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1.
Summary For astronomical seasons, Rubincam insolation deviations at latitude 65° N varied from 218.50 Wm−2 to 225.75 Wm−2 (3%). The periodicity of the insolation cycles varied from 36.7 Kyr to 44.7 Kyr (20%) due to phase shift. Phase shift of insolation variations can induce asymmetry of the insolation cycles, permitting rapid melting and prolonged glaciation of ice sheets to occur. For instance, an abnormal decrease of the insolation frequency during the longer period of glacial interval would prolong glaciation into deep ice age. In this study, we apply Rubincam’s insolation equations to investigate the phase shift effect of insolation variations on climate change. Using complex transforms of the changing insolation, we have detected a phase modulation signal in the insolation variations. As a result, an especially new and interesting series of the phase-related insolation pulsation is established. The phase modulated insolation is then introduced as a forcing function into energy balance climate models. Results of model computations shed new insights into the spectrum of the paleoclimatic proxy-data. It is shown that phase modulation of the insolation may provide an appropriate and complete external forcing mechanism to which the climate system would respond. The 100 Kyr cycle of the frequency modulation of the Rubincam’s insolation variations does seem adequate to change the climate. Received July 16, 1997 Revised May 18, 1998  相似文献   

2.
UV attenuation in the cloudy atmosphere   总被引：1，自引：0，他引：1  
Ultraviolet (UV) energy absorption plays a very important role in the Earth–atmosphere system. Based on observational data for Beijing, we suggest that some atmospheric constituents utilize or transfer UV energy in chemical and photochemical (C&P) reactions, in addition to those which absorb UV energy directly. These constituents are primarily volatile organic compounds (VOCs) emitted from both vegetative and anthropogenic sources. The total UV energy loss in the cloudy atmosphere for Beijing in 1990 was 78.9 Wm−2. This attenuation was caused by ozone (48.3 Wm−2), other compounds in the atmosphere (26.6 Wm−2) and a scattering factor (4.0 Wm−2). Our results for a cloudy atmosphere in the Beijing area show that the absorption due to these other compounds occurs largely through the mediation of water vapor. This fraction of energy loss has not been fully accounted for in previous models. Observations and previous models results suggest that 1) a cloudy atmosphere absorbs 25∼30 Wm−2 more solar shortwave radiation than models predict; and 2) aerosols can significantly decrease the downward mean UV-visible radiation and the absorbed solar radiation at the surface by up to 28 and 23 Wm−2, respectively. Thus, quantitative study of UV and visible absorption by atmospheric constituents involved in homogeneous and heterogeneous C&P reactions is important for atmospheric models.  相似文献   

3.
Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from 212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from 19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27% and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and radiation efficiency declined with elevation at an annual mean of 1.15 Wm−2/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline of 1.54 Wm−2/100 m and 0.34 Wm−2/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm−2 for the lowland site and 73 Wm−2 for the highest elevated site. Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation, cloud amount and screen temperature. Received March 5, 2001 Revised October 29, 2001  相似文献   

4.
Summary ?The LITFASS project (‘Lindenberg Inhomogeneous Terrain – Fluxes between Atmosphere and Surface: a Long-term Study’) of the Deutscher Wetterdienst (DWD, German Meteorological Service) aims to develop and to test a strategy for the determination and parameterisation of the area-averaged turbulent fluxes of heat, momentum, and water vapour over a heterogeneous land surface. These fluxes will be representative for an area of about 10 * 10 km2 (while the typical patch size is between 10−1 to 100 km2) corresponding to the size of a grid cell in the present operational numerical weather prediction model of the DWD. LITFASS consists of three components: – the development of a non-hydrostatic micro-α-scale model (the LITFASS local model – LLM) with a grid-size of about 100 * 100 m2, – experimental investigations of land surface – atmosphere exchange processes and boundary layer structure within a 20 * 20 km2 area around the Meteorological Observatory Lindenberg, – the assimilation of a data base as an interface between measurements and modelling activities. The overall project strategy was tested over a three-week period in June 1998 during the LITFASS-98 field experiment. This paper gives an overview on the LITFASS project, on the design and measurement program of the LITFASS-98 experiment, and on the weather conditions during the period of the experiment. Conclusions are formulated for the operational realisation of the LITFASS measurement concept and for future field experiments aimed at studying the land surface – atmosphere interaction in the Lindenberg area. Selected results from both experimental and modelling activities are presented in a series of companion papers completing this special issue of the journal. Received June 18, 2001; revised March 18, 2002; accepted April 2, 2002  相似文献   

