Variability Of Sensible Heat Fluxes Research Articles

Quantifying Spatial Heterogeneities of Surface Heat Budget and Methane Emissions over West-Siberian Peatland: Highlights from the Mukhrino 2022 Campaign

The study presents the first results from the multi-platform observational campaign carried out at the Mukhrino peatland in June 2022. The focus of the study is the quantification of spatial contrasts of the surface heat budget terms and methane emissions across the peatland, which arise due to the presence of microlandscape heterogeneities. It is found that surface temperature contrasts across the peatland exceeded 10 °C for clear-sky conditions both during day and night. Diurnal variation of surface temperature was strongest over ridges and drier hollows and was smallest over the waterlogged hollows and shallow lakes. This resulted in strong spatial variations of sensible heat flux (H) and Bowen ratio, while the latent heat varied much less. During the clear-sky days, H over ryam exceeded the one over the waterlogged hollow by more than a factor of two. The Bowen ratio amounted to about unity over ryam, which is similar to values over forests. Methane emissions estimated using the static-chamber method also strongly varied between various microlandscapes, being largest at a hollow within a ridge-hollow complex and smallest at a ridge. A strong nocturnal increase in methane mixing ratio was observed and was used in the framework of the atmospheric boundary layer budget method to estimate nocturnal methane emissions, which were found to be in the same order of magnitude as daytime emissions. Finally, the directions for further research are outlined, including the verification of flux-aggregation techniques, parameterizations of surface roughness and turbulent exchange, and land-surface model evaluation and development.

Air‐Sea Heat Fluxes Associated With Convective Cold Pools

AbstractHigh‐resolution air‐sea observations collected by a suite of remotely‐piloted uncrewed surface vehicles (USV) provide new insight on surface latent and sensible heat fluxes associated with convective cold pools over the eastern Pacific Intertropical Convergence Zone (ITCZ). Convective cold pools are linked to isolated, heavily precipitating cells over the ocean that yield downdrafts of evaporatively‐cooled air that spreads out horizontally in all directions and are characterized by strong air temperature gradients and horizontal wind speeds. Cold pool frontal signatures are identified based on air temperature depression criteria of −1.5°C in 10 min or less, resulting in a total of 276 events observed within the eastern Pacific ITCZ across 590 total seadays of data from 10 drones over the course of 3 years. Composite analysis reveals enhanced latent and sensible heat fluxes from the ocean to the atmosphere immediately following the frontal passage, and a decomposition of the bulk flux formulas indicates that variations in latent heat flux are largely influenced by the anomalous wind field (i.e., the cold pool gust front) acting on the mean background air‐sea moisture gradient, while variations in sensible heat flux are largely influenced by the mean background wind acting on the anomalous air‐sea temperature gradient (i.e., the cold pool temperature front). High‐frequency variations in wind speed and velocity are further explored in the context of “gustiness” and averaging intervals used in bulk flux algorithms. A case study is also presented for a convective cold pool event captured by a traveling mesoscale network, or mesonet, of USVs.

Land-sea difference of the planetary boundary layer structure and its influence on PM2.5 — Observation and numerical simulation

A comprehensive set of observations were collected along a sea-coast-inland area. According to these observations, the planetary boundary layer heights (PBLH) during winter and summer for sampling locations in sea, coast, and inland areas were 737 m, 483 m and 372 m, and 450 m, 645 m and 646 m, respectively. Little seasonal difference was observed for the daily variation of sensible heat flux over the sea, with the maximum of 55 W/m2 at 12:00 in winter and 27 W/m2 at 13:00 in summer. The duration of sea breeze was ∼10 h in summer and only 3 h in winter, extended to inland area ∼ 50 km and upward 920 m vertically. PM2.5 at coastal area was about 5 μg/m3 higher than inland during summer afternoon. Over the sea, PM2.5 mainly concentrated below 200 m in winter, increased with height at night and decreased with height in the morning below 300 m in summer. A typical land-sea breeze episode was analyzed with observed and simulated result. According to the observed data, the sea breeze extended to inland ∼50 km and upward 300 m vertically. From the simulating result, there was a clear temperature gradient between sea and land from surface to 400 m, when influenced by the sea breeze, the wind and temperature profiles showed different characteristics, and forming a high concentration center of PM2.5 at 300 m. These results provide insights into the structure of land-sea planetary boundary layer, and provide support for the prediction of heavy pollution episode.

