scholarly journals The open-ocean sensible heat flux and its significance for Arctic boundary layer mixing during early fall

2016 ◽  
Vol 16 (20) ◽  
pp. 13173-13184 ◽  
Author(s):  
Manisha Ganeshan ◽  
Dong L. Wu
Keyword(s):  
Heat Flux ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Surface Wind Speed ◽  
Open Ocean ◽  
The Other ◽  
Arctic Climate ◽  
The Arctic ◽  
Sensible Heat ◽  
Early Fall

Abstract. The increasing ice-free area during late summer has transformed the Arctic to a climate system with more dynamic boundary layer (BL) clouds and seasonal sea ice growth. The open-ocean sensible heat flux, a crucial mechanism of excessive ocean heat loss to the atmosphere during the fall freeze season, is speculated to play an important role in the recently observed cloud cover increase and BL instability. However, lack of observations and understanding of the resilience of the proposed mechanisms, especially in relation to meteorological and interannual variability, has left a poorly constrained BL parameterization scheme in Arctic climate models. In this study, we use multi-year Japanese cruise-ship observations from R/V Mirai over the open Arctic Ocean to characterize the surface sensible heat flux (SSHF) during early fall and investigate its contribution to BL turbulence. It is found that mixing by SSHF is favored during episodes of high surface wind speed and is also influenced by the prevailing cloud regime. The deepest BLs and maximum ocean–atmosphere temperature difference are observed during cold air advection (associated with the stratocumulus regime), yet, contrary to previous speculation, the efficiency of sensible heat exchange is low. On the other hand, the SSHF contributes significantly to BL mixing during the uplift (low pressure) followed by the highly stable (stratus) regime. Overall, it can explain  ∼  10 % of the open-ocean BL height variability, whereas cloud-driven (moisture and radiative) mechanisms appear to be the other dominant source of convective turbulence. Nevertheless, there is strong interannual variability in the relationship between the SSHF and the BL height which can be intensified by the changing occurrence of Arctic climate patterns, such as positive surface wind speed anomalies and more frequent conditions of uplift. This study highlights the need for comprehensive BL observations like the R/V Mirai for better understanding and predicting the dynamic nature of the Arctic climate.

scholarly journals The open ocean sensible heat flux and its significance for Arctic boundary layer mixing during early fall

2016 ◽  
Author(s):  
Manisha Ganeshan ◽  
Dong L. Wu
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Sensible Heat Flux ◽  
Open Ocean ◽  
Arctic Climate ◽  
The Arctic ◽  
Sensible Heat ◽  
Cloud Regime ◽  
Ocean Atmosphere ◽  
Early Fall

Abstract. The increasing ice-free area during late summer has transformed the Arctic to a climate system with more dynamic boundary layer clouds and seasonal sea ice growth. The open ocean sensible heat flux, a crucial mechanism of excessive ocean heat loss to the atmosphere during the fall freeze season, is speculated to play an important role in the recently observed cloud cover increase and boundary layer (BL) instability. However, lack of observations and understanding of the resilience of the proposed mechanisms, especially in relation to meteorological and interannual variability, has left a poorly constrained BL parameterization scheme in Arctic climate models. In this study, we use multi-year Japanese cruise ship observations from R/V Mirai over the open Arctic Ocean to characterize the surface sensible heat flux (SSHF) during early fall and investigate its contribution to BL turbulence. It is found that surface-generated turbulent mixing is favored during episodes of high wind speed, and is also influenced by the prevailing cloud regime. The maximum ocean-atmosphere temperature difference is observed during cold air advection (associated with the stratocumulus regime). Yet, contrary to previous speculation, the efficiency of turbulent heat exchange is low. The SSHF contribution to BL mixing is significant during the uplift (low-pressure) followed by the highly stable (stratus cloud) regime. Overall, the open ocean sensible heat flux can explain ~ 10 % of the BL height variability, whereas mechanisms such as cloud-driven turbulence appear to be dominant. Nevertheless, there is strong interannual variability in the strength of the ocean-atmosphere coupling. The changing occurrence of Arctic climate patterns, such as positive surface wind speed anomalies, can easily enhance the ocean's contribution to BL turbulence. This study highlights the need for comprehensive boundary layer observations such as the R/V Mirai for better understanding and predicting the dynamic nature of the Arctic climate.

scholarly journals A Study of Nocturnal Surface Wind Speed Overprediction by the WRF-ARW Model in Southeastern Texas

