Changes in Snow Mass Balance in the Canadian Rocky Mountains Caused by CO2 Rise: Regional Atmosphere Model Results

Abstract

This study investigates snow mass balance in the Canadian Rockies under a relatively conservative Intergovernmental Panel on Climate Change emission scenario for the twenty-first century through the use of regional atmosphere modelling. We dynamically downscale results from five 10-year subsets of general circulation model integrations to 6?km resolution to produce a physically consistent representation of the atmosphere at high elevations. Regional model results make evident greater warming with increasing elevation at low to mid-levels of the atmosphere, and a simple thermodynamic explanation of this process is presented. Simulated increases in atmospheric water vapour result in increases in cloud cover and precipitation at high elevations, which temporarily offset the effects of rising temperatures, but by 2100 all model elevations experience reductions in snow mass balance. A simple energy balance model produces reasonable estimates of changes in the elevation of equilibrium net snow accumulation, with increases between 185 and 197?m under an approximate 1.5°C rise in surface temperatures by 2100.     

The energy budget of the Sable Island ocean region

The heat exchange between ocean and atmosphere over cold water is studied by calculating all terms in the energy balance twice each day for the year 1971 for the Sable Island region.

The atmospheric long‐wave radiation is relatively constant because of frequent overcast and low clouds. The surface long‐wave balance is markedly negative in winter but slightly positive for a short time in summer, due to strong advection of warm moist air over the cold water. In winter, the turbulent fluxes are directed upwards and are strong, the upward fluxes beginning after the middle of August and lasting until mid‐March. The maximum daily values of latent heat flux are 400 to 500 ly day^?1 (194 to 242 W m^?2), about a third or a quarter of the magnitude over the warmer Gulf Stream water. The summer fluxes are fairly constant and directed downward.

The water of the Labrador Current in the Sable Island region warms substantially from March to September and conversely cools intensely in the period November‐January.

A comparison of the energy exchange for a current and for water without motion shows that the surface temperatures would be similar in summer, and the temperature drop would be about equal until November. From that time on, the surface temperature would level off for a water body with no current, but in actual conditions the surface temperature continues to drop to a late winter minimum of about 1°C.

Atmospheric advection of latent heat was calculated by assuming that the daily precipitation was always caused first by condensation of all locally evaporated water with any remainder being supplied by water‐vapour advection. The main cause for atmospheric heating in the Sable Island area was found to be condensation of imported water vapour. The region is, in summer, a marked sink for atmospheric heat and water content. For water it remains a sink even in winter. For sensible heat it becomes a source from November to March. The warming of the atmosphere is caused by release of latent heat of advected water vapour in the period February‐August. During the months September‐January the heat sources are both water‐vapour advection and surface turbulent terms.     

用卫星OLR资料估算中国大陆月降水量

利用国家卫星气象中心处理的NOAA下午轨道卫星的OLR资料,用Xie等在1998年的文章中提出的月降水量计算模式,计算了1991-2008年地理范围在10°～60°N、75°～150°E、分辨率为0.5°×0.5°的中国大陆月降水量,得出:用OLR月距平资料可以计算出月降水量,模式估算出的降水量通过与NCEP提供的18年月降水量陆地观测数据对比,精度为:冬季相对误差49.14%、绝对误差7.97 mm;春季相对误差37.60%、绝对误差14.97 mm;夏季相对误差27.37%、绝对误差31.61mm;秋季相对误差37.99%、绝对误差16.95 mm,可见精度效果并不是太好,造成误差的主要原因是降水机制不一,层状云降水特别是逆温层状云和连续阴天不下雨,以及月平均OLR不能完整地反映月内降水云和降水量是造成用OLR月距平估算月降水量的主要误差来源.通过对FY-2C卫星云分类产品的图像分析,得出中国南方冬季主要是层状云降水,OLR月距平值较高,用全球的A、B系数估算出的降水量偏低于实况,因此对中国大陆进行分区、分季节统计A、B系数,是解决OLR月距平估算月降水量精度问题的途径.     