Impacts of Anthropogenic Heat and Building Height on Urban Precipitation Over the Seoul Metropolitan area in Regional Climate Modeling

AbstractAn urban heat island (UHI) is a well‐known urban climatic feature; however, an opposite effect, i.e., an urban cool island (UCI), was recently observed at locations where high‐rises were concentrated. Here, we analyzed the impact of two urbanization factors (anthropogenic heat flux (AH) and building height (ZR)), which could affect the formation of UHIs and UCIs, on urban precipitation over the Seoul Metropolitan area. AH caused an increasing precipitation in urban and downwind areas, especially during daytime. AH directly contributed to the sensible heat flux, resulting in surface warming associated with an UHI. Surface heat was transferred within the planetary boundary layer by turbulence, which increased the low‐level instability, resulting in increased precipitation. In the experiments with the ZR, both surface temperature and precipitation increased (decreased) during the nighttime (daytime). The diurnal variation of ground heat flux intensified with an increasing ZR, inducing a smaller variation of sensible heat flux for surface energy balance. Changes in the ground heat flux could also explain occurrence of UCIs in cities with high‐rises. Tall buildings reduced daytime surface temperature, thereby facilitating atmospheric stabilization, and reducing precipitation. Also, the surface friction increased with a higher ZR, resulting in enhanced convergence in the western part of urban areas where dominant westerlies first met land, which was favorable for increased precipitation. In summary, a study of variations in AH and ZR demonstrated how UHIs and UCIs significantly altered precipitation, respectively. In particular, an UCI alleviated an increment in thermo‐driven daytime precipitation caused by an UHI.

Quantifying seasonal and diurnal contributions of urban landscapes to heat energy dynamics

Cooling energy consumption in urban areas is affected significantly by the dynamics of urban heat flux. However, we still lack a clear understanding of the quantitative contribution rate and underlying mechanism of typical urban landscapes to urban heat dynamics, especially in seasonal and diurnal patterns. Here we used a thermal infrared camera and portable meteorological instruments to examine the sensible heat flux (SHF) changes of five typical urban landscapes in Beijing based on surface temperature and concurrent microclimate conditions. Diurnal and seasonal variations of SHF were quantified by comparing changes in forenoon and afternoon in different seasons. Results showed that (1) walls and roads act as heat-source, while forests and water act as heat-sink in all seasons; however, grassland served as heat-sink in summer and spring-autumn, but it becomes a heat-source in winter. (2) The seasonal variation of sensible heat flux of the wall is the greatest, followed by water, while that of trees is the smallest. Besides, the highest sensible heat flux and the maximum variation among typical urban landscapes occur between noon and 2:00 pm. (3) The numerical contribution rate of typical landscapes to sensible heat flux varies with daytime (forenoon and afternoon) and seasonal changes, and these ratios can be used as parameters to adjust the numerical models to obtain more reliable results in surface-energy-flux-related studies. The results of this study can provide a reference for explaining controversial findings based on remote-sensing data, and provide insights into revealing the sensible heat flux mechanism of typical urban landscapes and cooling energy conservation in cities.

Observing the Whole-Canopy Short-Term Dynamic Response to Natural Step Changes in Incident Light: Characteristics of Tropical and Temperate Forests