2013 ◽  
Vol 52 (12) ◽  
pp. 2638-2653 ◽  
Author(s):  
Fong Ngan ◽  
Hyuncheol Kim ◽  
Pius Lee ◽  
Khalid Al-Wali ◽  
Bright Dornblaser
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Wind Speed ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Sea Breeze ◽  
Surface Wind Speed ◽  
Sensible Heat ◽  
Pressure System ◽  
Arw Model

AbstractThe overprediction of surface wind speed during nighttime by the Advanced Research core of the Weather Research and Forecasting (WRF-ARW) model was investigated for a period of the Second Texas Air Quality Study (28 May–3 July 2006). In coastal regions of southeastern Texas, the model had a significant increase of wind speed biases on the surface in the evening throughout the period, especially between 4 and 12 June. The synoptic pattern was a high pressure system centered over the Louisiana–Mississippi area that was subjected to a weak easterly–southeasterly flow in the lower troposphere. The weather conditions favorable for sea-breeze development brought a southerly–southwesterly onshore flow to the near-surface levels. In comparison with measurements, the downward sensible heat flux was overpredicted at night, which resulted in a warm bias in surface temperature. For the vertical wind profile on days with an evening wind bias, sea-breeze-driven nocturnal low-level jets (southerly–southwesterly) were present at around 300 m while another wind maximum was observed at higher levels (around 1.5–2 km), which were associated with a high pressure system centered on southeastern states. The vertical gradient of wind speed in the lowest 150 m was smoother in the model than it was in the observations; this could be attributed to excessive downward mixing. Sensitivities using different land surface and PBL parameterizations showed that the model's overprediction of nocturnal wind was still present despite improvements in the predictions of surface temperature and sensible heat flux.

scholarly journals Estimation of the Latent Heat Flux over Irrigated Short Fescue Grass for Different Fetches

Atmosphere ◽  
2021 ◽  
Vol 12 (3) ◽  
pp. 322
Author(s):  
Francesc Castellví ◽  
Pedro Gavilán
Keyword(s):  
Heat Flux ◽  
Latent Heat ◽  
Sensible Heat Flux ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
The Other ◽  
Sensible Heat ◽  
Surface Renewal ◽  
Residual Method ◽  
Flux Footprints

Often in agrometeorology the instrumentation required to estimate turbulent surface fluxes must be installed at sites where fetch is not sufficient for a sector of wind directions. For different integrated flux-footprints (IFFP) thresholds and taking as a reference the half-hourly latent heat fluxes (LE) measured with a large weighing lysimeter (LELys), the eddy covariance (EC) method and two methods based on surface renewal (SR) analysis to estimate LE were tested over short fescue grass. One method combined SR with the flux-gradient (profile) relationship, SR-P method, and the other with the dissipation method, SR-D method. When LE was estimated using traces of air moisture, good performances were obtained using the EC and the SR-P methods for samples with IFFP higher than 85%. However, the closest LE estimates were obtained using the residual method. For IFFP higher than 50%, the residual method combined with the sensible heat flux estimates determined using the SR-P method performed close to LELys and using the SR-D method good estimates were obtained for accumulated LELys. To estimate the sensible heat flux, the SR-D method can be recommended for day-to-day use by farmers because it is friendly and affordable.

scholarly journals A Variational Method for Computation of Sensible Heat Flux over the Arctic Sea Ice

2009 ◽  
Vol 26 (4) ◽  
pp. 838-845 ◽  
Author(s):  
Zuohao Cao ◽  
Jianmin Ma
Keyword(s):  
Heat Flux ◽  
Variational Method ◽  
Sea Ice ◽  
Variational Approach ◽  
Sensible Heat Flux ◽  
Arctic Sea Ice ◽  
Eddy Correlation ◽  
The Arctic ◽  
Sensible Heat ◽  
Arctic Sea

Abstract In this study, a variational approach was employed to compute surface sensible heat flux over the Arctic sea ice. Because the variational approach is able to take into account information from the Monin–Obukhov similarity theory (MOST) as well as the observed meteorological information, it is expected to improve the pure MOST-based approach in computation of sensible heat flux. Verifications using the direct eddy-correlation measurements over the Arctic sea ice during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment period of 1997/98 show that the variational method yields good agreement between the computed and the measured sensible heat fluxes. The variational method is also shown to be more accurate than the traditional MOST method in the computation of sensible heat flux over the Arctic sea ice.