Conditions for freezing precipitation at some airports of Russia and the CIS: IV. Airport of Nizhni Novgorod

Results of study of conditions for freezing precipitation (FP) at the airport of Nizhni Novgorod based on 20-year series of surface observations are described. The cloud tops are estimated from radiosonde data. It is found that the monthly mean FP frequency does not exceed 0.44%; the phenomenon occurs from October to February. Over 20 years, a total of 113 FP episodes were observed, or less than six episodes annually. Freezing precipitation is more frequent at night and in the morning and very rare in the afternoon, at surface air temperatures not exceeding 0°C and not below ?10°C; in half of the cases, the air temperature is within ?0.1 to ?2.0°C. Surface wind is most frequently from south or southwest, while in the lower 4-km layer, according to the radiosonde data, wind direction mostly veers with height from south to west and north. In the boundary layer, FP is often associated with low-level jet streams, most frequently of southwest direction in the cloudy layers. The warm layers within and below the clouds occur in more than 20% of the cases. The most typical precipitation is FP from “all cold” clouds. Using objective criteria of the fronts, synoptic situations, advection, and baroclinicity, it is shown that almost all cases of freezing rain are observed in frontal zones, while freezing drizzle is as frequent at the fronts as under airmass conditions. Both types of FP are associated mainly with high baroclinicity and warm advection. The results can be used to develop an objective method of FP forecasting.     

Comparison of artificial intelligence techniques for prediction of soil temperatures in Turkey

Soil temperature is a meteorological data directly affecting the formation and development of plants of all kinds. Soil temperatures are usually estimated with various models including the artificial neural networks (ANNs), adaptive neuro-fuzzy inference system (ANFIS), and multiple linear regression (MLR) models. Soil temperatures along with other climate data are recorded by the Turkish State Meteorological Service (MGM) at specific locations all over Turkey. Soil temperatures are commonly measured at 5-, 10-, 20-, 50-, and 100-cm depths below the soil surface. In this study, the soil temperature data in monthly units measured at 261 stations in Turkey having records of at least 20 years were used to develop relevant models. Different input combinations were tested in the ANN and ANFIS models to estimate soil temperatures, and the best combination of significant explanatory variables turns out to be monthly minimum and maximum air temperatures, calendar month number, depth of soil, and monthly precipitation. Next, three standard error terms (mean absolute error (MAE, °C), root mean squared error (RMSE, °C), and determination coefficient (R ²)) were employed to check the reliability of the test data results obtained through the ANN, ANFIS, and MLR models. ANFIS (RMSE 1.99; MAE 1.09; R ² 0.98) is found to outperform both ANN and MLR (RMSE 5.80, 8.89; MAE 1.89, 2.36; R ² 0.93, 0.91) in estimating soil temperature in Turkey.

     

Study on Radar Detection of One Stratiform Cloud Precipitation Process in the Central Part of the Tianshan Mountains in China

In order to investigate the physical structure characteristics of the clouds and precipitation over the Tianshan Mountains in summer, the Urumqi Institute of Desert Meteorology of China Meteorological Administration (CMA) carried out an atmospheric detection experiment in Bayanbulak from 1st to 31st August 2012 by means of a wind-profiling radar and a Doppler weather radar. Using the radar observation data, this paper analyzes the dynamic, thermodynamic, radar echo intensity and macro-micro structure characteristics of the 2–3 August precipitation process. The results show that: (1) The radar echo intensity of this rainfall process changes within 5–38 dBZ, and the precipitation cloud system is under the height of 6500 m, with notable 0 °C level echo bright band between 1200 m and 2000 m height. NCEP analysis data shows that the cloud top temperature ranges from ?25 °C to ?32 °C. These indicate the features of typical stratiform cold cloud precipitation. (2) Atmospheric motion during the precipitation process presents the multi-layer structure with wind velocity varying within the range of 3.0–8.0 m/s. The temperature advection is presented with the vertical structure distribution of “cold-warm-cold”, which indicates relative stability of the atmospheric stratification. (3) By retrieving and analyzing the raindrop size distributions below 0 °C level bright band within 600–1200 m height, when the precipitation evolve from early stage to its peak stage, the concentration of the tiny particle zone (D?≤?2.5 mm) changes a little while the concentration of the medium particle zone (2.5?<?D?≤?4 mm) and the concentration of the large particle(D?>?4 mm) increase considerably; but after peak stage the concentration in the medium particle zone and the concentration in the large particle zone decline first, then the concentration in the tiny particle zone reduces. (4) Raindrop size distribution data is used to calculate the precipitation intensity and the liquid water content, whose spatial-temporal variation characteristics are the same. During peak stage of the precipitation, the instantaneous precipitation intensity reaches 5.0 mm/h, and the liquid water content reaches 0.35 g/m³. This study would help deepen the understanding on the physical structure of the clouds and precipitation over the Tianshan Mountains in summer, and also provide some scientific basis for cloud seeding operation over this area.     