We examine the physical and biological responses of forest canopies to step changes in light caused by passing low cumulus clouds that intermittently block the direct solar beam. Using data obtained at a tropical rainforest and at a midlatitude deciduous forest, we estimate the course of sensible heat flux, net ecosystem exchange, evapotranspiration, and water-use efficiency in response to the rapid changes in the incident radiative flux. To describe these fluxes during the interval over which the effects of stomatal time delays can be most influential, eddy fluxes are estimated over minute or shorter intervals by invoking a conditional-sampling procedure based on forming a Reynolds-average ensemble. The most important differences between the two forests’ physical responses are in the thermal balances and heat-flux time response constants. During the initial period after the light transition the only mean variables that show appreciable changes are the blackbody and air temperatures, the other scalars being little affected. We find that a distinct transient thermal internal boundary layer appears ≈ 20 m thick above the temperate deciduous forest and ≈ 45 m thick above the tropical rainforest. At each forest, the effective thickness of the inferred thermal outer-canopy ‘big leaf’ is about 1 mm. Twenty minutes after the abrupt change in incident light, ensemble eddy-flux estimates approach those found using conventional time averaging, confirming the validity of the ensemble approach. Previously unrecognized transient maxima in net ecosystem exchange and evapotranspiration are evident 5–10 min following the shadow-to-light transition, longer than the average light interval between shadows observed on partly-cloudy days in each case. Short-term variations in sensible heat flux, net ecosystem exchange, and evapotranspiration approximate an exponential adjustment, implying that first-order time-dependent single-leaf models are adequate to describe whole-canopy processes in these cases, providing an experimental method for determining whole-canopy bulk stomatal time constants. During the sunlit interval (direct and diffuse radiative fluxes combined) net ecosystem exchange is enhanced, while under cloud shadow (only diffuse radiative flux) water-use efficiency increases. This light and shadow alternation provides a mechanism describing the observed enhanced net ecosystem exchange and water-use efficiency under certain types of partly-cloudy sky. We apply empirical flux-response curves to an idealized case of radiative flux varying in a regular on–off light and shadow pattern. For this case, an analytical solution for mean net ecosystem exchange flux as a function of diffuse-radiation fraction yields results that strongly resemble previous findings based on conventional time-averaged fluxes. Our analysis and modelling indicates that a well-known correlation between diffuse radiative flux and enhanced net ecosystem exchange and water-use efficiency on cloudy days is in many cases not causal, but rather to be a consequence of time averaging over light-and-shadow intervals. By linking processes associated with photosynthesis in fluctuating light at the leaf scale to the canopy scale, our efforts facilitate the scaling-up of leaf responses to the ecosystem scale.

Surface Energy Budget Observed for Winter Wheat in the North China Plain During a Fog–Haze Event

In recent winters, fog–haze events have occurred frequently over the North China Plain. To understand the characteristics of conventional meteorological conditions, the near-surface radiation balance, and the surface energy budget under different pollution levels, we analyzed data collected at an observation site in Gucheng, which is located in the Hebei province in North China, based on a campaign that ran from December 1 2016 to January 31 2017. We found that meteorological conditions with a lower wind speed, weakly unstable (stable) stratification, higher relative humidity, and lower surface pressure during the daytime (night-time) are associated with fog–haze events. On heavy pollution days (defined as days with a daily mean PM2.5 concentration > 150 μg m−3), the decrease in downward shortwave radiation (S↓) and the increase in downward longwave radiation (L↓) are significant. The mean S↓ (L↓) values on clean-air days (daily mean PM2.5 concentration < 75 μg m−3) and heavily polluted days was 222 (222) W m−2 and 124 (265) W m−2, respectively. Due to the negative (positive) radiative forcing of aerosols during the daytime (night-time), the daily maximum (night-time mean) net radiation (Rn) is negatively (positively) related to the daily mean PM2.5 concentration, the correlation coefficient between the daily maximum (night-time mean) Rn and daily mean PM2.5 concentration being − 0.47 (0.51). Diurnal variations in sensible heat flux (H) and latent heat flux (λE) are insignificant on heavily polluted days, the mean daily maximum H (λE) is only 40 (28) W m−2 on heavily polluted days, but reaches 90 (42) W m−2 on clean-air days. Additionally, the friction velocity, standard deviation of vertical velocity, and turbulent kinetic energy on heavily polluted days are also quantified.

Variability of sensible heat flux over the Bay of Bengal and its connection to Indian Ocean Dipole events

The Bay of Bengal (BOB) is known to possess complex thermodynamics which show distinct seasonal patterns. Surface heat fluxes in the BOB are very much dependant on upper ocean heat exchanges and wind. Sensible heat flux (SHF) is also one among those fluxes that depends on air-sea temperature difference and wind. However, this study further proves that a strong relationship exists between barrier layer thickness (BLT) and SHF variability that has not been focussed on in earlier literatures. This study also investigates the seasonal as well as inter-annual variability of SHF and its relationship with BLT and sea surface temperature (SST) patterns in more detail with statistical analyses. It is found that both SST and BLT are responsible for the evolution of SHF signal in the BOB although their effects are spatially distributed. During the post monsoon period, freshwater induced enhanced BLT is more related to SHF than the summer time when effect of SST is found to be dominant. During Indian Ocean Dipole (IOD) years, the correlation between SHF and BLT in the eastern BOB is more pronounced compared to SHF and SST. The western BOB however is dominated by SST variations for the respective IOD phase which also contribute to SHF signals there. Northernmost BOB shows high standard deviation due to river discharge effects.

