Observations Of Nocturnal Drainage Flow In A Shallow Gully

The formation of cold air drainage flows in a shallow gully is studied during CASES-99 (Cooperative Atmosphere-Surface Exchange Study). Fast and slow response wind and temperature measurements were obtained on an instrumented 10-m tower located in the gully and from a network of thermistors and two-dimensional sonic anemometers, situated across the gully. Gully flow formed on clear nights even with significant synoptic flow. Large variations in surface temperature developed within an hour after sunset and in situ cooling was the dominant factor in wind sheltered locations. The depth of the drainage flow and the height of the down-gully wind speed maximum were found to be largest when the external wind speed above the gully flow is less than 2 m s^-1. The shallow drainage current is restricted to a depth of a few metres, and is deepest when the stratification is stronger and the external flow is weaker. During the night the drainage flow breaks down, sometimes on several occasions, due to intermittent turbulence and downward fluxes of heat and momentum. The near surface temperature may increase by 6 ° C in less than 30 min due to the vertical convergence of downward heat flux. The mixing events are related to acceleration of the flow above the gully flow and decreased Richardson number. These warming events also lead to warming of the near surface soil and reduction of the upward soil heat flux. To examine the relative importance of different physical mechanisms that could contribute to the rapid warming, and to characterize the turbulence generated during the intermittent turbulent periods, the sensible heat budget is analyzed and the behaviour of different turbulent parameters is discussed.     

An Amplified Signal of Climatic Change in Soil Temperatures during the Last Century at Irkutsk, Russia

Climatic changes at the Earth's surface propagate slowly downward into theground and modify the ambient ground thermal regime. However, causes of soiltemperature changes in the upper few meters are not well documented. One majorobstacle to understanding the linkage between the soil thermal regime andclimatic change is the lack of long-term observations of soil temperatures andrelated climatic variables. Such measurements were made throughout the formerSoviet Union with some records beginning at the end of the 19th century. Inthis paper, we use records from Irkutsk, Russia, to demonstrate how the soiltemperature responded to climatic changes over the last century. Both airtemperature and precipitation at Irkutsk increased from the late 1890s to the1990s. Changes in air temperature mainly occurred in winter, while changes inprecipitation happened mainly during summer. There was an anti-correlationbetween mean annual air temperature and annual total precipitation, i.e., more(less) precipitation during cold (warm) years. There were no significanttrends of changes in the first day of snow on the ground in autumn, but snowsteadily disappeared earlier in spring, resulting in a reduction of the snowcover duration. A grass-covered soil experiences seasonal freezing for morethan nine months each year and the long-term average maximum depth ofseasonally frozen soils was about 177 cm with a range from 91 cm to 260 cm.The relatively lower soil temperature at shallow depths appears to representthe so-called `thermal offset' in seasonally frozen soils. Changes in meanannual air temperature and soil temperature at 40 cm depth were about the samemagnitude (2.0 °C to 2.5 °C) over the common period of record, but thepatterns of change were substantially different. Mean annual air temperatureincreased slightly until the 1960s, while mean annual soil temperatureincreased steadily throughout the entire period. This leads to the conclusionthat changes in air temperature alone cannot explain the changes in soiltemperatures at this station. Soil temperature actually decreased duringsummer months by up to 4 °C, while air temperature increased slightly.This cooling in the soil may be explained by changes in rainfall and hencesoil moisture during summer due to the effect of a soil moisture feedbackmechanism. While air temperature increased about 4 °C to 6 °C duringwinter, soil temperature increased by up to 9 °C. An increase in snowfallduring early winter (October and November) and early snowmelt in spring mayplay a major role in the increase of soil temperatures through the effects ofinsulation and albedo changes. Due to its relatively higher thermalconductivity compared to unfrozen soils, seasonally frozen ground may enhancethe soil cooling, especially in autumn and winter when thermal gradient isnegative.     

An improved temporal correction method for mobile measurement of outdoor thermal climates

Accurate temporal corrections for the spatial meteorological parameters obtained through mobile measurements are essential in the synchronous analysis of local urban climates. This paper discusses current temporal correction models and proposes an improved model by considering correlation coefficients that are influenced by the underlying surface conditions, and the distance between the stationary weather stations and the mobile location points during a mobile measurement. Together with four adjacent, simultaneously recording stationary weather stations, long-term mobile temperature and humidity measurements were taken along a 17-km transect covering 18 mobile location points through the University Town of Shenzhen. Using the multiple air temperature and relative humidity values of the mobile location points and stationary weather stations, the function equations for determining the correlation coefficients were obtained for application in the proposed temporal correction model. Further, three kinds of validation methods were applied to compare temporal correction models. Validation results showed that the temporal correction model proposed in this study was significantly more accurate and reliable compared to the other models.     

