scholarly journals A deep cyclone of African origin over the Western Mediterranean: diagnosis and numerical simulation

2002 ◽  
Vol 20 (1) ◽  
pp. 93-106 ◽  
Author(s):  
V. Homar ◽  
C. Ramis ◽  
S. Alonso
Keyword(s):  
Heat Flux ◽  
Heat Release ◽  
Sensible Heat Flux ◽  
Western Mediterranean ◽  
Sensible Heat ◽  
Latent Heat Release ◽  
Synoptic Scale ◽  
Deep Cyclone ◽  
Surface Sensible Heat Flux

Abstract. From 19 to 22 December 1979, a deep cyclone evolved over the Western Mediterranean. Gusty winds of more than 30 m/s, as well as a strong pressure decrease to about 990 hPa were recorded in Palma de Mallorca (Balearic Islands, Spain). ECMWF analyses are used for a diagnosis and numerical study of the case. Sensitivity experiments using the HIRLAM model are performed to assess the role of the surface sensible heat flux, latent heat release and orography on the genesis and evolution of the cyclone. At synoptic scale, the situation is governed by the instability of an upper-level short wave. The cyclone developed within a notable baroclinic environment, which resulted from a cold advection from the northwest towards North Africa. The baroclinicity at first stages of the cyclogenesis is quantified by means of the Eady model. At latter stages, the evolution of the potential vorticity structures at high levels reveals a wide tropopause fold over the cyclone, as well as the presence of a strong anomaly associated with the low-level system. Sensitivity experiments reveal a notable cyclogenetic role of the latent heat release throughout the atmosphere in the deepening of the low, whereas no significant effect of the surface sensible heat flux is obtained for the simulation interval. On the other hand, an unusual cyclolytic role can be attributed to the northern ranges of the Mediterranean basin. Effectively, the low enlargement and deepening is constrained by a "wall effect", which is a consequence of the interaction of the cyclonic flow and those northern mountainous systems.Key words. Meteorology and atmospheric dynamics (synoptic-scale meteorology, Mesoscale meteorology)

scholarly journals Atmospheric Dynamics Triggered by an Oceanic SST Front in a Moist Quasigeostrophic Model

2012 ◽  
Vol 69 (5) ◽  
pp. 1617-1632 ◽  
Author(s):  
Bruno Deremble ◽  
Guillaume Lapeyre ◽  
Michael Ghil
Keyword(s):  
Heat Flux ◽  
Heat Release ◽  
Latent Heat ◽  
Sensible Heat Flux ◽  
Sensible Heat ◽  
Latent Heat Release ◽  
Sst Front ◽  
Moist Processes ◽  
Surface Sensible Heat Flux ◽  
Sst Fronts

Abstract To understand the atmospheric response to a midlatitude oceanic front, this paper uses a quasigeostrophic (QG) model with moist processes. A well-known, three-level QG model on the sphere has been modified to include such processes in an aquaplanet setting. Its response is analyzed in terms of the upper-level atmospheric jet for sea surface temperature (SST) fronts of different profiles and located at different latitudes. When the SST front is sufficiently strong, it tends to anchor the mean atmospheric jet, suggesting that the jet’s spatial location and pattern are mainly affected by the latitude of the SST front. Changes in the jet’s pattern are studied, focusing on surface sensible heat flux and on moisture effects through latent heat release. It is found that latent heat release due to moist processes is modified when the SST front is changed, and this is responsible for the meridional displacement of the jet. Moreover, both latent heat release and surface sensible heat flux contribute to the jet’s strengthening. These results highlight the role of SST fronts and moist processes in affecting the characteristics of the midlatitude jet stream and of its associated storm track, particularly their positions.

Mechanisms of Along-Valley Winds and Heat Exchange over Mountainous Terrain

2010 ◽  
Vol 67 (9) ◽  
pp. 3033-3047 ◽  
Author(s):  
Juerg Schmidli ◽  
Richard Rotunno
Keyword(s):  
Heat Flux ◽  
Heat Exchange ◽  
Sensible Heat Flux ◽  
Volume Effect ◽  
Turbulent Heat ◽  
Relative Role ◽  
Sensible Heat ◽  
Dynamic Surface ◽  
Surface Sensible Heat Flux

Abstract The physical mechanisms leading to the formation of diurnal along-valley winds are investigated over idealized three-dimensional topography. The topography used in this study consists of a valley with a horizontal floor enclosed by two isolated mountain ridges on a horizontal plain. A diagnostic equation for the along-valley pressure gradient is developed and used in combination with numerical model simulations to clarify the relative role of various forcing mechanisms such as the valley volume effect, subsidence heating, and surface sensible heat flux effects. The full diurnal cycle is simulated using comprehensive model physics including radiation transfer, land surface processes, and dynamic surface–atmosphere interactions. The authors find that the basic assumption of the valley volume argument of no heat exchange with the free atmosphere seldom holds. Typically, advective and turbulent heat transport reduce the heating of the valley during the day and the cooling of the valley during the night. In addition, dynamically induced valley–plain contrasts in the surface sensible heat flux can play an important role. Nevertheless, the present analysis confirms the importance of the valley volume effect for the formation of the diurnal along-valley winds but also clarifies the role of subsidence heating and the limitations of the valley volume effect argument. In summary, the analysis brings together different ideas of the valley wind into a unified picture.