Seasonal variations in heat and carbon dioxide fluxes observed over a reed wetland in northeast China

Abstract Seasonal variations in sensible heat flux (Hs), latent heat flux (LE), and CO2 flux (Fc) during 2006 over a reed wetland ecosystem in Northeast China, as well as their relationships with environmental factors, were investigated based on micrometeorological observations and turbulence data, measured using the eddy covariance technique. The results showed that the LE values were significantly larger (>400 W m−2) in summer (June, July, and August) than those in other seasons because of the summertime abundant precipitation and strong evapotranspiration, whereas the Hs values were smaller ( 300 W m−2) and autumn (>200 W m−2). The cumulative evapotranspiration in 2006 was 577.1 mm that was mostly controlled by radiation at surface throughout the whole year but also limited by water supply during the non-growing season. Most of the Fc values ranged between −1.0 mg m−2 s−1 (sometimes close to −2.0 mg m−2 s−1) during daytime and 0.3 mg m−2 s−1 at night during the growing season (May to September) but varied around zero in the non-growing season, and the CO2 mass concentration was in the range of 600–800 mg m−3. Monthly cumulative CO2 flux for the growing season was negatively largest in July (−520 mg m−2 month−1) and smallest in May (−65 mg m−2 month−1), making this reed wetland a net CO2 sink in 2006. The daytime CO2 flux in the growing season was positively correlated with atmospheric stability |z/L| under unstable condition, photosynthetically active radiation (PAR), and wind speed, but depended less on air temperature, relative humidity and soil water content on a several-day time scale. However, over a longer time scale, a comparison of March–April conditions during 2005 and 2006 suggested that cooler conditions can result in reduced CO2 production before the growing season.

SENSIBLE HEAT TRANSFER ON ATMOSPHERIC-OCEANIC BOUNDARY IN THE OUTER AMBON BAY OF INDONESIA

Analysis of air-sea temperatures and sensible heat flux was conducted to investigate heat transfer process on the atmospheric-oceanic boundary for the outer Ambon Bay. The analysis used SST data derived from both satellite product and in situ measurement using linear regression method, as well as meteorological data such as air temperature and wind speed during daytime. The goals of the current work were to evaluate the relationship between SST and air temperature in the outer Ambon Bay, and to investigate the variation of sensible heat flux in association with seasonal variability of the bay. The major findings were: 1) SST was predominantly lower than airtemperature, resulting in the dominance of negative feedback process on the atmospheric-oceanic boundary layer of the bay; 2) the seasonal SST variability was influenced by land heating and upwelling in the Banda Sea; 3) land heating resulted in large gradient of air-sea temperatures, whereas cooler upwelled waters exerted an opposite effect.

Regional differences in the surface energy budget over China: an evaluation of a selection of CMIP5 models

The present study provides an evaluation of the regional differences over China in surface energy budget components as simulated by a selection of models from phase five of the Coupled Model Intercomparison Project (CMIP5), covering the period 1960–2005. Similarities and differences exist among the models in terms of both spatial and magnitude patterns. For climatology, the CMIP5 models show quite different spatial distributions of shortwave radiation and sensible heat flux. In terms of seasonal variation, the surface energy budgets are remarkably different between western and eastern China. The discrepancies in the seasonal variation of sensible heat flux are mainly attributable to temperature differences and wind speed, while those of shortwave radiation are caused by the seasonal variation in total cloud cover. Cloudiness is one of the most crucial parameters in estimating the surface energy budget. In addition, the study also reveals that the magnitudes of the various components show larger (more than two-fold) differences between western and eastern parts of China, especially in net longwave and upward shortwave radiation, as well as latent and sensible heat fluxes. The results for surface soil heat flux show that there is more incoming energy during spring and summer and more outgoing energy during fall and winter in both western and eastern China. Furthermore, compared to NCEP2 data, the ERA-40 reanalysis product produces results more similar to the multi-model ensemble mean for most components.