5.
Summary ?The analysis of ground-based measurements of solar erythemal ultraviolet (UV) irradiance with a Solar Light 501 biometer, and total (300–3000 nm) irradiance with an Eppley B&W pyranometer at the Argentine Antarctic Base “Almirante Brown”, Paradise Bay (64.9° S, 62.9° W, 10 m a.s.l.) is presented. Measurement period extends from February 16 to March 28 2000. A relatively high mean albedo and a very clean atmosphere characterise the place. Sky conditions were of generally high cloud cover percentage. Clear-sky irradiance for each day was estimated with model calculations, and the effect of the cloudiness was studied through the ratio of measured to clear-sky value (r). Two particular cases were analysed: overcast sky without precipitation and overcast sky with rain or slight snowfall, the last one presenting frequently dense fog. Total irradiance was more attenuated than UV by the homogeneous cloudiness, obtaining mean r values of 0.54 for erythemal irradiance and 0.30 for total irradiance in the first case (without precipitation) and 0.27 and 0.17 respectively in the second case (with precipitation). Mean r values for the complete period were 0.58 for erythemal irradiance and 0.43 for total irradiance. Erythemal and total daily insolations reduce quickly at this epoch due to the increase of the noon solar zenith angle and the decrease of daylight time. Additionally, they were strongly modulated by cloudiness. Measured maxima were 2.71 kJ/m2 and 18.42 MJ/m2 respectively. Measurements were compared with satellite data. TOMS-inferred erythemal daily insolation shows the typical underestimation with respect to ground measurements at regions of high mean albedo. Measured mean total daily insolation agrees with climatological satellite data for the months of the campaign. Received August 9, 2002; revised January 4, 2003; accepted January 28, 2003 Published online May 20, 2003  相似文献   

6.
Climate forcing by carbonaceous and sulfate aerosols   总被引：3，自引：0，他引：3  
 An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m-2 to −0.55 Wm-2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm-2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple scattering associated with clouds and high surface albedos can change the forcing from negative to positive. Received: 29 September 1997 / Accepted: 10 June 1998  相似文献   

7.
 A comprehensive dataset of direct observations is used to assess the representation of surface and atmospheric radiation budgets in general circulation models (GCMs). Based on combined measurements of surface and collocated top-of-the-atmosphere fluxes at more than 700 sites, a lack of absorption of solar radiation within the atmosphere is identified in the ECHAM3 GCM, indicating that the shortwave atmospheric absorption calculated in the current generation of GCMs, typically between 60 and 70 Wm-2, is too low by 10–20 Wm-2. The surface and atmospheric radiation budgets of a new version of the Max-Planck Institute GCM, the ECHAM4, differ considerably from other GCMs in both short- and longwave ranges. The amount of solar radiation absorbed in the atmosphere (90 Wm-2) is substantially larger than typically found in current GCMs, resulting in a lower absorption at the surface (147 Wm-2). It is shown that this revised disposition of solar energy within the climate system generally reduces the biases compared to the observational estimates of surface and atmospheric absorption. The enhanced shortwave absorption in the ECHAM4 atmosphere is due to an increase in both simulated clear-sky and cloud absorption compared to ECHAM3. The increased absorption in the cloud-free atmosphere is related to an enhanced absorption of water vapor, and is supported in stand-alone comparisons of the radiation scheme with synchronous observations. The increased cloud absorption, on the other hand, is shown to be predominantly spurious due to the coarse spectral resolution of the ECHAM4 radiation code, thus providing no physical explanation for the “anomalous cloud absorption” phenomenon. Quantitatively, however, an additional increase of atmospheric absorption due to clouds as in ECHAM4 is, at least at low latitudes, not in conflict with the observational estimates, though this does not rule out the possibility that other effects, such as highly absorbing aerosols, could equally contribute to close the gap between models and observations. At higher latitudes, however, the increased cloud absorption is not supported by the observational dataset. Overall, this study points out that not only the clouds, but also the cloud-free atmosphere might be responsible for the discrepancies between observational and simulated estimates of shortwave atmospheric absorption. The smaller absorption of solar radiation at the surface in ECHAM4 is compensated by an increased downward longwave flux (344 Wm-2), which is larger than in other GCMs. The enhanced downward longwave flux is supported by surface measurements and by a stand-alone validation of the radiation scheme for clear-sky conditions. The enhanced flux also ensures that a sufficient amount of energy is available at the surface to maintain a realistic intensity of the global hydrological cycle. In contrast, a one-handed revision of only the shortwave radiation budget to account for the increased shortwave absorption in GCM atmospheres may induce a global hydrological cycle that is too weak. Received: 26 February 1998 / Accepted: 18 May 1998  相似文献   

