Wouter H. Knap

Elevational changes in meteorological variables along a midlatitude glacier during summer

During the summer of 1994 a glaciometeorological experiment was carried out on the Pasterze (a glacier in Austria). This paper reports on the data from six energy-balance stations ranging in altitude from 2075 to 3225 m above sea level (asl). The wind regime was dominated by the glacier wind. On the tongue, directional constancies ranged between 0.94 and 0.97. Mean 2 m wind speed and specifie humidity were almost constant along the glacier. The variation in the 2 m temperature along the glacier cannot be described by the usually assumed constant decrease with elevation. On the tongue the 2 m temperature even increased with elevation. A much better description of the temperature distribution is given by a linear relation between the potential temperature and the distance along the flow line. This can be understood from a simple thermodynamic analysis of the glacier-wind layer. It is further shown that changes in clear-sky global radiation with elevation are due mainly to changes in local albedo and relief and hardly at all to changes in absolute optical path length and atmospheric water vapor and aerosol content. On the tongue the laterally averaged ice albedo is almost constant with elevation. The flux of incoming long-wave radiation during clear-sky conditions at U2 (2310 m asl) was 48 W/m2 higher than the flux at U5 (3225 m asl), on average. More than half of the difference was due to systematic differences in the shape of the temperature profile (probably the inversion depth is larger at U2). The rest can be ascribed to higher 2 m temperatures and larger amounts of upper hemisphere slopes at U2. The distributions of the meteorological variables and the parameterizations described in this paper might be incorporated in surface energy-balance models designed to simulate the surface mass balance.

Narrowband to broadband conversion of Landsat TM glacier albedos

In this paper we present an empirical relationship between the broadband glacier albedo (alpha) and the narrowband glacier albedos in Landsat TM bands 2 and 4 (alpha2 and alpha4, respectively). The relationship was established on the basis of multiple linear regression analysis of 112 ground-based simultaneous measurements of alpha, alpha2 and alpha4 made at 32 sites on the tongue of the Morteratschgletscher, Switzerland. The measurements were carried out over a representative set of glacier surface types ranging from completely debris-covered glacier ice (alpha=0.08)to dry snow (alpha=0.86). The regression model explains more than 99% of the variance of the broadband albedo and the root-mean-square value of the residuals is only 0.009. The relationship enables users of Landsat TM data to make an accurate estimate of the broadband albedo on the basis of narrowband albedos without having to classify the glacier surface.

The annual cycle of the surface energy balance of Antarctic blue ice

A 15-month meteorological data set was obtained from an automatic weather station over a blue ice area in Dronning Maud Land, Antarctica. The meteorological measurements are used as input for a surface energy balance model in order to compute the hourly varying surface fluxes and the (sub)surface temperatures to a depth of 10 m. The model reproduces reasonably well the directly measured temperatures in the upper meter of ice. Model results show that the net shortwave radiation is the largest positive term in the annual mean energy budget (42.2 W m−2). Other positive fluxes are the downward sensible heat flux (12.1 W m−2) and the upward subsurface energy flux (0.2 W m−2). Energy is lost by net longwave radiation (−49.1 W m−2) and the upward directed latent heat flux (−5.0 W m−2). We analyze the annual cycle of the surface heat fluxes on the basis of daily, monthly, and seasonally mean values. In addition, we calculate the surface energy budget for two distinctly different weather regimes, which are typical for this region. Finally, we demonstrate that the annual cycle of the turbulent fluxes can be explained in terms of the limiting values of the Bowen ratio.

