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Atsumu Ohmura

Abstract

In addition to the inherent problem of accumulating errors of measurement of net radiation and subsurface heat flux, the Bowen ratio energy balance method often produces totally unacceptable sensible and latent heat fluxes: wrong signs (directions) and extremely inaccurate magnitudes of the fluxes, or both. These problems are due to resolution limits of the instruments. Objective criteria to eliminate undesirable data are derived in general forms. An example is graphically presented for the common case of the psychometric tower with a 0.05°C resolution limit of temperature measurement.

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Atsumu Ohmura

Abstract

The close relationship between air temperature measured at standard screen level and the rate of melt on snow and ice has been widely used to estimate the rate of melt. The parameterization of the melt rate using air temperature usually takes a simple form as a function of either the mean temperature for the relevant period or positive degree-day statistics. The computation provides the melt rate with sufficient accuracy for most practical purposes. Because of its simplicity, it is often called a crude method and is rated as inferior to other more sophisticated methods such as the energy balance method. The method is often used with the justification that temperature data are easily available or that obtaining energy balance fluxes is difficult. The physical process responsible for the temperature effect on the melt rate is often attributed to the sensible heat conduction from the atmosphere. The simulation capacity of the temperature-based melt-index method, however, is too good to be called crude and inferior. The author investigated physical processes that make the air temperature so effective a parameter for melt rate. Air temperature has a more profound influence on melt than previously has been acknowledged. The influence of air temperature through the turbulent sensible heat flux is limited, however. The air temperature information is transferred to the surface mainly through longwave atmospheric radiation, which is by far the most important heat source for melt. Under cloudless-sky conditions, as much as 60% of the atmospheric emission is derived from within the first 100 m and 90% from the first 1 km of the atmosphere. When the sky is overcast with the cloud bottom within the first 1 km, more than 90% originates within this layer between the surface and the bottom of the cloud. When the sky is overcast with the cloud bottom higher than 1 km, the first 1 km of the atmosphere still makes up about 70% of the longwave irradiance at the surface, for which the air temperature measured at standard screen level is the single most influential factor. Wind speed is only weakly correlated with melt rate, because the main energy source for melting is longwave atmospheric radiation, followed by the absorbed global radiation, both of which are independent of the movement of the atmosphere.

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Hisashi Ozawa
and
Atsumu Ohmura

Abstract

Vertical heat transport through thermal convection of the earth’s atmosphere is investigated from a thermodynamic viewpoint. The postulate for convection considered here is that the global-mean state of the atmosphere is stabilized at a state of maximum entropy increase in a whole system through convective transport of sensible and latent heat from the earth’s surface into outer space. Results of an investigation using a simple vertical gray atmosphere show the existence of a unique set of vertical distributions of air temperature and of convective and radiative heat fluxes that represents a state of maximum entropy increase and that resembles the present earth. It is suggested that the global-mean state of the atmospheric convection of the earth, and that of other planets, is stabilized so as to increase entropy in the universe at a possible maximum rate.

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Christian Plüss
and
Atsumu Ohmura

Abstract

Longwave radiation in snow-covered alpine environments was investigated based on LOWTRAN7 calculations. The irradiance from the sky and from the surrounding topography were determined separately in order to detect the influence of the topography on longwave radiation balance. Sensitivity studies showed that the irradiance from the surrounding terrain is determined primarily by the atmospheric conditions within the investigated area and by the surface temperature of the surrounding terrain. In snow-covered environments, the air temperature is usually above the snow surface temperature and the effects of the air between the topography and the receiving surface may be relevant.

Longwave irradiance from the surrounding terrain is an important component of the energy balance at the snow surface on inclined slopes and should be considered for areal investigations. A simple parameterization that accounts for the effects of the air is proposed for efficient calculation of longwave irradiance from snow-covered topography.

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Atsumu Ohmura
,
Martin Wild
, and
Lennart Bengtsson

Abstract

A high-resolution GCM is found to simulate precipitation and surface energy balance of high latitudes with high accuracy. This opens new possibilities to investigate the future mass balance of polar glaciers and its effect on sea level. The surface mass balance of the Greenland and the Antarctic ice sheets is simulated using the ECHAM3 GCM with TI06 horizontal resolution. With this model, two 5-year integrations for the present and doubled carbon dioxide conditions based on the boundary conditions provided by the ECHAM1/T21 transient experiment have been conducted. A comparison of the two experiments over Greenland and Antarctica shows to what extent the effect of climate change on the mass balance on the two largest glaciers of the world can differ. On Greenland one sees a slight decrease in accumulation and a substantial increase in melt, while on Antarctica a large increase in accumulation without melt is projected. Translating the mass balances into terms of sea-level equivalent. the Greenland discharge causes a sea level rise of 1. 1 mm yr−1, while the accumulation on Antarctica tends to lower it by 0.9 mm yr−1. The change in the combined mass balance of the two continents is almost zero. The sea level change of the next century can be affected more effectively by the thermal expansion of seawater and the mass balance of smaller glaciers outside of Greenland and Antarctica.

