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Warm Air (warm + air)
Selected AbstractsForced depression of leaf hydraulic conductance in situ: effects on the leaf gas exchange of forest treesFUNCTIONAL ECOLOGY, Issue 4 2007T. J. BRODRIBB Summary 1Recent work on the hydraulic conductance of leaves suggests that maximum photosynthetic performance of a leaf is defined largely by its plumbing. Pursuing this idea, we tested how the diurnal course of gas exchange of trees in a dry tropical forest was affected by artificially depressing the hydraulic conductance of leaves (Kleaf). 2Individual leaves from four tropical tree species were exposed to a brief episode of forced evaporation by blowing warm air over leaves in situ. Despite humid soil and atmospheric conditions, this caused leaf water potential (,leaf) to fall sufficiently to induce a 50,74% drop in Kleaf. 3Two of the species sampled proved highly sensitive to artificially depressed Kleaf, leading to a marked and sustained decline in the instantaneous rate of CO2 uptake, stomatal conductance and transpiration. Leaves of these species showed a depression of hydraulic and photosynthetic capacity in response to the ,blow-dry' treatment similar to that observed when major veins in the leaf were severed. 4By contrast, the other two species sampled were relatively insensitive to Kleaf manipulation; photosynthetic rates were indistinguishable from control (untreated) leaves 4 h after treatment. These insensitive species demonstrate a linear decline of Kleaf with ,leaf, while Kleaf in the two sensitive species falls precipitously at a critical water deficit. 5We propose that a sigmoidal Kleaf vulnerability enables a high diurnal yield of CO2 at the cost of exposing leaves to the possibility of xylem cavitation. Linear Kleaf vulnerability leads to a relatively lower CO2 yield, while providing better protection against cavitation. [source] Spring northward retreat of Eurasian snow cover relevant to seasonal and interannual variations of atmospheric circulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2003Hiroaki Ueda Abstract An observational study is made of the seasonal and interannual variations of spring snow-disappearance over the Eurasian continent and the circulation mechanisms causing those variations. The spring northward retreat of the snow boundary over the East European Plain (EEP) between 30 and 60° E is faster (0.4° per day) than to the east of the Ural Mountain range (0.3° per day). These migrations of the snow boundary lag behind the appearance of the surface air temperature 0 °C by about 1 to 5 pentads. The analyses of the atmospheric heat and moisture budgets showed that the seasonal intrusion of warm air associated with southwesterly winds is primarily responsible for the rapid snowmelt in March and April over the EEP. In addition, the adiabatic heating of descending air plays a secondary role in the snowmelt in mid-March. On an interannual time scale, horizontal warm advection also plays an essential role in the spring northward retreat of snow cover extent. The present study confirms the previous finding that the surface air temperature anomalies, produced during the seasonal snow-disappearance period, diminished in May, suggesting a weak dynamical linkage between the EEP snow cover and Asian summer monsoon. Copyright © 2003 Royal Meteorological Society [source] Use of METEOSAT water-vapour images for the diagnosis of a vigorous stratospheric intrusion over the central MediterraneanMETEOROLOGICAL APPLICATIONS, Issue 3 2000K Lagouvardos The diagnosis of a vigorous dry intrusion over the central Mediterranean is performed using water-vapour images from METEOSAT. This dry intrusion was located on the rear side of a cold front (propagating from Italy to Greece) and played an important role in the onset of thunderstorms over the western Greek coasts. A combination of satellite imagery and potential vorticity analyses showed that the dry air originated in the lower-stratospheric and higher-tropospheric layers. The interaction of the dry air with the moist air masses within the warm conveyor belt ahead of the cold front (overrun of warm air by low equivalent potential temperature air) produced a potentially unstable region over the area of reported thunderstorms. Copyright © 2000 Royal Meteorological Society [source] Föhn as a response to changing upstream and downstream air massesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 635 2008Georg J. Mayr Abstract Observations of föhn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow föhn, when it passes through passes in the mountains, or deep föhn, when it overflows the Alpine crest. First, the föhn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the föhn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow föhn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross-barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the föhn layer had an even stronger cross-barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep föhn never developed. An examination of other well-documented MAP föhn cases confirmed the conclusion from the 30 October event that shallow and deep föhns , at least for the central Alps , are mostly a response to differences in air masses between the upstream and downstream side. A cross-barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side. Copyright © 2008 Royal Meteorological Society [source] The sting at the end of the tail: Damaging winds associated with extratropical cyclonesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004K. A. Browning Abstract Strong surface winds often accompany the low-level jets that occur along the cold fronts of extratropical cyclones, but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone. The most damaging extratropical cyclones go through an evolution that involves the formation of a bent-back front and cloud head separated from the main polar-front cloud band by a dry slot. When the cyclone attains its minimum central pressure, the trailing tip of the cloud head bounding the bent-back front forms a hook which goes on to encircle a seclusion of warm air. The most damaging winds occur near the tip of this hook,the sting at the end of the tail. Observations of the Great Storm of October 1987 in south-east England are re-examined in some detail to study this phenomenon. The cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations. Air within these circulations leaves the hooked tip of the cloud head (and enters the dry slot) much faster than the rate of travel of the cloud-head tip, implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds. The circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds. Regardless of how important this role may be, the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds. Copyright © 2004 Royal Meteorological Society [source] | ||||||||||