Temperature regimes and turbulent heat fluxes across a heterogeneous canopy in an Alaskan boreal forest

AbstractWe evaluate local differences in thermal regimes and turbulent heat fluxes across the heterogeneous canopy of a black spruce boreal forest on discontinuous permafrost in interior Alaska. The data were taken during an intensive observing period in the summer of 2013 from two micrometeorological towers 600 m apart in a central section of boreal forest, one in a denser canopy (DC) and the other in a sparser canopy, but under approximately similar atmospheric boundary layer (ABL) flow conditions. Results suggest that on average 34% of the half‐hourly periods in a day are nonstationary, primarily during night and during ABL transitions. Also, thermal regimes differ between the two towers; specifically between midnight and 0500 Alaska Standard Time (AKST) it is about 3°C warmer at DC. On average, the sensible heat flux at DC was greater. For midday periods, the difference between those fluxes exceeded 30% of the measured flux and over 30 W m−2 in magnitude more than 60% of the time. These differences are due to higher mechanical mixing as a result of the increased density of roughness elements at DC. Finally, the vertical distribution of turbulent heat fluxes verifies a maximum atop the canopy crown (2.6 h) when compared with the subcanopy (0.6 h) and above canopy (5.1 h), where h is the mean canopy height. We argue that these spatial and vertical variations of sensible heat fluxes result from the complex scale aggregation of energy fluxes over a heterogeneous canopy.

Heat flux variations over sea ice observed at the coastal area of the Sejong Station, Antarctica

This study presents variations of sensible heat flux and latent heat flux over sea ice observed in 2011 from the 10-m flux tower located at the coast of the Sejong Station on King George Island, Antarctica. A period from July to September was selected as a sea ice period based on daily record of sea state and hourly photos looking at the Marian Cove in front of the Sejong Station. For the sea ice period, mean sensible heat flux is about −11 Wm−2, latent heat flux is about +2 W m−2, net radiation is −12 W m−2, and residual energy is −3 W m−2 with clear diurnal variations. Estimated mean values of surface exchange coefficients for momentum, heat and moisture are 5.15 × 10−3, 1.19 × 10−3, and 1.87 × 10−3, respectively. The observed exchange coefficients of heat shows clear diurnal variations while those of momentum and moisture do not show diurnal variation. The parameterized exchange coefficients of heat and moisture produces heat fluxes which compare well with the observed diurnal variations of heat fluxes.

Warming and drying climate over Loess plateau area in China and its effect on land surface energy exchange

China's Loess plateau, as one of the areas in the world, is vulnerable to ecological environment and sensitive to climate change. The characteristic of land surface process has been changing in this area, and it affects the formation and evolution of regional weather and climate. However, limited by observational land surface data, exchange process variation of the surface energy of the Loess plateau and its mechanism of response to climate change is limited. In this study, we combine the observed experimental data with the land surface model simulated data, test the previous four land surface models. It shows the CLM is reliable in Loess plateau. Then we use the observed data and model simulated data to analyze the variation of regional climate and the characteristics of land surface energy exchange, and discuss the effect of climate change on land surface energy exchange. It shows that Loess plateau is warming and drying in recent decades. It induces the increase in solar radiation, surface reflected radiation and surface longwave upward radiation. This makes the net radiation decrease. Correspondingly, surface latent heat flux, sensible heat flux and soil heat flux all have a decreasing trend. But the distribution ratio of components of surface heat flux is basically unchanged. In addition, the annual variation of sensible heat flux is controlled by solar radiation, and latent heat flux is controlled by solar radiation and precipitation. The annual variation of each component of surface heat flux is sensitive to precipitation, not sensitive to temperature. The climate acidification affected surface energy balance farmore than temperature rise.

