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1.
A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics
of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave
(SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the
model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties
are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear
competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the
atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of
our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible.
Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features
of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit
a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air.
The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis
data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of
the low-latitude climate's sensitivity to the “runaway greenhouse” effect.
Received: 7 December 1999 / Accepted: 19 February 2001 相似文献
2.
Summary The greenhouse effect has been investigated predominantly with satellite measurements, but more than 90% of the greenhouse radiative flux affecting Earths surface temperature and humidity originates from a 1000 meter layer above the surface. Here we show that substantial improvements on surface longwave radiation measurements and very good agreement with radiative transfer model calculations allow the clear-sky greenhouse effect be determined with measured surface longwave radiation and calculated longwave outgoing radiation at the top of the atmosphere. The cloud radiative forcing is determined by measured net longwave fluxes and added to the clear-sky greenhouse effect to determine the all-sky greenhouse effect. Longwave radiation measurements at different altitudes were used to determine the clear-sky and all-sky annual and seasonal greenhouse effect and altitude gradients over the Alps. Linear altitude gradients are measured for clear-sky situations, whereas the all-sky greenhouse effect is strongly influenced by varying, cloud amounts at different altitudes. Large diurnal and seasonal variations show the importance of surface heating and cooling effects and demonstrate the strong coupling of the greenhouse effect to surface temperature and humidity. 相似文献
3.
利用毫米波云雷达、微波辐射计联合反演方法,对2015年11月11日安徽寿县的一次层状云过程的云参数进行了反演,将所得云参数加入到SBDART辐射传输模式中,进行辐射通量计算,并将计算的地面辐射通量与观测的地面辐射通量进行了对比分析。研究表明:1)利用毫米波雷达和微波辐射计数据联合反演的云参数比较可靠;2)利用SBDART模式并结合反演的云参数,可以准确实时地计算地面及其他高度层的长短波辐射通量;3)在反演的云参数中,光学厚度对地面各种辐射通量的影响是最大的,云层的光学厚度越大,到达地面的太阳短波辐射越小,地面反射短波辐射也越小。另外云底温度越高,云体向下发射的红外长波辐射越大。地面向上的长波辐射是地面温度的普朗克函数,随地面温度而变;4)云对地面的短波辐射强迫为负值,对地面有降温的作用。云对地面的长波辐射强迫是一个正值,对地面有一个增温的作用;5)云对地面的净辐射强迫随时间变化很大,它的正负与太阳高度角和云参数有关。 相似文献
4.
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
5.
We examine 800-year time series of internally generated variability in both a coupled ocean-atmosphere model where water
vapor anomalies are not allowed to interact with longwave radiation and one where they are. The ENSO-like phenomenon in the
experiment without water vapor feedback is drastically suppressed both in amplitude and geographic extent relative to the
experiment with water vapor feedback. Surprisingly, the reduced amplitude of ENSO-related sea surface temperature anomalies
in the model without water vapor feedback cannot be attributed to greater longwave damping of sea surface temperature. (Differences
between the two experiments in radiative feedback due to clouds counterbalance almost perfectly the differences in radiative
feedback due to water vapor.) Rather, the interaction between water vapor anomalies and longwave radiation affects the ENSO-like
phenomenon through its influence on the vertical structure of radiative heating: Because of the changes in water vapor associated
with it, a given warm equatorial Pacific sea surface temperature anomaly is associated with a radiative heating profile that
is much more gravitationally unstable when water vapor feedback is present. The warm sea surface temperature anomaly therefore
results in more convection in the experiment with water vapor feedback. The increased convection, in turn, is related to a
larger westerly wind-stress anomaly, which creates a larger decrease in upwelling of cold water, thereby enhancing the magnitude
of the original warm sea surface temperature anomaly. In this manner, the interaction between water vapor anomalies and longwave
radiation magnifies the air-sea interactions at the heart of the ENSO phenomenon; without this interaction, the coupling between
sea surface temperature and wind stress is effectively reduced, resulting in smaller amplitude ENSO episodes with a more limited
geographical extent.
Received: 26 March 1999 / Accepted: 25 October 1999 相似文献
6.