Lake ice records used to detect historical and future climatic changes

Historical ice records, such as freeze and breakup dates and the total duration of ice cover, can be used as a quantitative indicator of climatic change if long homogeneous records exist and if the records can be calibrated in terms of climatic changes. Lake Mendota, Wisconsin, has the longest uninterrupted ice records available for any lake in North America dating back to 1855. These records extend back prior to any reliable air temperature data in the midwestern region of the U.S. and demonstrate significant warming of approximately 1.5 °C in fall and early winter temperatures and 2.5 °C in winter and spring temperatures during the past 135 years. These changes are not completely monotonie, but rather appear as two shorter periods of climatic change in the longer record. The first change was between 1875 and 1890, when fall, winter, and spring air temperatures increased by approximately 1.5 °C. The second change, earlier ice breakup dates since 1979, was caused by a significant increase in winter and early spring air temperatures of approximately 1.3 °C. This change may be indicative of shifts in regional climatic patterns associated with global warming, possibly associated with the Greenhouse Effect.With the relationships between air temperature and freeze and break up dates, we can project how the ice cover of Lake Mendota should respond to future climatic changes. If warming occurs, the ice cover for Lake Mendota should decrease approximately 11 days per 1 °C increase. With a warming of 4 to 5 °C, years with no ice cover should occur in approximately 1 out of 15 to 30 years.     

Formation and Characteristics of Coastal Internal Boundary Layers During Onshore Flows

The development and characteristics of coastal internal boundary layers were investigated in 28 tests. These were made at all seasons and in both gradient and sea-breeze flows but only during mid-day periods. Measurements of turbulence and temperature were taken from a light aircraft which flew traverses across Long Island at successive altitudes parallel to the wind direction. These were used to locate the boundary between modified and unmodified air as a function of height and distance from the coast. The same measurements plus tower measurements of wind, turbulence and temperature, pilot balloon soundings and measurements of land and water surface temperatures by a remote sensing IR thermometer were used to quantify the characteristics of the modified and unmodified air. The boundary layer slope was steep close to the land-water interface and became shallower with downwind distance. Growth of the boundary layer was initially slower with stable lapse rates upwind than with neutral or unstable conditions over the water. An equilibrium height was found in many tests except under conditions of free convection when the internal boundary layer merged into the mixed layer inland and with sea-breeze conditions. The equilibrium height depended on downwind conditions and was greater with low wind speeds and strong land surface heating than with stronger winds and small land-water temperature differences. Current theoretical models are not adequate to predict the height of the boundary layer at the altitudes and distances studied but reasonably good predictions were given by an empirical model developed earlier. Wind speed in the modified air averaged about 70% of that at the coast but turbulence levels were several times higher both near the surface and aloft. These findings have important implications for diffusion from coastal sites.     

Formation and characteristics of coastal internal boundary layers during onshore flows

The development and characteristics of coastal internal boundary layers were investigated in 28 tests. These were made at all seasons and in both gradient and sea-breeze flows but only during mid-day periods. Measurements of turbulence and temperature were taken from a light aircraft which flew traverses across Long Island at successive altitudes parallel to the wind direction. These were used to locate the boundary between modified and unmodified air as a function of height and distance from the coast. The same measurements plus tower measurements of wind, turbulence and temperature, pilot balloon soundings and measurements of land and water surface temperatures by a remote sensing IR thermometer were used to quantify the characteristics of the modified and unmodified air. The boundary layer slope was steep close to the land-water interface and became shallower with downwind distance. Growth of the boundary layer was initially slower with stable lapse rates upwind than with neutral or unstable conditions over the water. An equilibrium height was found in many tests except under conditions of free convection when the internal boundary layer merged into the mixed layer inland and with sea-breeze conditions. The equilibrium height depended on downwind conditions and was greater with low wind speeds and strong land surface heating than with stronger winds and small land-water temperature differences. Current theoretical models are not adequate to predict the height of the boundary layer at the altitudes and distances studied but reasonably good predictions were given by an empirical model developed earlier. Wind speed in the modified air averaged about 70% of that at the coast but turbulence levels were several times higher both near the surface and aloft. These findings have important implications for diffusion from coastal sites.     