An analysis of heavy precipitation caused by a retracing plateau vortex based on TRMM data

In this paper, we study a persistent heavy precipitation process caused by a special retracing plateau vortex in the eastern Tibetan Plateau during 21–26 July 2010 using tropical rainfall measuring mission (TRMM) data. Results show that during the whole heavy rainfall process, the precipitation rate of convective cloud is steady for all four phases of the plateau vortex movement. Compared with the convective precipitation clouds, the stratiform precipitation clouds have a higher fraction of area, a comparable ratio of contribution to the total precipitation, and a much lower precipitation rate. Precipitation increases substantially after the vortex moves out of the Tibetan Plateau, and Sichuan Province has the most extensive precipitation, which occurs when the vortex turns back westward. A number of strong convective precipitation cloud centers appear at 3–5 km. With strong upward motion, the highest rain top can reach up to 15 km. In various phases of the vortex evolution, there is always more precipitable ice than precipitable water, cloud ice water and cloud liquid water. The precipitating cloud particles increase significantly in the middle and lower troposphere when the vortex moves eastward, and cloud ice particles increase quickly at 6–8 km when the vortex retraces westward. The center of the latent heat release is always prior to the center of the vortex, and the vortex moves along the latent heat release areas. Moreover, high latent heat is released at 5–8 km with maximum at 7 km. Also, the latent heat release is more significant when the vortex moves out of the Tibetan Plateau than over the Tibetan Plateau.     

Observational Characteristics of Cloud Vertical Profiles over the Continent of East Asia from the CloudSat Data

The CloudSat satellite data from June 2006 to April 2011 are used to investigate the characteristics of cloud vertical profiles over East Asia(20°-50°N,80°-120°E),with particular emphasis on the profiles of precipitative clouds in comparison with those of nonprecipitative clouds,as well as the seasonal variations of these profiles.There are some obvious differences between the precipitative and nonprecipitative cloud profiles.Generally,precipitative clouds mainly locate below 8 km with radar reflectivity in the range of-20 to 15 dBZ and maximum values appearing within 2-4-km height,and the clouds usually reach the ground;while nonprecipitative clouds locate in the layers of 4-12 km with radar reflectivity between-28 and 0 dBZ and maximum values within 8-10-km height.There are also some differences among the liquid precipitative,solid precipitative,and possible drizzle precipitative cloud profiles.In precipitative clouds,radar reflectivity increases rapidly from 11 to 7 km in vertical,implying that condensation and collision-coalescence processes play a crucial role in the formation of large-size drops.The frequency distribution of temperature at-15℃ is consistent with the highest frequency of radar reflectivity in solid precipitative clouds,which suggests that the temperatures near-15℃ are conductive to deposition and accretion processes.The vertical profiles of liquid precipitative clouds show almost the same distributions in spring,summer,and autumn but with differences in winter at mainly lower levels.In contrast,the vertical profiles of solid precipitative clouds change from spring to winter with an alternate double and single high-frequency core,which is consistent with variations of the frequency distribution of temperature at-15℃.The vertical profiles of nonprecipitative clouds show a little change with season.The observations also show that the precipitation events over East Asia are mostly related to deep convective clouds and nimbostratus clouds.These results are expected to be useful for evaluation of weather and climate models and for improvement of microphysical parameterizations in numerical models.     

On the role of the planetary boundary layer in cyclogenesis over the ocean: Research note

Abstract

The importance of boundary‐layer convergence of mass and water vapour on the pressure tendency is studied. Mass convergence tends to fill the low while moisture convergence deepens it, owing to the subsequent release of latent heat. It is estimated that at dew points above about 18°C, the moisture convergence effect becomes dominant; that is, the C1SK mechanism is important at high dew points.     

Thermal features of the Mackenzie basin from NOAA AVHRR observations for summer 1994

Abstract

A series of mid‐afternoon Advanced Very High Resolution Radiometer (AVHRR) thermal radiance scenes were assembled in order to develop a better understanding of the complex energy and water processes leading to variations in surface temperature. An in‐depth knowledge of the temperature variability is of interest to land surface process modelling and its application to the Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS).