The latitude-altitude distributions of radiative fluxes and heating rates are investigated by utilizing CloudSat satellite data over China during summer. The Tibetan Plateau causes the downward shortwave fluxes of the lower atmosphere over central China to be smaller than the fluxes over southern and northern China by generating more clouds. The existence of a larger quantity of clouds over central China reflects a greater amount of solar radiation back into space. The vertical gradients of upward shortwave radiative fluxes in the atmosphere below 8 km are greater than those above 8 km. The latitudinal-altitude distributions of downward longwave radiative fluxes show a slantwise decreasing trend from low latitudes to high latitudes that gradually weaken in the downward direction. The upward longwave radiative fluxes also weaken in the upward direction but with larger gradients. The maximum heating rates by solar radiation and cooling rates by longwave infrared radiation are located over 28-40°N at 7-8 km mean sea level (MSL), and they are larger than the rates in the northern and southern regions. The heating and cooling rates match well both vertically and geographically. 相似文献
7.
An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is
composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ
l
) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model,
and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy
air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections
for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation
due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model
is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite
well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net
radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation
has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In
addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction
of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling. 相似文献
8.
将包含多形状冰晶粒子的冰云辐射参数化方案应用于全球气候模式中,详细讨论了冰云粒子从球形假定到多形状假定的变化对辐射场和气候场的影响。结果显示,冰晶粒子形状假定的引入对冰云光学厚度、辐射通量和加热率以及温度场均有明显的影响。采用新的冰云方案使得全球平均云光学厚度值降低0.28（23%）;热带地区降低最为明显,其差异绝对值可达1.02,而在中高纬度陆地地区,两者的冰云光学厚度差别较小。冰晶粒子形状假定改变将导致全球平均的大气顶出射长波辐射通量增加5.52 W/m2（2.3%）。与观测资料的比较表明,多形状冰晶粒子假定明显减小了球形粒子假定对长波出射辐射的低估。对大气加热率廓线的模拟显示,多形状冰晶粒子假定会减弱短波辐射对大气的加热作用,同时增强长波辐射对大气的冷却作用;在热带对流层中高层,这两种影响尤为显著。冰晶粒子形状假定的改变对温度场有明显的影响,热带地区的对流层高层大气温度降低幅度可超过1.5 K。研究表明,冰晶粒子形状假定的改变对模拟的辐射和温度场均有重要的影响。 相似文献
9.
S. H. Franchito V. Brahmananda Rao R. Ramos da Silva 《Meteorology and Atmospheric Physics》1998,69(1-2):23-38
Summary A parameterization of shortwave and longwave radiation fluxes derived from detailed radiative transfer models is included in a global primitive equation statistical-dynamical model (SDM) with two bulk atmospheric layers. The model is validated comparing the model simulations with the observed mean annual and seasonal zonally averaged climate. The results show that the simulation of the shortwave and longwave radiation fluxes matches well with the observations. The SDM variables such as surface and 500 hPa temperatures, zonal winds at 250 hPa and 750 hPa, vertical velocity at 500 hPa and precipitation are also in good agreement with the observations. A comparison between the results obtained with the present SDM and those with the previous version of the model indicates that the model results improved when the parameterization of the radiative fluxes based on detailed radiative transfer models are included into the SDM.The SDM is used to investigate its response to the greenhouse effect. Sensitivity experiments regarding the doubling of CO2 and the changing of the cloud amount and height are performed. In the case 2×CO2 the model results are consistent with those obtained from GCMs, showing a warming of the climate system. An enhancement of the greenhouse effect is also noted when the cloud layer is higher. However, an increase of the cloud amount in all the latitude belts provokes an increase of the surface temperature near poles and a decrease in all the other regions. This suggests that the greenhouse effect overcomes the albedo effect in the polar latitudes and the opposite occurs in other regions. In all the experiments the changes in the surface temperature are larger near poles, mainly in the Southern Hemisphere.With 8 Figures 相似文献
10.
在研究水云的微物理性质与光学性质关系的基础上，提出了三种用于长波区域窄带和宽带水云光学性质计算的参数化方案，研究了应用不同的水云光学性质参数化方案和不同的宽带数目对云辐射性质的影响。分析了云滴的散射作用对有云大气中长波辐射通量和冷却率的影响。 相似文献
11.
Towards constraining climate sensitivity by linear analysis of feedback patterns in thousands of perturbed-physics GCM simulations 总被引：2，自引：1，他引：1
Benjamin M. Sanderson C. Piani W. J. Ingram D. A. Stone M. R. Allen 《Climate Dynamics》2008,30(2-3):175-190
A linear analysis is applied to a multi-thousand member “perturbed physics" GCM ensemble to identify the dominant physical
processes responsible for variation in climate sensitivity across the ensemble. Model simulations are provided by the distributed
computing project, climate prediction.net . A principal component analysis of model radiative response reveals two dominant independent feedback processes, each
largely controlled by a single parameter change. The leading EOF was well correlated with the value of the entrainment coefficient—a
parameter in the model’s atmospheric convection scheme. Reducing this parameter increases high vertical level moisture causing
an enhanced clear sky greenhouse effect both in the control simulation and in the response to greenhouse gas forcing. This
effect is compensated by an increase in reflected solar radiation from low level cloud upon warming. A set of ‘secondary’
cloud formation parameters partly modulate the degree of shortwave compensation from low cloud formation. The second EOF was
correlated with the scaling of ice fall speed in clouds which affects the extent of cloud cover in the control simulation.