scholarly journals Evaluations on Profiles of the Eddy Diffusion Coefficients through Simulations of Super Typhoons in the Northwestern Pacific

2016 ◽  
Vol 2016 ◽  
pp. 1-14 ◽  
Author(s):  
Jimmy Chi Hung Fung ◽  
Guangze Gao
Keyword(s):  
Heat Flux ◽  
Diffusion Coefficients ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Eddy Diffusivity ◽  
Surface Wind Speed ◽  
Eddy Diffusion ◽  
Northwestern Pacific ◽  
Sea Level Pressure ◽  
Pbl Scheme

The modeling of the eddy diffusion coefficients (also known as eddy diffusivity) in the first-order turbulence closure schemes is important for the typhoon simulations, since the coefficients control the magnitude of the sensible heat flux and the latent heat flux, which are energy sources for the typhoon intensification. Profiles of the eddy diffusion coefficients in the YSU planetary boundary layer (PBL) scheme are evaluated in the advanced research WRF (ARW) system. Three versions of the YSU scheme (original, K025, and K200) are included in this study. The simulation results are compared with the observational data from track, center sea-level pressure (CSLP), and maximum surface wind speed (MWSP). Comparing with the original version, the K200 improves the averaged mean absolute errors (MAE) of track, CSLP, and MWSP by 6.0%, 3.7%, and 23.1%, respectively, while the K025 deteriorates the averaged MAEs of track, CSLP, and MWSP by 25.1%, 19.0%, and 95.0%, respectively. Our results suggest that the enlarged eddy diffusion coefficients may be more suitable for super typhoon simulations.

scholarly journals On Boundary Layer Separation in the Lee of Mesoscale Topography

2007 ◽  
Vol 64 (2) ◽  
pp. 401-420 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle ◽  
Shouping Wang ◽  
Ronald B. Smith
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Wind Speed ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Lower Troposphere ◽  
Boundary Layer Separation ◽  
Surface Wind Speed ◽  
Trapped Waves ◽  
Layer Separation

Abstract The onset of boundary layer separation (BLS) forced by gravity waves in the lee of mesoscale topography is investigated based on a series of numerical simulations and analytical formulations. It is demonstrated that BLS forced by trapped waves is governed by a normalized ratio of the vertical velocity maximum to the surface wind speed; other factors such as the mountain height, mountain slope, or the leeside speedup factor are less relevant. The onset of BLS is sensitive to the surface sensible heat flux—a positive heat flux tends to increase the surface wind speed through enhancing the vertical momentum mixing and accordingly inhibits the occurrence of BLS, and a negative heat flux does the opposite. The wave forcing required to cause BLS decreases with an increase of the aerodynamical roughness zo; a larger zo generates larger surface stress and weaker surface winds and therefore promotes BLS. In addition, BLS shows some sensitivity to the terrain geometry, which modulates the wave characteristics. For a wider ridge, a higher mountain is required to generate trapped waves with a wave amplitude comparable to that generated by a lower but narrower ridge. The stronger hydrostatic waves associated with the wider and higher ridge play only a minor role in the onset of BLS. It has been demonstrated that although hydrostatic waves generally do not directly induce BLS, undular bores may form associated with wave breaking in the lower troposphere, which in turn induce BLS. In addition, BLS could occur underneath undular jump heads or associate with trapped waves downstream of a jump head in the presence of a low-level inversion.

scholarly journals High-Latitude Contribution to Global Variability of Air–Sea Sensible Heat Flux

2012 ◽  
Vol 25 (10) ◽  
pp. 3515-3531 ◽  
Author(s):  
Xiangzhou Song ◽  
Lisan Yu
Keyword(s):  
Heat Flux ◽  
Air Temperature ◽  
High Latitude ◽  
Large Scale ◽  
Decadal Variability ◽  
Sensible Heat Flux ◽  
The Arctic ◽  
High Latitudes ◽  
Sensible Heat ◽  
Arctic Oscillation Index

