based temperature inversion, have the potential to warm the surface more strongly than near-surface fog or thin low-level boundary layer clouds because of a higher cloud base temperature. For MCAOs it is found that the thermodynamic profile of the southward-moving air mass adapts only slowly to the warmer ocean surface.

Brown carbon aerosols from burning of boreal peatlands: microphysical properties, emission factors, and implications for direct radiative forcing

Abstract. The surface air warming over the Arctic has been almost twice as much as the global average in recent decades. In this region, unprecedented amounts of smoldering peat fires have been identified as a major emission source of climate-warming agents. While much is known about greenhouse gas emissions from these fires, there is a knowledge gap on the nature of particulate emissions and their potential role in atmospheric warming. Here, we show that aerosols emitted from burning of Alaskan and Siberian peatlands are predominantly brown carbon (BrC) – a class of visible light-absorbing organic carbon (OC) – with a negligible amount of black carbon content. The mean fuel-based emission factors for OC aerosols ranged from 3.8 to 16.6 g kg−1. Their mass absorption efficiencies were in the range of 0.2–0.8 m2 g−1 at 405 nm (violet) and dropped sharply to 0.03–0.07 m2 g−1 at 532 nm (green), characterized by a mean Ångström exponent of ≈ 9. Electron microscopy images of the particles revealed their morphologies to be either single sphere or agglomerated “tar balls”. The shortwave top-of-atmosphere aerosol radiative forcing per unit optical depth under clear-sky conditions was estimated as a function of surface albedo. Only over bright surfaces with albedo greater than 0.6, such as snow cover and low-level clouds, the emitted aerosols could result in a net warming (positive forcing) of the atmosphere.

An image-based four-source surface energy balance model to estimate crop evapotranspiration from solar reflectance/thermal emission data (SEB-4S)

A remote sensing-based surface energy balance model is developed to explicitly represent the energy fluxes of four surface components of agricultural fields including bare soil, unstressed green vegetation, non-transpiring green vegetation, and standing senescent vegetation. Such a four-source representation (SEB-4S) is achieved by a consistent physical interpretation of the edges and vertices of the polygon (named T−fvg polygon) obtained by plotting surface temperature (T) as a function of fractional green vegetation (fvg) and the polygon (named T−α polygon) obtained by plotting T as a function of surface albedo (α). To test the performance of SEB-4S, a T−α image-based model and a T−fvg image-based model are implemented as benchmarks. The three models are tested over a 16km by 10km irrigated area in northwestern Mexico during the 2007–2008 agricultural season. Input data are composed of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) thermal infrared, Formosat-2 shortwave, and station-based meteorological data. The fluxes simulated by SEB-4S, the T−α image-based model, and the T−fvg image-based model are compared on seven ASTER overpass dates with the in situ measurements collected at six locations within the study domain. The evapotranspiration simulated by SEB-4S is significantly more accurate and robust than that predicted by the models based on a single (either T−fvg or T−α) polygon. The improvement provided with SEB-4S reaches about 100Wm−2 at low values and about 100Wm−2 at the seasonal peak of evapotranspiration as compared with both the T−α and T−fvg image-based models. SEB-4S can be operationally applied to irrigated agricultural areas using high-resolution solar/thermal remote sensing data, and has potential to further integrate microwave-derived soil moisture as additional constraint on surface soil energy and water fluxes.

Detection of natural oil seeps signature from SST and ATI in South Yellow Sea combining ASTER and MODIS data

Daytime Advanced Spaceborne Thermal Emission and reflection Radiometer (ASTER) data and the consequent nighttime Moderate Resolution Imaging Spectroradiometer (MODIS) data have been utilized to retrieve the sea surface temperature (SST) and apparent thermal inertia (ATI). They were used to detect the signature of natural oil seepage in Yingge Sea Basin (YSB) and South Yellow Sea (SYS). In this paper, ATI was used for the first time to detect the signature of natural oil seepage. ATI is an approximation of the real thermal inertia and can be expressed as a function of surface albedo and temperature difference between day and night. Surface albedo can be calculated by weighting spectral reflectivities in visible and near-infrared bands from ASTER data. The spectral reflectivities can be obtained after performing atmospheric correction using the 6S model. An iterative self-consistent split-window algorithm was employed to retrieve the daytime SSTs from ASTER data and nighttime SSTs from MODIS data. Because of the spatial resolution difference between ASTER and MODIS data, the nighttime SST derived from MODIS data was downscaled from 1 km to 90 m using the pixel block intensity modulation (PBIM) method. Two study areas, YSB and SYS, were selected to detect the possible signature of the natural oil seepage. The approximate SST and ATI values in the YSB where natural oil seepage actually developed were derived in advance. A look-up table of albedo, temperature difference and ATI for different sea waters with or without natural oil seepage was constructed. These were then used as prior knowledge to enclose the possible signatures of natural oil seepage in SYS. Our results show that there are some signatures of natural oil seepage in the study area in SYS. Compared with SST, ATI was more sensitive to the signature of natural oil seepage. The reason for this might be that the temperature difference was used in the calculation of the ATI, which decreased the influence of the SST accuracy on ATI values.

Aerosol optical thickness retrieval over the Atlantic Ocean using GOES imager data

An algorithm based on simulated satellite signal calculated by the 6S radiative transfer model has been developed in order to retrieve the aerosol optical thickness of dust over the Atlantic ocean. The algorithm is applied to the visible channel of Geostationary Operational Environmental Satellite (GOES 8) images. The inversion uses a look-up table giving the satellite signal intensity as a function of surface albedo, viewing geometry, solar illumination and the optical properties of the aerosols. The study consists of assessing the feasibility of monitoring and mapping the transport of suspended particles across the Atlantic from the Sahara to the Caribbean. The study area is between 10 and 25 N and 30 and 65 W. The optical thickness of aerosols has been calculated over a period of 11 days between 10th and 20th of June 1997 for the 14:15 UT GOES image acquisition. The calculated aerosol optical thickness ranges from 0.0 to 0.81 with an important event of dust presence occurring between 13th and 16th of June. The retrieved aerosol optical thickness is in good agreement with the values obtained between 14h UT and 15h UT from ground based sun photometer measurements on the island of Guadeloupe, and a coefficient correlation ( R 2) of 0.88 has been found between the data sets.

The Effect of Variable Surface Albedo on the Atmospheric Circulation in Desert Regions

A desert circulation model, which includes a vertically parameterized boundary layer, is used to investigate flow in the northern Negev, Israel. An equation is derived relating the vertical velocity at the top of the boundary layer to net radiative heating, convective heating and friction at the ground. Since the net radiative heating is a function of the surface albedo, it is possible to calculate the vertical velocity at the top of the boundary layer as a function of surface albedo, all other things being equal. Calculations using data for the northern Negev desert on 21 June and 21 December, 1974, show sinking motion for high albedos, with a gradual decrease and change toward rising motion for lower albedos. The implication of this result is that a lowering of the surface albedo in a desert region could lead to increased vertical velocity and possibly to increased rainfall.