8.
 A general circulation model is used to examine the effects of reduced atmospheric CO2, insolation changes and an updated reconstruction of the continental ice sheets at the Last Glacial Maximum (LGM). A set of experiments is performed to estimate the radiative forcing from each of the boundary conditions. These calculations are used to estimate a total radiative forcing for the climate of the LGM. The response of the general circulation model to the forcing from each of the changed boundary conditions is then investigated. About two-thirds of the simulated glacial cooling is due to the presence of the continental ice sheets. The effect of the cloud feedback is substantially modified where there are large changes to surface albedo. Finally, the climate sensitivity is estimated based on the global mean LGM radiative forcing and temperature response, and is compared to the climate sensitivity calculated from equilibrium experiments with atmospheric CO2 doubled from present day concentration. The calculations here using the model and palaeodata support a climate sensitivity of about 1 Wm-2 K-1 which is within the conventional range. Received: 8 February 1997 / Accepted: 4 June 1997  相似文献   

9.
《Atmospheric Research》2005,73(1-2):23-36
Three cirrus cloud cases have been remotely sounded near Paris by a ground-based backscatter lidar and broadband radiometers. Some cirrus properties (optical depth, emissivity, height) are derived from these measurements and used to compare radiative transfer calculations to surface and METEOSAT observations of broadband irradiances.For a useful comparison, the three cirrus cases were selected to have different morphologies and optical properties: June 29, 1993—thin cirrus cloud (thickness 1.5 km, optical depth 0.22); September 6, 1993—thick cirrus cloud (thickness 5 km, optical depth 2.7); and November 16, 1993—inhomogeneous and geometrically thick cirrus cloud (thickness 3.5–6.5 km) but optically thin (optical depth 0.82).At surface, the differences between measurements and model range from 1.5 to 4 Wm−2 for longwave fluxes, and from 20 to 70 Wm−2 for shortwave fluxes.At the top of the atmosphere, the differences between METEOSAT measurements and model are in fair agreement for longwave fluxes (up to 50 Wm−2). However, unexpected high differences are found for shortwave fluxes (up to 144 Wm−2) due to cirrus clouds heterogeneities and uncertainties in their microphysical properties and especially the occurrence of high reflectivity due to horizontally oriented ice crystals at the cloud top, which are not taken into account by the Model presently.  相似文献   

10.
Summary During the Basel Urban Boundary Layer Experiment (BUBBLE) conducted in 2002, micrometeorological in-situ data were collected for different sites using a variety of instruments. This provides a unique data set for urban climate studies. Nevertheless, the spatial distribution of energy and heat fluxes can only be taken into account with remote sensing methods or numerical models. Therefore, multiple satellite images from different platforms (NOAA-AVHRR, MODIS and LANDSAT ETM+) were acquired, processed and analysed. In addition, a high resolution digital elevation model (DEM) and a 1 m resolution digital surface model (DSM) of a large part of the city of Basel was utilized. This paper focuses on the calculation and modelling of the ground (or storage) heat flux density using remotely sensed data combined with in-situ measurements using three different approaches. First, an empirical regression function was generated to estimate the storage heat flux from NDVI values second approach used the Objective Hysteresis Model (OHM) which is often used for in-situ measurements. The last method used information of the geometric parameters of urban street canyons, computed from the high resolution digital urban surface model. Modelled and measured data are found to be in agreement within ±30 Wm−2 and result in a coefficient of determination (R2) of 0.95.  相似文献   

11.
The Summer Surface Energy Balance of the High Antarctic Plateau   总被引：1，自引：0，他引：1  
The summertime surface energy balance (SEB) at Kohnen station, situated on the high Antarctic plateau (75°00′ S, 0°04′ E, 2892m above sea level) is presented for the period of 8 January to 9 February 2002. Shortwave and longwave radiation fluxes were measured directly; the former was corrected for problems associated with the cosine response of the instrument. Sensible and latent heat fluxes were calculated using the bulk method, and eddy-correlation measurements and the modified Bowen ratio method were used to verify these calculated fluxes. The calculated sub-surface heat flux was checked by comparing calculated to measured snow temperatures. Uncertainties in the measurements and energy-balance calculations are discussed. The general meteorological conditions were not extraordinary during the period of the experiment, with a mean 2-m air temperature of −27.5°C, specific humidity of 0.52×10−3kg kg−1 and wind speed of 4.1ms−1. The experiment covered the transition period from Antarctic summer (positive net radiation) to winter (negative net radiation), and as a result the period mean net radiation, sensible heat, latent heat and sub-surface heat fluxes were small with values of −1.1, 0.0, −1.0 and 0.7 Wm−2, respectively. Daily mean net radiation peaked on cloudy days (16 Wm−2) and was negative on clear-sky days (minimum of −19 W m−2). Daily mean sensible heat flux ranged from −8 to +10 Wm−2, latent heat flux from −4 to 0 Wm−2 and sub-surface heat flux from −8 to +7 Wm−2.  相似文献   