Clear‐sky land surface temperatures are estimated by applying a split window technique to remove atmospheric effects. A maximum land surface temperature map of the Mackenzie basin at 1‐km scale for summer 1994 is produced. The patterns are related to land surface features and elevation. The basin's maximum land surface temperature patterns can be subdivided into three land zones (≥ 35°C, 33–34°C and 27–32°C) and a water dominated zone (20.5°C on average). The highest maximum temperature zone (≥35°C) corresponds to a combination of minimal vegetation, drier soils and low terrain. This zone is not in the southern part of the basin as might be speculated in the absence of these data, but in a wide low elevation corridor from west of Great Bear Lake along the Mackenzie River down to 50°N, 120°W. The maximum land surface temperatures tend to decrease with increasing vegetation density and surface moisture; they also decrease with elevation at a rate of –4.5°C km^–1. This is confirmed by weather station data. The AVHRR data extend this relationship to the 1200 – 2200 m altitude ranges, where there are no station data. The data suggest that elevation and land cover should be taken into account in the objective analysis (spatial interpolation) of station data.     

Physical retrievals of over-ocean rain rate from multichannel microwave imagery. Part I: Theoretical characteristics of normalized polarization and scattering indices

Summary Microwave rain rate retrieval algorithms have most often been formulated in terms of the raw brightness temperatures observed by one or more channels of a satellite radiometer. Taken individually, single-channel brightness temperatures generally represent a near-arbitrary combination of positive contributions due to liquid water emission and negative contributions due to scattering by ice and/or visibility of the radiometrically cold ocean surface. Unfortunately, for a given rain rate, emission by liquid water below the freezing level and scattering by ice particles above the freezing level are rather loosely coupled in both a physical and statistical sense. Furthermore, microwave brightness temperatures may vary significantly (30–70 K) in response to geophysical parameters other than liquid water and precipitation. Because of these complications, physical algorithms which attempt to directly invert observed brightness temperatures have typically relied on the iterative adjustment of detailed microphysical profiles or cloud models, guided by explicit forward microwave radiative transfer calculations.In support of an effort to develop a significantly simpler and more efficient inversion-type rain rate algorithm, the physical information content of two linear transformations of single-frequency, dual-polarization brightness temperatures is studied: thenormalized polarization difference P of Petty and Katsaros (1990, 1992), which is intended as a measure of footprint-averaged rain cloud transmittance for a given frequency; and ascattering index S (similar to the polarization corrected temperature of Spencer et al., 1989) which is sensitive almost exclusively to ice. A reverse Monte Carlo radiative transfer model is used to elucidate the qualitative response of these physically distinct single-frequency indices to idealized 3-dimensional rain clouds and to demonstrate their advantages over raw brightness temperatures both as stand-alone indices of precipitation activity and as primary variables in physical, multichannel rain rate retrieval schemes.As a byproduct of the present analysis, it is shown that conventional plane-parallel analyses of the well-known footprint-filling problem for emission-based algorithms may in some cases give seriously misleading results.With 11 Figures     

Impacts of boreal hydroelectric reservoirs on seasonal climate and precipitation recycling as simulated by the CRCM5: a case study of the La Grande River watershed,Canada

Located in northern Quebec, Canada, eight hydroelectric reservoirs of a 9782-km² maximal area cover 6.4% of the La Grande watershed. This study investigates the changes brought by the impoundment of these reservoirs on seasonal climate and precipitation recycling. Two 30-year climate simulations, corresponding to pre- and post-impoundment conditions, were used. They were generated with the fifth-generation Canadian Regional Climate Model (CRCM5), fully coupled to a 1D lake model (FLake). Seasonal temperatures and annual energy budget were generally well reproduced by the model, except in spring when a cold bias, probably related to the overestimation of snow cover, was seen. The difference in 2-m temperature shows that reservoirs induce localized warming in winter (+0.7 ± 0.02 °C) and cooling in the summer (−0.3 ± 0.02 °C). The available energy at the surface increases throughout the year, mostly due to a decrease in surface albedo. Fall latent and sensible heat fluxes are enhanced due to additional energy storage and availability in summer and spring. The changes in precipitation and runoff are within the model internal variability. At the watershed scale, reservoirs induce an additional evaporation of only 5.9 mm year⁻¹ (2%). We use Brubaker’s precipitation recycling model to estimate how much of the precipitation is recycled within the watershed. In both simulations, the maximal precipitation recycling occurs in July (less than 6%), indicating weak land-atmosphere coupling. Reservoirs do not seem to affect this coupling, as precipitation recycling only decreased by 0.6% in July.