Clear‐sky shortwave radiative closure for the Cabauw Baseline Surface Radiation Network site, Netherlands

[1] In this paper a clear-sky shortwave closure analysis is presented for the Baseline Surface Radiation Network (BSRN) site of Cabauw, Netherlands (51.97N, 4.93E). The analysis is based on an exceptional period of fine weather during the first half of May 2008, resulting in a selection of 72 comparisons, on 6 days, between BSRN measurements and Doubling Adding KNMI (DAK) model simulations of direct, diffuse, and global irradiances. The data span a wide range of aerosol properties, water vapor columns, and solar zenith angles. The model input consisted of operational Aerosol Robotic Network (AERONET) aerosol products and radiosonde data. The wavelength dependence of the aerosol optical thickness, single scattering albedo, and asymmetry parameter was taken into account. On the basis of these data, excellent closure was obtained: the mean differences between model and measurements are 2 W/m 2 (+0.2%) for the direct irradiance, 1 W/m 2 (+0.8%) for the diffuse irradiance, and 2 W/m 2 (+0.3%) for the global irradiance. The good results were obtained because of proper specification of the DAK model input and the high quality of the AERONET and BSRN measurements. The sensitivity of the achieved closure to uncertainties in the aerosol optical thickness, single scattering albedo, and asymmetry parameter was examined. Furthermore, several sensitivity experiments related to the wavelength dependence of the aerosol optical properties and the treatment of water vapor were performed. It appeared that a correct description of the wavelength dependence of the aerosol optical properties is important for achieving broadband closure. However, broadband closure can also be obtained by means of using spectrally averaged values of the single scattering albedo and the asymmetry parameter. Cancellation of errors in different parts of the solar spectrum also contributes to the achieved closure.

Climate sensitivity of the ice cap of King George Island, South Shetland Islands, Antarctica

A two-dimensiona l vertically integrated ice-flow model has been used to simulate the current state of the ice cap of King George Island, South Shetland Islands, Antarctica, as well as the sensitivity of this state to climate change. The model was forced by an energy-balance model that generates the specific mass balance from climatological input data of two research stations. It proved difficult to simulate satisfactorily the entire geometry of the present-day ice cap. Nevertheless, it was possible to simulate a steady-state ice cap whose volume and areal extent approximate the (estimated) current situation. Several experiments have indicated that this state is highly sensitive to climate change. The model predicts that cooling by l K will increase the ice volume by 10% and warming by l K will decrease it by 36%. A 10% change in precipitation will alter the ice volume by less than 8%. Application of the IPCC-90 Business-as-Usual scenario leads to a 55% reduction in the ice volume by the year AD 2100, compared to the present-day situation. The response of the ice cap to warming is therefore totally different from the response of the main Antarctic ice sheet which is believed to gain mass by increasing temperatures.

Modeling total and polarized reflectances of ice clouds: evaluation by means of POLDER and ATSR-2 measurements

Four ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earths Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 microm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > approximately 16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 microm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 microm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs.

Cloud Thermodynamic-Phase Determination From Near-Infrared Spectra of Reflected Sunlight

Abstract A simple method for the determination of the thermodynamic phase of clouds over ocean from near-infrared spectra of reflected sunlight is presented. The method is based on thresholding the parameter S1.67 (in percent), which is defined as the ratio of the difference between the spectral reflectivities at 1.70 and 1.64 μm to the reflectivity at 1.64 μm. Radiative transfer calculations for different cloudy atmospheres over ocean are presented to show that S1.67 ≈ 0 for water clouds and S1.67 > 0 for ice clouds and mixed-phase clouds. It is shown that S1.67 is sensitive to the presence of ice particles in clouds, and depends primarily on ice-cloud optical thickness and crystal size. The method is relatively independent of viewing and solar geometry because it is based on spectral absorption properties rather than scattering properties of clouds. The method is thoroughly analyzed using near-infrared reflectivity spectra made by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) over a we...