A long-term Climatology of Planetary Boundary Layer Height over the Tibetan Plateau revealed by ERA5

2020 ◽  
Author(s):  
Nils Slättberg ◽  
Deliang Chen
Keyword(s):  
Heat Flux ◽  
Monsoon Season ◽  
Sensible Heat Flux ◽  
Dry Season ◽  
The Tibetan Plateau ◽  
Climate Variables ◽  
Sensible Heat ◽  
Boundary Layer Height ◽  
Surface Sensible Heat Flux

<p>The Planetary Boundary Layer Height (PBLH) is important for the exchange of energy, water, and momentum between the surface and the free atmosphere, making it a significant factor in studies of surface climate and atmospheric circulation. Over the Tibetan Plateau (TP) - a vast elevated heat source exerting significant influence on the Asian monsoon systems - the climate is changing rapidly. Among the many climate variables expected to change as global temperatures rise is the PBLH which, in addition to temperature profile, mechanical turbulence production, vertical velocity, and horizontal advection, is highly dependent on the surface sensible heat  fluxes. Our understanding of PBLH over the TP is very limited, although scattered estimates has indicated that it sometimes reach unusual heights – up to the vicinity of the tropopause. Long-term assessment of PBLH covering the whole TP is hampered by the fact that observations are scarce in time and space. This study takes advantage of a recently available high-resolution reanalysis (ERA5) for 1979-2018 to create a multi-decadal climatology of PBLH over the TP, and assess the seasonality, interannual variation and long-term trend of PBLH in relation to other climate variables such as tropopause height and surface sensible heat flux as well as large-scale atmospheric circulation. </p><p>The results show that the most prominent feature of the PBLH trend is a large region of decline in the central TP during the monsoon season. Notably, this is a region where the temperature increase is smaller than in the rest of the region, and the precipitation shows a statistically significant increasing trend. Increased cloudiness may therefore have decreased the surface heating and thus the sensible heat flux and PBLH. Assessing the spatially averaged trends for the first and second halves of the period separately reveals that the monsoon season PBLH does in fact increase during the first half of the period. In the dry season in contrast, the spatially averaged PBLH decreases by almost 30 meter per decade during the first half of the period and increases slightly in the second. Although none of the spatially averaged PBLH trends are statistically significant at the 95% level, it can be noted that the shift from decreasing to increasing PBLH for the dry season is in accordance with a recent study of spring sensible heat flux over the TP. The aforementioned study found that although the sensible heat flux has been declining because of wind speed decreases, it has recently started to recover in response to an increased difference between the ground surface temperature and the air temperature. Given that the PBLH is highly dependent on the surface sensible heat flux, this decline and recovery may very well have produced the PBLH trends for the dry season. In the monsoon season, with cloudy conditions and less solar radiation reaching the ground, other factors are likely of greater importance for the PBLH.</p>

A Comparison between Moist and Dry Tropical Cyclones: The Low Effectiveness of Surface Sensible Heat Flux in Storm Intensification

2021 ◽  
Author(s):  
Zhanhong Ma ◽  
Jianfang Fei
Keyword(s):  
Heat Flux ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Equivalent Potential Temperature ◽  
Sensible Heat ◽  
Budget Analysis ◽  
Equivalent Potential ◽  
Surface Sensible Heat Flux

AbstractRecent numerical modeling studies demonstrate that dry tropical cyclones can be stably sustained via supply of surface sensible heat flux. This raises questions of whether surface sensible heat flux (SHX) and latent heat flux (LHX) have the same effect on the intensity evolution of tropical cyclones. An estimation of equivalent potential temperature budget in the boundary layer shows that LHX leads to larger increase in equivalent potential temperature than SHX even when they possess the same magnitude. By formulating these two kinds of surface heat fluxes with the same mathematical framework, the simulated intensifications of moist and dry tropical cyclones are compared, with the former driven exclusively by LHX and the latter by SHX. Results show significantly larger intensification rates for the tropical cyclone driven by LHX than that by SHX, revealing low effectiveness of SHX in the intensification of tropical cyclones. The diabatic heating in the moist tropical cyclone occurs accompanying the convection, while it is merely pronounced near the surface in the dry tropical cyclone and is decoupled from the dry convection. A new surface pressure tendency equation is proposed, without incorporating implicit pressure tendency term on the right-hand side. The budget analysis indicates that the SHX is less effective than LHX in lowering surface central pressure and therefore in tropical cyclone intensification. A series of sensitivity experiments suggest that the threshold of energy input required for spinning up a tropical cyclone is lower in the form of LHX than that of SHX.

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