     

一次东北冷涡云降水垂直结构特征分析

利用辽宁阜新国家站(121.7458°E,42.0672°N)的毫米波云雷达(8 mm)和微雨雷达(12.5 mm)对2020年8月12—13日东北冷涡影响下的一次降水过程进行了观测,分析了云降水的垂直结构特征并探讨了降水机制.结果表明:本次过程中,云水平方向发展不均匀,以层状云和层积混合云为主,云内有时还嵌有对流泡....     

Stream temperature sensitivity to climate warming in California’s Sierra Nevada: impacts to coldwater habitat

Water temperature influences the distribution, abundance, and health of aquatic organisms in stream ecosystems, so understanding the impacts of climate warming on stream temperature will help guide management and restoration. This study assesses climate warming impacts on stream temperatures in California’s west-slope Sierra Nevada watersheds, and explores stream temperature modeling at the mesoscale. We used natural flow hydrology to isolate climate induced changes from those of water operations and land use changes. A 21 year time series of weekly streamflow estimates from WEAP21, a spatially explicit rainfall-runoff model were passed to RTEMP, an equilibrium temperature model, to estimate stream temperatures. Air temperature was uniformly increased by 2°C, 4°C, and 6°C as a sensitivity analysis to bracket the range of likely outcomes for stream temperatures. Other meteorological conditions, including precipitation, were unchanged from historical values. Raising air temperature affects precipitation partitioning into snowpack, runoff, and snowmelt in WEAP21, which change runoff volume and timing as well as stream temperatures. Overall, stream temperatures increased by an average of 1.6°C for each 2°C rise in air temperature, and increased most during spring and at middle elevations. Viable coldwater habitat shifted to higher elevations and will likely be reduced in California. Thermal heterogeneity existed within and between basins, with the high elevations of the southern Sierra Nevada and the Feather River watershed most resilient to climate warming. The regional equilibrium temperature modeling approach used here is well suited for climate change analysis because it incorporates mechanistic heat exchange, is not overly data or computationally intensive, and can highlight which watersheds are less vulnerable to climate warming. Understanding potential changes to stream temperatures from climate warming will affect how fish and wildlife are managed, and should be incorporated into modeling studies, restoration assessments, and licensing operations of hydropower facilities to best estimate future conditions and achieve desired outcomes.     

On the Factors Controlling the Snow Surface and 2-m Air Temperatures Over the Arctic Sea Ice in Winter

Factors controlling the magnitudes of, and short-term variations in, the potential temperatures of the snow surface and the air at the height of 2 m θ_S and θ_{2 m} over Arctic sea ice in winter are analysed. The study addresses the winters of 1986–1987 and 1987–1988, and is based on the temperature, wind, and cloud observations made by Russian drifting ice stations. It also relies on the ERA40 re-analyses of the European Centre for Medium-Range Weather Forecasts, which were utilised to calculate the lateral heat advection at the sites of the ice stations. The cloud cover and wind speed were more important than the heat advection in controlling the magnitudes of θ_{2 m} and θ_S, while on a time scale of 24 h, during steady forcing conditions, the heat advection was the most important factor affecting the changes in θ_S and θ_{2 m}. During changing conditions, and considering individual factors separately, the monthly mean 24-h temperature changes were less than ± 5 °C: the effect of the cloud cover was the largest, and that of the heat advection was the smallest. When simultaneous changes in the three factors were analysed, the seasonal mean temperature changes were even of the order of ±15 °C, with the strongest warming events exceeding 35 K in a single day. The difference θ_S − θ_{2 m} reached its lowest seasonal mean values during conditions of clear skies (−1.3 °C), light winds (−1.3 °C) and warm-air advection (−0.8 °C). θ_S and θ_{2 m} followed each other closely, even during major synoptic-scale temperature variations.     