Abstract The study examined global variability of air–sea sensible heat flux (SHF) from 1980 to 2009 and the large-scale atmospheric and ocean circulations that gave rise to this variability. The contribution of high-latitude wintertime SHF was identified, and the relative importance of the effect of the sea–air temperature difference versus the effect of wind on decadal SHF variability was analyzed using an empirical orthogonal function (EOF) approach. The study showed that global SHF anomalies are strongly modulated by SHF at high latitudes (poleward of 45°) during winter seasons. Decadal variability of global wintertime SHF can be reasonably represented by the sum of two leading EOF modes, namely, the boreal wintertime SHF in the northern oceans and the austral wintertime SHF in the southern oceans. The study also showed that global wintertime SHF is modulated by the prominent modes of the large-scale atmospheric circulation at high latitudes. The increase of global SHF in the 1990s is attributable to the strengthening of the Southern Hemisphere annular mode index, while the decrease of global SHF after 2000 is due primarily to the downward trend of the Arctic Oscillation index. This study identified the important effects of wind direction and speed on SHF variability. Changes in winds modify the sea–air temperature gradient by advecting cold and dry air from continents and by imposing changes in wind-driven oceanic processes that affect sea surface temperature (SST). The pattern of air temperature anomalies dominates over the pattern of SST anomalies and dictates the pattern of decadal SHF variability.

scholarly journals The effect of misleading surface temperature estimations on the sensible heat fluxes at a high Arctic site – the Arctic turbulence experiment 2006 on Svalbard (ARCTEX-2006)

2009 ◽  
Vol 9 (4) ◽  
pp. 16913-16939 ◽  
Author(s):  
J. Lüers ◽  
J. Bareiss
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Temperature Profile ◽  
Sensible Heat Flux ◽  
Transport Processes ◽  
The Arctic ◽  
Accurate Estimation ◽  
Physical Parameters ◽  
Turbulent Heat ◽  
Sensible Heat

Abstract. The observed rapid climate warming in the Arctic requires improvements in permafrost and carbon cycle monitoring, accomplished by setting up long-term observation sites with high-quality in-situ measurements of turbulent heat, water and carbon fluxes as well as soil physical parameters in an Arctic landscape. But accurate quantification and well adapted parameterizations of turbulent fluxes in polar environments presents fundamental problems in soil-snow-ice-vegetation-atmosphere interaction studies. One of these problems is the accurate estimation of the surface or aerodynamic temperature T(0) required to force most of the bulk aerodynamic formula currently used. Results from the Arctic-Turbulence-Experiment (ARCTEX-2006) performed on Svalbard during the winter/spring transition 2006 helped to better understand the physical exchange and transport processes of energy. The existence of an untypical temperature profile close to the surface in the Arctic spring at Svalbard could be proven to be one of the major issues hindering estimation of the appropriate surface temperature. Thus, it is essential to adjust the set-up of measurement systems carefully when applying flux-gradient methods that are commonly used to force atmosphere-ocean/land-ice models. The results of a comparison of different sensible heat-flux parameterizations with direct measurements indicate that only the use of a hydrodynamic three-layer temperature-profile model achieves enough accuracy for heat flux calculations as it reliably reproduces the temporal variability of the surface temperature.

scholarly journals Near-surface profiles of aerosol number concentration and temperature over the Arctic Ocean

2011 ◽  
Vol 4 (3) ◽  
pp. 3017-3053 ◽  
Author(s):  
A. Held ◽  
D. A. Orsini ◽  
P. Vaattovaara ◽  
M. Tjernström ◽  
C. Leck
Keyword(s):  
Heat Flux ◽  
Arctic Ocean ◽  
Particle Deposition ◽  
Deposition Velocity ◽  
Sensible Heat Flux ◽  
Number Concentration ◽  
The Arctic ◽  
Sensible Heat ◽  
Central Arctic Ocean ◽  
Near Surface

Abstract. Temperature and particle number concentration profiles were measured at small height intervals above open and frozen leads and snow surfaces in the central Arctic. The device used was a gradient pole designed to investigate potential particle sources over the central Arctic Ocean. The collected data was fitted according to basic logarithmic flux-profile relationships to calculate the sensible heat flux and particle deposition velocity. Independent measurements by the eddy covariance technique were conducted at the same location. General agreement was observed between the two methods when logarithmic profiles could be fitted to the gradient pole data. In general, snow surfaces behaved as weak particle sinks with a maximum deposition velocity vd = 1.3 mm s−1 measured with the gradient pole. The lead surface behaved as a weak particle source before freeze-up with an upward flux Fc = 5.7 × 104 particles m−2 s−1, and as a relatively strong heat source after freeze-up, with an upward maximum sensible heat flux H = 13.1 W m−2. Over the frozen lead, however, we were unable to resolve any significant aerosol profiles.

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