The most prominent feature in the EOF was an increase in longwave cloud forcing. The two leading EOFs account for 70% of the
ensemble variance in λ—the global feedback parameter. Linear predictors of feedback strength from model climatology are applied
to observational datasets to estimate real world values of the overall climate feedback parameter. The predictors are found
using correlations across the ensemble. Differences between predictions are largely due to the differences in observational
estimates for top of atmosphere shortwave fluxes. Our validation does not rule out all the strong tropical convective feedbacks
leading to a large climate sensitivity. 相似文献
12.
Combining ERBE and ISCCP data to assess clouds in the Hadley Centre, ECMWF and LMD atmospheric climate models 总被引：3，自引：2，他引：3
This study compares radiative fluxes and cloudiness fields from three general circulation models (the HadAM4 version of the
Hadley Centre Unified model, cycle 16r2 of the ECMWF model and version LMDZ 2.0 of the LMD GCM), using a combination of satellite
observations from the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP).
To facilitate a meaningful comparison with the ISCCP C1 data, values of column cloud optical thickness and cloud top pressure
are diagnosed from the models in a manner consistent with the satellite view from space. Decomposing the cloud radiative effect
into contributions from low-medium- and high-level clouds reveals a tendency for the models' low-level clouds to compensate
for underestimates in the shortwave cloud radiative effect caused by a lack of high-level or mid-level clouds. The low clouds
fail to compensate for the associated errors in the longwave. Consequently, disproportionate errors in the longwave and shortwave
cloud radiative effect in models may be taken as an indication that compensating errors are likely to be present. Mid-level
cloud errors in the mid-latitudes appear to depend as much on the choice of the convection scheme as on the cloud scheme.
Convective and boundary layer mixing schemes require as much consideration as cloud and precipitation schemes when it comes
to assessing the simulation of clouds by models. Two distinct types of cloud feedback are discussed. While there is reason
to doubt that current models are able to simulate potential `cloud regime' type feedbacks with skill, there is hope that a
model capable of simulating potential `cloud amount' type feedbacks will be achievable once the reasons for the remaining
differences between the models are understood.
Received: 23 January 2000 / Accepted: 24 January 2001 相似文献
13.
Xiaodong XIE Xiaoxian HUANG Tijian WANG Mengmeng LI Shu LI Pulong CHEN 《Journal of Meteorological Research》2018,(3)
Carbon dioxide(CO_2) is an important greenhouse gas that influences regional climate through disturbing the earth's energy balance. The CO_2 concentrations are usually prescribed homogenously in most climate models and the spatiotemporal variations of CO_2 are neglected. To address this issue,a regional climate model(RegCM4) is modified to investigate the non-homogeneous distribution of CO_2 and its effects on regional longwave radiation flux and temperature in East Asia. One-year simulation is performed with prescribed surface CO_2 fluxes that include fossil fuel emission, biomass burning, air-sea exchange, and terrestrial biosphere flux. Two numerical experiments(one using constant prescribed CO_2 concentrations in the radiation scheme and the other using the simulated CO_2 concentrations that are spatially non-homogeneous) are conducted to assess the impact of non-homogeneous CO_2 on the regional longwave radiation flux and temperature. Comparison of CO_2 concentrations from the model with the observations from the GLOBALVIEW-CO_2 network suggests that the model can well capture the spatiotemporal patterns of CO_2 concentrations. Generally, high CO_2 mixing ratios appear in the heavily industrialized eastern China in cold seasons, which probably relates to intensive human activities. The accommodation of non-homogeneous CO_2 concentrations in the radiative transfer scheme leads to an annual mean change of -0.12 W m~(-2) in total sky surface upward longwave flux in East Asia. The experiment with non-homogeneous CO_2 tends to yield a warmer lower troposphere.Surface temperature exhibits a maximum difference in summertime, ranging from -4.18 K to 3.88 K, when compared to its homogeneous counterpart. Our results indicate that the spatial and temporal distributions of CO_2 have a considerable impact on regional longwave radiation flux and temperature, and should be taken into account in future climate modeling. 相似文献
14.