Effect of aerosols on the downward shortwave irradiances at the surface: Measurements versus calculations with MODTRAN4.1

[1] A detailed analysis of measurements and model calculations of clear-sky shortwave irradiances at the surface is presented for a set of 18 cases collected during 3 cloudless days in the Netherlands in 2000. The analysis is focused on the influence of the optical and physical properties of aerosols on simulations of direct and diffuse downward solar irradiance at the surface. The properties of aerosols in the boundary layer are derived from surface measurements, under the assumption that all aerosol is confined to a well-mixed atmospheric boundary layer. The simulations of the irradiances are performed with the radiative transfer model MODTRAN 4, version 1.1. The analysis reveals no discernable differences between model and measurement for the direct irradiance, but several significant differences for the diffuse irradiance. The model always overestimates the diffuse irradiance measurements by 7 to 44 Wm � 2 (average: 25 Wm � 2 ). On the basis of an estimated uncertainty in the differences of 18 Wm � 2 , it appears that for 13 out of 18 cases the model significantly overestimates the measurements. This number decreases if instrumental errors (e.g., pyranometer zero-offset) and assumptions on the model input (e.g., wavelength-independent surface albedo) are considered. Nevertheless, the analysis presented here points to a persistent and significant positive model-measurement difference for the diffuse irradiance, which typically amounts to 1–4% of the top-ofatmosphere irradiance, and does not depend on the solar zenith angle. The reason for the discrepancy may be found in the presence of ultrafine absorbing aerosol particles that were not detected by the surface instrument for measuring aerosol absorption. It is also possible that these particles are not present near the surface, due to dry deposition, but do contribute to the total extinction if they are situated higher up in the boundary layer. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: aerosol-radiation interaction, shortwave closure

Remote sensing of the albedo and detection of the slush line on the Greenland ice sheet

Advanced very high resolution radiometer images of a section of the Greenland ice sheet (western part between 64.5° and 70.5°N with a size of 667 (N-S) by 700 (W-E) km) were processed in order to retrieve the surface albedo. The images were for the summer seasons of 1990–1997 inclusive, and results were compared to simultaneous mass balance measurements carried out along the transect (67°N). The main findings are as follows: First, many images show a distinct transition from low spatial variability of the albedo at higher elevations to higher spatial variability of the albedo at lower elevations. It is argued that these transitions most likely correspond to transitions at the surface (“slush line,” in short) from the area that is uniformly covered by snow to a mosaic of snow patches, slush, and ice. Second, 70% of the interannual variations in the mean specific mass balance can be explained by interannual variations in the maximum elevation of the slush line (i.e., the “slush limit”). However, a higher percentage (82%) can be explained by interannual variations in the sum of positive degree-days. Third, in 3 years with a relatively large amount of melt the inland migration of the slush line stopped or slowed considerably down at a fixed distance from the ice sheet margin. We believe that this so-called “maximum slush line” coincides with an abrupt change in the density profile. The maximum slush line forms an important limitation for the use of satellite data to monitor interannual variations in the mass balance. Its position is a climate indicator and was determined for the entire section of the Greenland ice sheet investigated.

The Surface Albedo Of The Vatnajökull Ice Cap, Iceland: A Comparison Between Satellite-Derived And Ground-Based Measurements

The temporal and spatial variations in the surface albedo of the Vatnajökull ice cap, Iceland, are investigated. A time series of the surface albedo is composed for the summer of 1996 using satellite radiance measurements from the Advanced Very High Resolution Radiometer (AVHRR). This time series is compared with ground measurements carried out during a glacio-meteorological experiment during the same summer on the ice cap. The AVHRR is able to reproduce the development in time of the surface albedo fairly well. The large systematic differences found for some of the stations on the ice are attributed to sub-pixel-scale variations in the albedo. An attempt is made to confirm this hypothesis using satellite radiance measurements carried out by the Thematic Mapper (TM) and measurements made with a portable albedometer. The TM has a pixel size of 30 × 30 m whereas the pixel size of the AVHRR is 1 × 1 km. Although the TM measurements show greater variability in the albedo than do the AVHRR measurements, the large systematic difference remains. Measurements with the portable albedometer show a large spread in the albedo at sites with large systematic differences. This implies that the scale of the albedo variations is smaller than the scale of the AVHRR and TM pixels.