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B. J. Garnier
and
Atsumu Ohmura

Abstract

A formula has been developed whereby variations in the shortwave radiation income on different slopes in any latitude can be easily determined from a knowledge of surface geometry and the sun's declination. This is achieved by expressing slope characteristics and the sun's position as unit vectors in the coordinates of a common system, and multiplying the cosine of the angle between the two vectors by a factor involving the solar constant, atmospheric transmissivity and the optical air mass. Integration of the formula permits the topographic variations of direct shortwave radiation income over specified areas to be calculated for daily or longer periods from radiation observations at a single site. Thus, a device has been developed which has applications in many aspects of pure and applied microclimatology, and which differs from other similar formulae in that it is at the same time both integrable and applicable under different atmospheric transmissivities.

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Martin Wild
,
Atsumu Ohmura
, and
Ulrich Cubasch

Abstract

The changes in the surface energy fluxes calculated with a general circulation model under increased levels of carbon dioxide concentration are analyzed and related to the simulation of these fluxes under present-day conditions. It is shown that the errors in the simulated fluxes under present climate are often of similar or larger magnitude than the simulated changes of these quantities. A similar relationship may be found in climate change experiments of many GCMs. Although this does not imply that the projected changes of the fluxes are wrong, more accurate absolute values would improve confidence in GCM-simulated climate change scenarios.

The global mean increase in the downward component of the longwave radiation, which is the direct greenhouse forcing at the surface, is on the order of 10 W m−2 at the time of doubled carbon dioxide in a transient coupled atmosphere–ocean scenario experiment. This is an amount similar to the underestimation of this quantity in the present-day simulations compared to surface observations. Thus, it is only with doubled carbon dioxide concentration that the simulated greenhouse forcing at the surface reaches the values observed at present.

The simulated shortwave radiation budget at the surface is less affected by the increased levels of carbon dioxide than the longwave budget on the global scale. Regionally and seasonally, the changes in the incoming shortwave radiation at the surface can exceed 20 W m−2, mainly due to changes in cloud amounts. The projected changes, however, are generally of smaller magnitude than the systematic errors in the control run at the majority of 720 observation sites.

The positive feedback between excessive radiation and surface processes leading to excessive summer dryness and temperatures over continental surfaces in the control run is enhanced in the doubled carbon dioxide experiment, resulting in a massive increase in the projected surface temperature.

In the high-resolution T106 time-slice scenario experiment performed in this study the global mean latent heat flux and associated intensity of the hydrological cycle is slightly decreased rather than increased with doubled carbon dioxide. A reduction in surface wind speed in the T106 scenario is suggested as a major factor for the reverse of sign.

The improved representation of the orography with T106 resolution allows a better estimate of the projected changes of surface energy fluxes in mountain areas, as demonstrated for the European Alps.

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Martin Wild
,
Atsumu Ohmura
,
Hans Gilgen
, and
Erich Roeckner

Abstract

The surface radiative fluxes of the ECHAM3 General Circulation Model (GCM) with T2 1, T42, and T 106 resolutions have been validated using observations from the Global Energy Balance Archive (GEBA, World Climate Program-Water Project A7). GEBA contains the most comprehensive dataset now available for worldwide instrumentally measured surface energy fluxes.

The GCM incoming shortwave radiation at the surface has been compared with more than 700 long-term monitoring stations. The ECHAM3 models show a clear tendency to overestimate the global annual-mean incoming shortwave radiation at the surface due to an underestimation of atmospheric absorption. The model-calculated global-mean surface shortwave absorption around 165 W m−2 is estimated to be too high by 10–15 W m−2. A similar or higher overestimate is present in several other CYCMS. Deficiencies in the clear-sky absorption of the ECHAM3 radiation scheme are proposed as a contributor to the flux discrepancies. A stand-alone validation of the radiation scheme under clear-sky conditions revealed overestimates of up to 50 W m−2 for daily maximum values of incoming shortwave fuxes. Further, the lack of shortwave absorption by the model clouds is suggested to contribute to the overestimated surface shortwave radiation.

There are indications that the incoming longwave radiation at the surface is underestimated in ECHAM3 and other GCMS. This largely offsets the overestimated shortwave flux in the global mean, so that the 102 W m-’ calculated in ECHAM3 for the surface net radiation is considered to be a realistic value. A common feature of several GCMs is, therefore, a superficially correct simulation of global mean net radiation, as the overestimate in the shortwave balance is compensated by an underestimate in the longwave balance.