Mean seasonal and spatial variability in global surface air temperature

Summary Using terrestrial observations of shelter-height air temperature and shipboard measurements, a global climatology of mean monthly surface air temperature has been compiled. Data were obtained from ten sources, screened for coding errors, and redundant station records were removed. The combined data base consists of 17 986 independent terrestrial station records and 6 955 oceanic grid-point records. These data were then interpolated to a 0.5° of latitude by 0.5° of longitude lattice using a spherically-based interpolation algorithm. Spatial distributions of the annual mean and intra-annual variance are presented along with a harmonic decomposition of the intra-annual variance.With 7 Figures     

九龙站在西南低涡/晴朗天气背景下观测资料的对比分析

本文基于中国气象局成都高原气象研究所2017年西南涡加密观测资料,对比分析了西南低涡主要源地九龙站在西南低涡/晴朗这两种不同天气背景下,各个基本物理量的日变化特征及存在的差异,认为:西南涡天气背景下,边界层高度较低,而晴朗天气状况下,边界层大气湍流运动强烈,对流混合伸展高度非常惊人,能超过3000m。西南涡天气背景下,夜间大气较为暖湿,气压显著偏低,上层大气风速较大,便于动量下传。晴朗天气背景下,大气较为干燥,温度日较差偏大,大气压较高,白天湍流混合强。越靠近地面差异越显著,差值随高度的增加而逐渐减小。     

Fluxes of gases and particles above a deciduous forest in wintertime

Eddy-correlation measurements of the vertical fluxes of ozone, carbon dioxide, fine particles with diameter near 0.1 m, and particulate sulfur, as well as of momentum, heat and water vapor, have been taken above a tall leafless deciduous forest in wintertime. During the experimental period of one week, ozone deposition velocities varied from about 0.1 cm s^–1 at night to more than 0.4 cm s^-1 during the daytime, with the largest variations associated primarily with changes in solar irradiation. Most of the ozone removal took place in the upper canopy. Carbon dioxide fluxes were directed upward due to respiration and exhibited a strong dependence on air temperature and solar heating. The fluxes were approximately zero at air temperatures less than 5 °C and approached 0.8 mg m^–2 s^–1 when temperatures exceeded 15 °C during the daytime. Fine-particle deposition rates were large at times, with deposition velocities near 0.8 cm s^–1 when turbulence levels were high, but fluxes directed upward were found above the canopy when the surface beneath was covered with snow. Diffusional processes seemed to dominate fine-particle transfer across quasilaminar layers and subsequent deposition to the upper canopy. Deposition velocities for particulate sulfur were highly variable and averaged to a value small in magnitude as compared to similar measurements taken previously over a pine forest in summer.     

Cloud water content effects on computing spectral characteristics of temperature fields and turbulent heat fluxes in the cumulus cloud zones as derived from airborne observations

Analyzed are the earlier obtained data on the dynamic structure of cumulus (convective) clouds in order to estimate cloud water content effects on the air temperature, computed turbulent fluxes, and their spectral characteristics. It is demonstrated that the cloud and precipitation droplets falling to the aircraft temperature sensor provoke an error in temperature measurements. Developed is a method for taking into account these effects and for computing the correct air temperature fluctuations in the cloud, wavelet spectra of the temperature, and wavelet cospectra of heat fluxes. The use of the method enabled to construct a consistent picture of horizontal heat flux distribution in the cumulus zone depending on the wave number and cloud location. It is shown that mesoscale eddies with the scale of more than 500 m are the main factors of heat exchange between clouds and the ambient space, and the role of turbulence consists only in the mixing inside the cloud. The conclusion is made that the temperature adjustment to the water content of the cloud is needed for correct interpretation of the results of aircraft observations of dynamic structure of cumulus clouds.     

Obtaining Potential Virtual Temperature Profiles,Entrainment Fluxes,and Spectra from Mini Unmanned Aerial Vehicle Data

We present a simple but effective small unmanned aerial vehicle design that is able to make high-resolution temperature and humidity measurements of the atmospheric boundary layer. The air model used is an adapted commercial design, and is able to carry all the instrumentation (barometer, temperature and humidity sensor, and datalogger) required for such measurements. It is fitted with an autopilot that controls the plane’s ascent and descent in a spiral to 1800 m above ground. We describe the results obtained on three different days when the plane, called Aerolemma-3, flew continuously throughout the day. Surface measurements of the sensible virtual heat flux made simultaneously allowed the calculation of all standard convective turbulence scales for the boundary layer, as well as a rigorous test of existing models for the entrainment flux at the top of the boundary layer, and for its growth. A novel approach to calculate the entrainment flux from the top-down, bottom-up model of Wynagaard and Brost is used. We also calculated temperature fluctuations by means of a spectral high-pass filter, and calculated their spectra. Although the time series are small, tapering proved ineffective in this case. The spectra from the untapered series displayed a consistent −5/3 behaviour, and from them it was possible to calculate a dimensionless dissipation function, which exhibited the expected similarity behaviour against boundary-layer bulk stability. The simplicity, ease of use and economy of such small aircraft make us optimistic about their usefulness in boundary-layer research.     