Estimation and numerical simulation of atmospheric surface layer parameters at Mangalore, west coast of India

AbstractAtmospheric surface layer meteorological observations obtained from 20 m height meteorological tower at Mangalore University campus, situated along the west coast of India, are used to estimate the surface layer scaling parameters of roughness length (zo) and drag coefficient (CD) and the surface layer fluxes of sensible heat and momentum. These parameters are computed using the simple flux‐profile relationships under the framework of Monin–Obukhov (M–O) similarity theory. The estimated values of zo are higher (1.35–1.54 m) than the reported values in the literature (&gt;0.4–0.9 m), probably due to the undulating topography surrounding the location. The magnitude of CD is high for low wind speed (&lt;1.5 m/s) and found to be in the range of 0.005–0.03. The variations of sensible heat fluxes (SHFs) and momentum fluxes (MFs) are also discussed. Relatively high fluxes of heat and momentum are observed during typical days on 26–27 February 2004 and 10–11 April 2004, due to the daytime unstable atmospheric conditions. Stable or near neutral conditions prevail after 1700 h IST with negative SHF. A mesoscale model PSU/NCAR MM5 is run using a high‐resolution (1 km) grid over the study region to examine the influence of complex topography on the surface layer parameters and the simulated fluxes are compared with the estimated values. Spatial variations of the friction velocity (u*), CD, surface fluxes, planetary boundary layer(PBL) height and surface winds are observed according to the topographic variations in the simulation. Copyright © 2011 Royal Meteorological Society

Comparisons of energy balance and evapotranspiration between flooded and aerobic rice fields in the Philippines

Abstract The seasonal and annual variability of sensible heat flux (H), latent heat flux (LE), evapotranspiration (ET), crop coefficient (Kc) and crop water productivity (WPET) were investigated under two different rice environments, flooded and aerobic soil conditions, using the eddy covariance (EC) technique during 2008–2009 cropping periods. Since we had only one EC system for monitoring two rice environments, we had to move the system from one location to the other every week. In total, we had to gap-fill an average of 50–60% of the missing weekly data as well as those values rejected by the quality control tests in each rice field in all four cropping seasons. Although the EC method provides a direct measurement of LE, which is the energy used for ET, we needed to correct the values of H and LE to close the energy balance using the Bowen ratio closure method before we used LE to estimate ET. On average, the energy balance closure before correction was 0.72 ± 0.06 and it increased to 0.99 ± 0.01 after correction. The G in both flooded and aerobic fields was very low. Likewise, the energy involved in miscellaneous processes such as photosynthesis, respiration and heat storage in the rice canopy was not taken into consideration. Average for four cropping seasons, flooded rice fields had 19% more LE than aerobic fields whereas aerobic rice fields had 45% more H than flooded fields. This resulted in a lower Bowen ratio in flooded fields (0.14 ± 0.03) than in aerobic fields (0.24 ± 0.01). For our study sites, evapotranspiration was primarily controlled by net radiation. The aerobic rice fields had lower growing season ET rates (3.81 ± 0.21 mm d−1) than the flooded rice fields (4.29 ± 0.23 mm d−1), most probably due to the absence of ponded water and lower leaf area index of aerobic rice. Likewise, the crop coefficient, Kc, of aerobic rice was significantly lower than that of flooded rice. For aerobic rice, Kc values were 0.95 ± 0.01 for the vegetative stage, 1.00 ± 0.01 for the reproductive stage, 0.97 ± 0.04 for the ripening stage and 0.88 ± 0.03 for the fallow period, whereas, for flooded rice, Kc values were 1.04 ± 0.04 for the vegetative stage, 1.11 ± 0.05 for the reproductive stage, 1.04 ± 0.05 for the ripening stage and 0.93 ± 0.06 for the fallow period. The average annual ET was 1301 mm for aerobic rice and 1440 mm for flooded rice. This corresponds to about 11% lower total evapotranspiration in aerobic fields than in flooded fields. However, the crop water productivity (WPET) of aerobic rice (0.42 ± 0.03 g grain kg−1 water) was significantly lower than that of flooded rice (1.26 ± 0.26 g grain kg−1 water) because the grain yields of aerobic rice were very low since they were subjected to water stress. The results of this investigation showed significant differences in energy balance and evapotranspiration between flooded and aerobic rice ecosystems. Aerobic rice is one of the promising water-saving technologies being developed to lower the water requirements of the rice crop to address the issues of water scarcity. This information should be taken into consideration in evaluating alternative water-saving technologies for environmentally sustainable rice production systems.