12.
A suite of active and passive remote sensing instruments and in-situ sensors deployed at the SIRTA Observatory (Instrumented Site for Atmospheric Remote Sensing Research), near Paris, France, for a period of six months (October 2006–March 2007) document simultaneously radiative, microphysical and dynamic processes driving the continental-fog life cycle. The study focuses on a 6-day period between 23 and 29 December 2006 characterized by several stratus-cloud lowering and lifting events and almost 18 h of visibility below 1 km. Conceptual models and different possible scenarios are presented here to explain the formation, the development and the dissipation phases of three major stratus–fog events and to quantify the impact of each driving process. For example, slowly evolving large-scale conditions characterized by a slow continuous cloud-base lowering, followed by a rapid transient period conductive to fog formation and dissipation, are observed for cases 1 and 3. During this stable period, continuous cloud-top radiative cooling (≈ −160 Wm−2) induces a progressive and slow lowering of the cloud base: larger droplets at cloud top (cloud reflectivity approximately equals to −20 dBZ) induce slow droplet fall to and beyond cloud base (Doppler velocity ≈ −0.1 ms−1), cooling the sub-cloud layer by evaporation and lowering the saturation level to 100 m (case 1) or to the surface (cases 2 and 3). Suddenly, a significant increase in Doppler velocity magnitude ≈ −0.6 ms−1 and of turbulent kinetic energy dissipation rate around 10−3 m2s−3 occurs at cloud base (case 1). These larger cloud droplets reach the surface leading to fog formation over 1.5 h. The Doppler velocity continues to increase over the entire cloud depth with a maximum value of around −1 ms−1 due to the collection of fog droplets by the drizzle drops with high collection efficiency. As particles become larger, they fall to the ground and lead to fog dissipation. Hence, falling particles play a major role in both the formation and also in the dissipation of the fog. These roles co-exist and the balance is driven by the characteristics of the falling particles, such as the concentration of drizzle drops, the size distribution of drizzle drops compared to fog droplets, Doppler velocity and thermodynamic state close to the surface.  相似文献   

13.
Summary The west coast of the Indian peninsula receives very heavy rainfall during the summer Monsoon (June–September) season with average rainfall over some parts exceeding 250 cm. Heavy rainfall events with rainfall more than 15 cm day−1 at one or more stations along the west coast of India occur frequently and cause considerable damage. A special observational programme, Arabian Sea Monsoon Experiment, was carried out during the monsoon season of 2002 to study these events. The spatial and temporal distributions of intense rainfall events, presented here, were used for the planning of this observational campaign. The present study using daily rainfall data for summer monsoon season of 37 years (1951–1987) shows that the probability of getting intense rainfall is the maximum between 14° N–16° N and near 19° N. The probability of occurrence of these intense rainfall events is high from mid June to mid August, with a dip in early July. It has been believed for a long time that offshore troughs and vortices are responsible for these intense rainfall events. However, analysis of the characteristics of cloud systems associated with the intense rainfall events during 1985–1988 using very high resolution brightness temperature data from INSAT-IB satellite shows that the cloud systems during these events are characterized by large spatial scales and high cloud tops. Further study using daily satellite derived outgoing longwave radiation (OLR) data over a longer period (1975–1998) shows that, most of these events (about 62%) are associated with systems organized on synoptic and larger scales. We find that most of the offshore convective systems responsible for intense rainfall along the west coast of India are linked to the atmospheric conditions over equatorial Indian Ocean.  相似文献   