A meteorological 80-m tower near Rotterdam

Construction, observation site and instrumentation are described of an 80-m meteorological observation tower, erected in Vlaardingen, The Netherlands, for studies in connection with air pollution problems along the Nieuwe Waterweg. From measurements made regularly in the period April 1967–April 1968 summaries are given of wind profile parameters and gustiness averages. Case histories are presented of an air pollution episode lasting several days, and of an hourly average wind direction difference of 120 ° over 70 m.     

Measurements of the surface layer turbulence in the urban area

The turbulence characteristics measured in the surface layer over a real underlying surface are presented. The measurements are carried out at the 3- and 16-m levels using acoustic anemometers; at the lower level, the measurements are doubled. The diurnal cycle is obtained of the temperature flux and friction velocity. To obtain the temperature flux, so-called “acoustic” temperature is used as calculated from the sound speed data with corrections for air humidity and pressure. The normalized characteristics of turbulence are presented as dependent on dimensionless height z/L. The values of temperature flux and friction velocity increase with height: their day-integral values at 16-m level exceed those at 3-m level for about 20 and 35%, respectively.     

Two-dimensional field of thermal turbulence at the edge of an escarpment

Acoustic sounder measurements of the temperature structure parameter were obtained at the edge of an escarpment which is part of a ridge of mountains. These measurements indicate that in mountainous terrain, the daytime two-dimensional field of thermal turbulence is strongly affected by relative sun-slope orientation and wind direction out to ranges of at least 200–300 m. For the geometry of this site, westerly flow results in a field which tends to decrease rapidly to the west in the morning with a much less rapid decrease in the afternoon. At night, easterly flow results in significantly higher thermal turbulence compared to that obtained during westerly flow.These measurements show an increase in thermal turbulence at horizontal ranges of 100–200 m to the west of the escarpment during early afternoon on days with deep mixed layers. It is conjectured that this is due to the mountain upslope wind.     

The analysis of results of remote sensing monitoring of the temperature profile in lower atmosphere in Bergen (Norway)

Considered is the application of MTP-5 meteorological temperature profiler used for the remote sensing of vertical profiles of the air temperature in the planetary boundary layer and the lower one-kilometer layer of the atmosphere. The measurements were carried out in Bergen (Norway) in 2011–2012. The obtained dataset of temperature profiles has temporal resolution of five minutes and vertical resolution of 50 m. The MTP-5 data are complemented with the measurements of the air temperature and the wind taken at two automatic weather stations and with the measurements of the rain intensity made with the rain radar. Studied is the impact of meteorological conditions and precipitation on the MTP-5 readings. It is revealed that formation of a thin water film (of ice or, to a smaller degree, of sleet) on the surface of the sensor cover of MTP-5 has a significant impact on the data of the temperature monitoring. The removal of intensive precipitation (the precipitation rate is >0.2 mm/hour) improved the reliability and quality of the temperature profile monitoring. In particular, it is demonstrated that significant air pollution and stably stratified atmospheric conditions which lead to low temperatures are reliably monitored with this instrument.     

Observations of the convective plume of a lake under cold-air advective conditions

Moderating effects of Lake Apopka, Florida on downwind surface temperatures were evaluated under cold-air advective conditions. Point temperature measurements north and south of the lake and data obtained from a thermal scanner flown at 1.6 km indicate that surface temperatures directly downwind may be higher than surrounding surface temperatures by as much as 5 °C under conditions of moderate winds (~4 m s^–1). No substantial temperature effects were observed with surface wind speed less than 1 m s^–1. Fluxes of sensible and latent heat from Lake Apopka were calculated from measurements of lake temperature, net radiation, relative humidity and air temperature above the lake. Bulk transfer coefficients and the Bowen ratio were calculated and found to be in agreement with reported data for non-advective conditions.IFAS Journal Series No. 1006.     