The maximum temperatures and heat waves in Serbia during the summer of 2007

An extraordinary heat wave occurred in Serbia from July 14 to July 24 in 2007. Record values of the maximum temperatures were observed over almost the whole territory of Serbia and in Smederevska Palanka, a temperature of 44.9°C was registered, which was the absolute maximum value ever recorded. The highest increase over the previous absolute maximum temperature, dating back to 1888, of 3.1°C was registered in Belgrade. The Warm Spell Duration Indicator, from which the duration and severity of the heat waves are estimated, was applied to the series of the daily maximum temperatures in Smederevska Palanka. An analysis of the daily maximum temperatures and heat waves during the summer of 2007 revealed significant changes in the trends of anomalies and extreme (90%) quantiles. In addition, the main characteristics of the heat wave and the circulation conditions which caused the heat wave in Serbia during the summer 2007 were analyzed.     

Simulation of urban climate with high-resolution WRF model: A case study in Nanjing, China

In this study, urban climate in Nanjing of eastern China is simulated using 1-km resolution Weather Research and Forecasting (WRF) model coupled with a single-layer Urban Canopy Model. Based on the 10-summer simulation results from 2000 to 2009 we find that the WRF model is capable of capturing the high-resolution features of urban climate over Nanjing area. Although WRF underestimates the total precipitation amount, the model performs well in simulating the surface air temperature, relative humidity, and precipitation frequency and inter-annual variability. We find that extremely hot events occur most frequently in urban area, with daily maximum (minimum) temperature exceeding 36°C (28°C) in around 40% (32%) of days. Urban Heat Island (UHI) effect at surface is more evident during nighttime than daytime, with 20% of cases the UHI intensity above 2.5°C at night. However, The UHI affects the vertical structure of Planet Boundary Layer (PBL) more deeply during daytime than nighttime. Net gain for latent heat and net radiation is larger over urban than rural surface during daytime. Correspondingly, net loss of sensible heat and ground heat are larger over urban surface resulting from warmer urban skin. Because of different diurnal characteristics of urban-rural differences in the latent heat, ground heat and other energy fluxes, the near surface UHI intensity exhibits a very complex diurnal feature. UHI effect is stronger in days with less cloud or lower wind speed. Model results reveal a larger precipitation frequency over urban area, mainly contributed by the light rain events (< 10 mm d^?1). Consistent with satellite dataset, around 10?C20% more precipitation occurs in urban than rural area at afternoon induced by more unstable urban PBL, which induces a strong vertical atmospheric mixing and upward moisture transport. A significant enhancement of precipitation is found in the downwind region of urban in our simulations in the afternoon.     

Criteria for the Onset of Significant Concentrations of Ice Particles in cumulus clouds

Field observations and measurements, reported from many locations around the world, are used to deduce the cloud depths (and cloud top temperatures) required for the Onset of Significant Concentrations of Ice Particles (OSCIP) in cumulus clouds. The results show that the OSCIP will generally occur if cloud top temperatures are between about −4° and −10°C. The results are compared with the criteria for the OSCIP proposed by Hobbs and Rangno (1985) and for the formation of high ice particle concentrations by riming-splintering.     

A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to ?7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between ?50 W m^?2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m^?2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.     

GATE A/B‐scale budget analysis

Abstract

Using GATE A/B‐scale U.S.S.R. ship data, heat and moisture budget analyses have been carried out for the three‐day period: 0000 GMT, 7 September to 2400 GMT, 9 September, 1974. The period has been subdivided into an undisturbed period (0000 GMT, 7 September ‐1800 GMT, 8 September) and a disturbed period (1800 GMT, 8 September ‐ 2400 GMT, 9 September) based on surface precipitation and satellite cloud observations. During the undisturbed period, precipitation was very light (1–3 mm day^‐1). On the other hand, the precipitation rate became well over 10 mm day^‐1 during the disturbed period. A/B‐scale heat and moisture budget results for both periods are presented.

It is found that during the undisturbed period, cumulus clouds have heating and moistening effects in the lower troposphere below 700 mb, and cooling and moistening effects in the upper troposphere above 600mb. In the disturbed period, clouds have strong heating and drying effects throughout the entire cloud layer.

Using the diagnostic scheme developed by Cho (1977), the collective properties of cumulus clouds for both the undisturbed and the disturbed periods are also determined. During the undisturbed period, some clouds reached as high as the 300‐mb level, however, little precipitation was produced. The total cloud mass flux is found to be negative in the upper troposphere and can be attributed to downdrafts induced by the evaporation of cloud liquid water. On the other hand, total cloud mass flux for the disturbed period is positive throughout the entire troposphere.