Seasonal and zonal analyses show that the largest overestimate in the incoming shortwave radiation of ECHAM3 is found at low latitudes year round and in midlatitude summer, while at high latitudes and in midlatitude winter the solar input is underestimated. As a result, the meridional gradient of incoming shortwave radiation becomes too large. The zonal discrepancies of the duxes are consistent with differences between the simulated cloud amount and a cloud climatology based on surface observations. The shortwave discrepancies are further visible in the net radiation where the differences show a similar latitudinal dependency including the too strong meridional gradient.

On the global and zonal scale, the simulated fuxes are rather insensitive to changes in horizontal resolution. The systematic large-scale model deviations dominate the effects of increased horizontal resolution.

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Martin Wild
,
Atsumu Ohmura
,
Hans Gilgen
,
Jean-Jacques Morcrette
, and
Anthony Slingo

Abstract

The longwave radiation emitted by the atmosphere toward the surface [downward longwave radiation (DLR)] is a crucial factor in the exchange of energy between the earth surface and the atmosphere and in the context of radiation-induced climate change. Accurate modeling of this quantity is therefore a fundamental prerequisite for a reliable simulation and projection of the surface climate in coupled general circulation models (GCM).

DLR climatologies calculated in a number of GCMs and in a model in assimilation mode (reanalysis) are analyzed using newly available data from 45 worldwide distributed observation sites of the Global Energy Balance Archive (GEBA) and the Baseline Surface Radiation Network (BSRN). It is shown that substantial biases are present in the GCM-calculated DLR climatologies, with the GCMs typically underestimating the DLR (estimated here to be approximately 344 W m−2 globally). The biases are, however, not geographically homogeneous, but depend systematically on the prevailing atmospheric conditions. The DLR is significantly underestimated particularly at observation sites with cold and dry climates and thus little DLR emission. This underestimation gradually diminishes toward sites with more moderate climates; at sites with warm or humid atmospheric conditions and strong DLR emission, the GCM-calculated DLR is in better agreement with the observations or even overestimates them. This is equivalent to creating an excessively strong meridional gradient of DLR in the GCMs.

The very same tendencies are independently found in stand-alone calculations with the GCM radiation codes in isolation, using observed atmospheric profiles of temperature and humidity for cloud-free conditions as input to the radiation schemes. A significant underestimation of DLR is calculated by the radiation schemes when driven with clear-sky atmospheric profiles of temperature and humidity representative for cold and dry climates, whereas the DLR is no longer underestimated by the radiation schemes with prescribed clear-sky profiles representative for a hot and humid atmosphere. This suggests that the biases in the GCM-calculated DLR climatologies are predominantly induced by problems in the simulated emission of the cloud-free atmosphere.

The same biases are also found in the DLR fluxes calculated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model in assimilation mode (reanalysis), in which the biases in the atmospheric thermal and humidity structure are minimized. This gives further support that the biases in the DLR are not primarily due to errors in the model-predicted atmospheric temperature and humidity profiles that enter the radiative transfer calculations, but rather are due to the radiation schemes themselves. A particular problem in these schemes is the accurate simulation of the thermal emission from the cold, dry, cloud-free atmosphere.

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C. Saskia Bourgeois
,
Atsumu Ohmura
,
Karl Schroff
,
Hans-Jörg Frei
, and
Pierluigi Calanca

Abstract

This work presents a new field goniospectrometer developed at the Institute for Atmospheric and Climate Science (IAC) of the Swiss Federal Institute of Technology (ETH; Switzerland). The goniospectrometer was built to study the hemispherical directional reflectance factor (HDRF) of snow, but can also be applied to other surfaces with moderate surface roughness.

The IAC ETH goniospectrometer measures HDRFs with high spatial resolution. The goniometer is exclusively built of straight parts, thus ensuring a high pointing accuracy. The two robotic arms are controlled automatically with step motors, whereby the step size can be defined by the user. With the default grid size of 15° in zenith and azimuth, the time needed to collect one complete HDRF dataset is 11 min, corresponding to a change of less than 4° in solar zenith and azimuth angles.

The spectrometer comprises two probes. The first probe is equipped with a 3° foreoptic and is used for taking a spectrum of the reflected radiance; the second is placed on a tripod, has a 2π foreoptic, and simultaneously records a spectrum of the incoming irradiance. Both probes measure in the spectral range from 350 to 1050 nm, with a resolution of approximately 3 nm at around 700 nm.

The performance of the new goniospectrometer was tested at the Greenland Environmental Observatory Summit Station (72°35′N, 34°30′W, 3203 m ASL) during the summer of 2004.

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