R. Bintanja 《Theoretical and Applied Climatology》1997,56(1-2):1-24
Summary In this paper a simple climate model is presented which is used to perform some sensitivity experiments. The atmospheric part is represented by a vertically and zonally averaged layer in which the surface air temperature, radiative fluxes at the surface and at the top of the atmosphere, the turbulent fluxes between atmosphere and surface and the snow cover are calculated. This atmospheric layer is coupled to a two-dimensional advection-diffusion ocean model in which the zonal overturning pattern is prescribed. The ocean model evaluates the temperature distribution, the amount of sea-ice and the meridional and vertical heat fluxes. The present-day climate simulated by the model compares reasonably well with observations of the seasonal and latitudinal distribution of temperature, radiation, surface alebdo, sea-ice and snow cover and meridional energy fluxes. Then, the sensitivity of the model-simulated present-day climate to perturbations in the incident solar radiation at the top of the atmosphere is investigated. The temperature response displays large latitudinal and seasonal variations, which is in qualitative agreement with results obtained with other climate models. It is found that the seasonal variation of sea-ice cover (and hence, the effective oceanic heat capacity) is one of the most important elements determining seasonal variations in climate sensitivity. Differences in sensitivity between the seasonal and annual mean version of the model are discussed. Finally, the equilibrium response to perturbations in some selected model variables is presented; these variables include meridional diffusion coefficients, drag coefficient, sea-ice thickness, atmospheric CO2-concentration and cloud optical thickness.With 13 Figures 相似文献
15.
城市气溶胶对边界层热量收支的影响 总被引：1，自引：1，他引：1
本文利用长、短波辐射模式和地面能量平衡模式，计算了烟雾层状况下的边界层和地面热量收支情况，计算表明，烟地务层的全天辐射效应使低层大气上部辐射能量收入为正，中下部辐射能量收入为负，总的结果是使低层大气稍稍冷却并使稳定增加，计算还表明，烟雾层造成的地面接收短波辐射能量的减少量可由大气逆辐射的增加量来补偿，烟雾层使得地面温度振幅变小。 相似文献
16.
Xiaodong Xie Xiaoxian Huang Tijian Wang Mengmeng Li Shu Li Pulong Chen 《Acta Meteorologica Sinica》2018,32(3):456-468
Carbon dioxide (CO2) is an important greenhouse gas that influences regional climate through disturbing the earth’s energy balance. The CO2 concentrations are usually prescribed homogenously in most climate models and the spatiotemporal variations of CO2 are neglected. To address this issue, a regional climate model (RegCM4) is modified to investigate the non-homogeneous distribution of CO2 and its effects on regional longwave radiation flux and temperature in East Asia. One-year simulation is performed with prescribed surface CO2 fluxes that include fossil fuel emission, biomass burning, air–sea exchange, and terrestrial biosphere flux. Two numerical experiments (one using constant prescribed CO2 concentrations in the radiation scheme and the other using the simulated CO2 concentrations that are spatially non-homogeneous) are conducted to assess the impact of non-homogeneous CO2 on the regional longwave radiation flux and temperature. Comparison of CO2 concentrations from the model with the observations from the GLOBALVIEW-CO2 network suggests that the model can well capture the spatiotemporal patterns of CO2 concentrations. Generally, high CO2 mixing ratios appear in the heavily industrialized eastern China in cold seasons, which probably relates to intensive human activities. The accommodation of non-homogeneous CO2 concentrations in the radiative transfer scheme leads to an annual mean change of–0.12 W m–2 in total sky surface upward longwave flux in East Asia. The experiment with non-homogeneous CO2 tends to yield a warmer lower troposphere. Surface temperature exhibits a maximum difference in summertime, ranging from–4.18 K to 3.88 K, when compared to its homogeneous counterpart. Our results indicate that the spatial and temporal distributions of CO2 have a considerable impact on regional longwave radiation flux and temperature, and should be taken into account in future climate modeling. 相似文献
17.
对云层垂直结构的准确参数化描述是数值天气预报模式准确计算长短波辐射通量、辐射加热率廓线、云反射率、云辐射效应等参数的重要基础,但地基观测数据无法对模式预报的云层垂直分层情况进行验证。文章基于卫星资料Collection5版本的MODIS云产品MOD06,利用国际上能够较准确判别云层垂直分层的一个新算法,以反演的高、中、低云发生频率和云顶气压结果,评估美国国家环境预报中心(NCEP)北美中尺度模式NAM的云层垂直结构。2006年7～10月北美地区(153°～48°W,12°～62°N)的评估结果表明:①卫星反演和模式预报的高、中、低云的云量区域分布比较相似,尤其是高云。热带太平洋地区模式预报高云量大于卫星反演值。模式预报的低云在墨西哥及北美大陆、大西洋地区更多。②卫星反演和模式预报中云发生频率的差异最小,模式预报高云和低云发生频率峰值比卫星反演的峰值更大,且云顶出现的高度更高。③模式预报中云量和低云量的纬度平均值比卫星反演的高,尤其是低云量。NAM的云参数化有待于进一步改进。 相似文献
18.