14.
Summary  Turbulent fluxes of CO2 were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period with snow cover, fluxes of CO2 to the atmosphere were small, typically 0.005 mg CO2 m−2 s−1 (0.41 g CO2 m−2 d−1), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the onset of vegetation growth, fluxes of CO2 to the atmosphere were typically 0.1 mg CO2 m−2 s−1 in the afternoon, and daily sums reached values up to almost 9 g CO2 m−2 d−1. After 4 July, downward fluxes of CO2 increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values of about −0.23 mg m−2 s−1 at midday and max values of daily sums of −12 g CO2 m−2 d−1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO2 m−2. Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements. On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO2 m−2 yr−1. Received October 6, 1999 Revised May 2, 2000  相似文献   

15.
Summary The concept of effective cloud cover, elaborated on the basis of an assumption that changes in the net radiation at the top of the atmosphere are mainly caused by changing cloudiness, has been used to deduce solar surface radiation from satellite data. It has been shown that the method permits a calculation of solar surface absorption distributions that agree well with the results obtained by other authors and that the existing disagreement can be to a great extent ascribed to the differences in the data sets and analysis periods. The method allows use of early satellite measurements to get longer time series of the surface radiation budget. In this study, it has been applied to the Nimbus-7 ERB WFOV data for 1979–1986.The net solar flux at the TOA (top of the atmosphere) can be partitioned into absorption at the surface and within the atmosphere. The geographical distributions of all the three quantities as well as the zonal averages of the surface absorption for January and July have been described. Special objectives of the present study are to estimate the interannual standard deviation for the 8-year period and to analyse the shortwave cloud-radiative forcing distributions at the surface and especially within the atmosphere.The standard deviation of the TOA and the surface solar absorption shows a temporal asymmetry, being much larger in January than in July. Noticeable is the disappearance of the wintertime strong variability over the central Pacific in July. As can be expected, the strong variability areas coincide with the strong variability areas of the cloud amount, showing the values up to 27 Wm–2 at the surface.According to our estimate, the shortwave cloud forcing at the surface is everywhere stronger than that at the TOA, so that the cloud forcing of the atmosphere is negative. This means that in the belt of 58.5° N–58.5° S a cloudy atmosphere absorbs more solar energy than a cloud-free atmosphere. Our mean annual value of the atmospheric cloud forcing for this belt is –11 Wm–2 which is somewhat stronger than that obtained by other investigators. It must be stressed that this value is within the uncertainty limits.Shortwave cloud forcing of the atmosphere is the strongest in the lower latitude areas of heavy cloudiness above the continents and negligible in the midlatitudes in winter. This gives evidence that the value of the shortwave cloud forcing of the atmosphere is modified by a combination of cloud absorption and cloud albedo.With 4 Figures  相似文献   

16.
Summary ?By analyzing normalized variables, it was found that the latitudinal secular variations of the rainwater deuterium fractionation ratio δ2H, oxygen fractionation ratio δ18O, vapor pressure, and surface temperature were almost non-linear, occurred in parallel, and decreased with latitude. The rate of depletion around the equator is asymmetric and smaller to the south of 45° S than to the north of 45° N. In the east Mediterranean, the rate of change of δ18O with height was found to be −.2‰ per 100 m and that of δ2H is comparable with the dry lapse rate in the atmosphere. Analysis of the annual time series of δ2H at Alexandria has indicated that variations show sinusoidal waveform with a major cycle of two years that accounts for 68% of the total variance. Although the quasi-biannual cycle in the atmosphere has small amplitude in the lower layers of the atmosphere at East Mediterranean latitudes, the major cycle in annual series of δ2H or δ18O may be linked to the quasi-biannual oscillation in the atmosphere. It was also found that the first three Empirical Orthogonal Functions (EOF) account for 72% of the seasonal variation of δ2H and share 68% of the seasonal variation of δ18O. Share of variances of monthly EOF in the months of the year indicate that the main underlying factors that cause fractionation processes for δ2H and δ18O are similar across the east Mediterranean especially in late winter and early spring. Received May 13, 2002; revised July 8, 2002; accepted August 6, 2002  相似文献   