Estimation of atmospheric surface layer parameters and numerical simulation using MM5 at Mangalore, West Coast of India

Atmospheric surface layer meteorological observations obtained from 20-m-high meteorological tower at Mangalore, situated along the west coast of India are used to estimate the surface layer scaling parameters of roughness length (z o) and drag coefficient (C D), surface layer fluxes of sensible heat and momentum. These parameters are computed using the simple flux–profile relationships under the framework of Monin–Obukhov (M–O) similarity theory. The estimated values of z o are higher (1.35–1.54 m) than the values reported in the literature (>0.4–0.9 m) probably due to the undulating topography surrounding the location. The magnitude of C D is high for low wind speed (<1.5 m s−1) and found to be in the range 0.005–0.03. The variations of sensible heat fluxes (SHF) and momentum fluxes are also discussed. Relatively high fluxes of heat and momentum are observed during typical days on 26–27 February 2004 and 10–11 April 2004 due to the daytime unstable atmospheric conditions. Stable or near neutral conditions prevail after 1700 h IST with negative SHF. A mesoscale model PSU/NCAR MM5 is run using a high-resolution (1 km) grid over the study region to examine the influence of complex topography on the surface layer parameters and the simulated fluxes are compared with estimated values. Spatial variations of the frictional velocity (u *), C D, surface fluxes, planetary boundary layer (PBL) height and surface winds are noticed according to the topographic variations in the simulation.

An assessment of the quality of surface sensible heat flux derived from reanalysis data through comparison with station observations in Northwest China

The present study compares seasonal and interdecadal variations in surface sensible heat flux over Northwest China between station observations and ERA-40 and NCEP-NCAR reanalysis data for the period 1960–2000. While the seasonal variation in sensible heat flux is found to be consistent between station observations and the two reanalysis datasets, both land-air temperatures difference and surface wind speed show remarkable systematic differences. The sensible heat flux displays obvious interdecadal variability that is season-dependent. In the ERA-40 data, the sensible heat flux in spring, fall, and winter shows interdecadal variations that are similar to observations. In the NCEP-NCAR reanalysis data, sensible heat flux variations are inconsistent with and sometimes even opposite to observations. While surface wind speeds from the NCEP-NCAR reanalysis data show interdecadal changes consistent with station observations, variations in land-air temperature difference differ greatly from the observed dataset. In terms of land-air temperature difference and surface wind speed, almost no consistency with observations can be identified in the ERA-40 data, apart from the land-air temperature difference in fall and winter. These inconsistencies pose a major obstacle to the application in climate studies of surface sensible heat flux derived from reanalysis data.

Difference in the Interdecadal Variability of Spring and Summer Sensible Heat Fluxes over Northwest China

AbstractThe present study investigates the difference in interdecadal variability of the spring and summer sensible heat fluxes over Northwest China by using station observations from 1960 to 2000. It was found that the spring sensible heat flux over Northwest China was greater during the period from the late 1970s to the 1990s than during the period from the 1960s to the mid-1970s. The summer sensible heat flux was smaller in the late 1980s through the 1990s than it was in the 1970s through the early 1980s. Both the spring and summer land-air temperature differences over Northwest China displayed an obvious interdecadal increase in the late 1970s. Both the spring and summer surface wind speeds experienced an obvious interdecadal weakening in the late 1970s. The change in the surface wind speed played a more important role in the interdecadal variations in sensible heat flux during the summer, whereas the change in the land-air temperature difference was more important for the interdecadal variations in s...

Spatial variations of sensible heat flux over an urban area measured using Doppler lidar

AbstractThe terrain and morphology of an urban area have a major impact upon the distribution of sensible heat flux and consequently the energy balance of the boundary layer. The spatial distribution of sensible heat flux is difficult to measure, particularly in cities. In this article the potential of a scanning Doppler lidar to make these measurements is explored. The measurements of sensible heat flux are interpreted using a simple trajectory model and an energy balance model. The lidar does seem to offer a practical measurement technique, although the system employed in this study does not have sufficient resolution to provide detailed values over small areas. Copyright © 2008 Royal Meteorological Society