不同形状冰晶权重假定对冰云光学和辐射特性的影响 总被引：1，自引：0，他引：1
在BCC_RAD辐射传输模式和包含多形状冰晶粒子的冰云光学性质参数化方案的基础上,详细分析了不同冰晶粒子权重选取对冰云光学和辐射特性的影响。结果显示,不同形状冰晶粒子权重的选取对长波带平均消光系数、单次散射比、不对称因子和短波带平均不对称因子均有较大的影响。冰晶粒子权重选取对长波辐射通量有很大影响:对长波向下辐射通量,权重选择不同可在云底处造成高达10.50 W/m2的差别;对长波向上辐射通量,权重选择不同可在云顶处造成高达15.05 W/m2的差别。冰晶粒子权重选择对短波辐射通量也存在较大影响:对短波向下辐射通量,权重选择不同可在云底处造成高达12.48 W/m2的差别;对短波向上辐射通量,权重选择不同可在云顶处造成高达10.23 W/m2的差别。冰晶粒子权重选择对长波加热率影响较大,在云顶处和云底处分别可达1.31和-2.06 K/d。研究表明,不同形状冰晶粒子权重的选取对冰云光学性质和辐射计算均有较大的影响,在长波区间尤其明显。 相似文献
19.
Feedbacks in the Land-Surface and Mixed-Layer Energy Budgets 总被引：2，自引：1，他引：1
A mixed-layer model of the surface energy budget and the planetary boundary layer (PBL) is developed, based on the prognostic equations for soil temperature, mixed layer potential temperature and specific humidity and the growth and abrupt collapse of the PBL. Detailed parameterizations of the longwave radiative fluxes are included. The feedbacks in the uncoupled (i.e. surface energy budget with non-responding PBL) and coupled land surface and atmospheric mixed-layer energy budgets are examined. A simplified, time continuous, version of the model, in which the specific humidity budget is the balance of evapotranspiration and dry-air entrainment, and the PBL height is given by the lifted condensation level, is shown to be in good agreement with the complete model. By forcing the simplified model with daily mean rather than periodic solar radiation, an equilibrium model state is achieved where the fluxes are in close agreement with the daily mean fluxes corresponding to the periodic forcing. The model also agrees favorably with measurements from the FIFE field experiment. Feedbacks are examined using the equilibrium model state. The uncoupled and coupled model sensitivities with respect to the minimal stomatal resistance and the atmospheric specific humidity not only differ in magnitude, but in sign as well. This results puts into question the extent to which uncoupled land-surface models that are forced with atmospheric variables may be used in sensitivity studies. 相似文献
20.
Jeremy Price 《Boundary-Layer Meteorology》2011,139(2):167-191
Data from several cases of radiation fog occurring at the Met Office field site at Cardington, Bedfordshire, UK have been
analyzed with a view to elucidating the typical evolution in its static stability from formation to dissipation. Typically
the early stages of radiation fog are characterized by a stable thermal profile and a relatively shallow depth. However, when
the fog reached approximately 100 m depth it was observed to become optically thick (to longwave radiation), with a subsequent
change over several hours to a saturated adiabatic stability profile. At this time turbulence levels were seen to increase
significantly. The mechanisms involved appear to be radiative cooling from fog top and a positive heat flux to the atmosphere
from the soil. The importance of this change in stability for numerical modelling of fog episodes is discussed. Several case
studies are made to gain some insight into how common this transition is. Droplet spectra were measured at 2-m height for
many of the cases considered, and their evolution is discussed. It is found that distributions fall into an initial phase
with small drop sizes (approximately ≤ 10 μm diameter) and concentration, and a mature phase with the appearance of much larger
drop sizes with a mean diameter of approximately 15−20 μm. It is found that the appearance of the mature phase does not coincide
with the change in stability from stable to saturated adiabatic, but there is some evidence that once a saturated adiabatic
profile is established, the droplet spectra variations are significantly less than for the stable period. The observed evolution
of these spectra brings into question the suitability of microphysical schemes that assume constant spectral shape, drop diameter,
and number density. 相似文献