17.
Summary The effect of clouds on longwave radiation budget at the top and base of the atmosphere is studied by using the HIRS2/MSU-retrieved temperature and humidity fields, and cloud fields and the International Satellite Cloud Climatology Project-produced fields. Detailed studies are carried out at four selected sites: one at Equatorial Eastern Pacific (ITCZ) area, one at Libyan Desert (Libya), one at Ottawa, Montreal (Ottawa), and one at central Europe (Europe). The monthly mean differences in outgoing longwave radiation (OLR) (the ISCCP-based OLR minus the HIRS2-based OLR), ranging from –2.8 Wm–2 at ITCZ to –15.4 Wm–2 at Ottawa, are less than the monthly mean differences in surface downward flux, ranging from –2.7 Wm–2 at Libya to 40.6 Wm–2 at the ITCZ. The large differences in surface downward flux are mainly due to large differences in cloud amount and moisture in the low levels of the atmosphere.Monthly mean OLR and surface downward flux can be derived either (1) from instantaneous temperature, humidity, and cloud fields over a month period or (2) from monthly mean temperature, humidity, and cloud fields. The monthly mean OLR and surface downward flux derived from the first approach is compared with the second. The differences in OLR are small, ranging from –0.05 Wm–2 to 6.2 Wm–2, and the differences in surface downward flux is also small, ranging from 0.4 Wm–2 to 6.4 Wm–2.List of Acronyms AVHRR Advanced Very High Resolution radiometer - ERB Earth Radiation Budget - ERBE Earth Radiation Budget Experiment - FGGE First Global GARP Experiment - GARP Global Atmospheric Research Program - GCM General Circulation Model - GISS Goddard Institute for Space Studies - GLA Goddard Laboratory for Atmospheres - GMS Geostationary Meteorological Satellite - GOES Geostationary Operational Environmental Satellite - HIRS2 High Resolution Infrared Radiation Sounder/2 - ISCCP International Satellite Cloud Climatology Project - IR Infrared - MSU Microwave Sounding Unit - NFOV Narrow Field of View - NOAA National Oceanic and Atmospheric Administration - NESDIS National Environmental Satellite Data Information Service - TOVS TIROS Operational Vertical Sounder With 4 Figures  相似文献   

18.
Summary  Cloud bands that extend from the ITCZ along the subtropical jet toward the subtropics are known as ‘tropical plumes’. At times rainstorms develop at their subtropical edges. One such rainstorm swept eastern North Africa and the Middle East on 23–24 December 1988, with rainfall comparable with the annual averages there. This study examines the storm using the ECMWF initialized data together with surface observations and satellite imageries. The analysis indicates that the storm developed at the inflection region ahead of a pronounced trough in the subtropical jet, with which a mid-latitude trough was merged. Two ageostrophic effects taking place along the jet ahead of the trough contributed to the intensity of the rainstorm. One was associated with acceleration at the jet entrance, located at tropical latitudes, which contributed to the enhancement of both tropical convection and the southerly wind component, which enhanced the moisture tropical transport toward the subtropics. The second was the enhanced near-tropospheric divergence associated with positive vorticity advection at the inflection region itself. Since both effects have a quadratic dependence on wind speed, the observed jet speed, 50% larger than its average value, explains the observed divergence at the inflection point at the 200 hPa level, over 6 × 10−5 s−1, and the vertical velocity at the 700 hPa level, about 10−1 ms−1. It is suggested here that the merging of a mid-latitude with the trough in the subtropical jet, with which the tropical plume is associated, is the cause for the intensification of the subtropical jet and hence of its related rainstorms. Received May 24, 2000  相似文献   

19.
Summary  Three cyclones developing between 28 August and 6 September 1995 were studied with respect to the temporal evolution of their water budget components. The cyclones were simulated with the regional model REMO. Water budget values were determined from hourly model output for circle areas with 500 km radius around the pressure minimum. The results show a maximum liquid water path of about 0.12 kg m−2 and a maximum ice water path of 0.16 kg m−2. In the vertical cloud structure the medium cloud layer disappears at the end of the life cycle for all three cyclones. The release of precipitation onto the Baltic Sea drainage basin is different for each cyclone. It lies between 13 and 22 · 1012 kg. This is about 50% of the total precipitation in the whole area for the strongest cyclone and 65% for the others. The P — E (precipitation minus evaporation) is 15 · 1012 kg for two of the cyclones and 10 · 1012 kg for the third one. Received August 7, 2000 Revised March 19, 2001  相似文献   

20.
Summary ?Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above ground level, νval, and the large-scale flow velocity, νlar, are between νval/νlar ≈ 0.1 and νval/νlar ≈ 1. The νval/νlar ratio exhibits a strong dependence on thermal stratification, δ, and decreases from νval/νlar ≈ 1 at δ = 0 K m−1 to νval/νlar ≈ 0.2 at δ = 0.0075 K m−1. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the νval/νlar ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The νval/νlar ratio is characterized by small values in the direct lee side (νval/νlar ≈ 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (νval/νlar ≈ 0.8). Received October 22, 2001; revised June 18, 2002; accepted June 23, 2002  相似文献   

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