Surface Characteristics Observed over the Central Tropical Indian Ocean During Indoex IFP99

The present study is based on the observations carried out over the IndianOcean from the Indian research vessel ORV Sagar Kanya during the intensive field phase of the Indian Ocean Experiment in January–March 1999. The study area spanned from 15°N to 20°S in the central Indian Ocean. Near surface variations and surface fluxes along the cruise track are presented. A comparison of near surface characteristics over the Indian Ocean and tropical west Pacific has been made. It is observed that the average difference between the sea surface temperature and air temperature at 10 m height was 0.7 °C over the study area, nearly half of that observed over the tropical west Pacific. A comparison between observed and NCEP reanalysissurface data has been made. We find good agreement between ship measured andNCEP reanalysis surface pressure, specific humidity and wind fields.On the other hand, surface air temperature in the reanalysis tends to be lowcompared to observations. The components of the net surface heat flux comparebetter in the north Indian Ocean than in the southern Indian Ocean.     

Analysis of Coastal Fog from a Ship During the C-FOG Campaign

This work presents ship-based measurements of fog off St John’s, Newfoundland, on 13 September 2018 during the Coastal Fog field campaign. The measurements included cloud-particle spectra, cloud-base height and aerosol backscatter, radiation, turbulence, visibility, and sea-surface temperature. Radiosonde soundings were made at intervals of less than 2 h. Fog occurred in two episodes during the passage of an eastward-moving synoptic low-pressure system. The boundary-layer structure during the first fog episode consisted of three layers, separated by two saturated temperature inversions, and capped by a subsidence inversion. The lowest layer was fog, and the upper layers were cloud. The second fog episode consisted of one well-mixed fog layer capped by a subsidence inversion. Low wind speeds and stable stratification maintained low surface-layer turbulence during fog. Droplet size distributions had typical bimodal distributions. The visibility correlated with the droplet number concentration and liquid water content. The air temperature was higher than the sea-surface temperature for the first 30 min of the first fog episode but was lower than the sea for the remainder of all fog. The sensible heat flux was upward, from sea to air, for the first 62% of the first fog episode and then reversed to downward, from air to sea, for the remainder of the first fog episode and the second fog episode. The counter-gradient heat fluxes observed (i.e., opposite to what is expected from the instantaneous air–sea temperature difference) appear to be related to turbulence, entrainment, and stratification in the fog layer that overwhelmed the influence of the air–sea temperature difference. While the synoptic-scale dynamics preconditioned the area for fog formation, the final step of fog appearance in this case was nuanced by stratification–turbulence interactions, local advective processes, and microphysical environment.

     

Projected global climate change impact on water temperatures in five north central U.S. streams

The effect of projected global climate change due to a doubling of atmospheric CO₂ on water temperatures in five streams in Minnesota was estimated using a deterministic heat transport model. The model calculates heat exchange between the atmosphere and the water and is driven by climate parameters and stream hydrologic parameters. The model is most sensitive to air temperature and solar radiation. The model was calibrated against detailed measurements to account for seasonally variable shading and wind sheltering. Using climate projections from the GISS, GFDL and OSU GCMs as input; stream temperature simulations predict a warming of freely flowing river reaches by 2.4 °C to 4.7 °C when atmospheric CO₂ doubles. In small shaded streams water temperatures are predicted to rise by an additional 6 °C in summer if trees along stream banks should be lost due to climate change or other human activities (e.g. logging). These projected water temperature changes have significant consequences for survival and growth of fishes. Simulation with the complete heat budget equations were also used to examine simplified water temperature/air temperature correlations.     

Internal gravity waves in a stably stratified boundary layer

Often, a combination of waves and turbulence is present in the stably stratified atmospheric boundary layer. The presence of waves manifest itself in the vertical profiles of variances of fluctuations and in low-frequency contributions to the power spectra. In this paper we study internal waves by means of a linear stability analysis of the mean profiles in a stably stratified boundary layer and compare the results with observed vertical variance profiles of fluctuating wind and temperature along a 200 m mast. The linear stability analysis shows that the observed mean flow is unstable for disturbances in a certain frequency and wavenumber domain. These disturbances are expected to the detectable in the measurements. It is shown that indeed the calculated unstable frequencies are present in the observed spectra. Furthermore, the shape of the measured vertical variance profiles, which increase with height, is explained well by the calculated vertical structure of the amplitude of unstable Kelvin-Helmholtz waves, confirming the contribution of waves to the variances. Because turbulence and waves have quite distinct transport properties, estimates of diffusion from measurements of variances would strongly overestimate this diffusion. Therefore it is important to distinguish between them.