Low Pressure Disturbances Research Articles

A modeling study of an extreme rainfall event along the northern coast of Taiwan on 2 June 2017

Abstract. In this study, the extreme rainfall event on 2 June 2017 along the northern coast of Taiwan is studied from a modeling perspective. While a peak amount of 645 mm was observed, two 1 km experiments produced about 400 and 541 mm, respectively, using different initial and boundary conditions, and thus are compared to isolate the key reasons for a higher total amount in the second run. While the conditions in the frontal intensity and its slow movement are similar in both runs, the frontal rainband remains stationary for a long period in this second run due to a frontal disturbance that acts to enhance the prefrontal southwesterly flow and focuses its convergence with the postfrontal flow right across the coastline. Identified as the key difference, this low-pressure disturbance is supported by the observation, and without it in the first run, multiple slow-moving rainbands pass through the coastal region and produce more widely spread but less concentrated rainfall, resulting in the lower peak amount by comparison. To explore and test the effects of Taiwan's topography in this event, two additional 1 km runs are also used. It is found that the removal of the terrain in northern Taiwan allowed the postfrontal cold air to move more inland and the rainfall became less concentrated, in agreement with a recent study. Also, when the entire island topography of Taiwan is removed, the result showed significant differences. In this case, the blocking and deflecting effects on the prefrontal flow are absent, and the heavy rainfall in northern Taiwan does not occur.

Numerical studies of the current response at Ormen Lange to a travelling storm

Mean temporal circulation at Ormen Lange (OL) off mid-Norway is strongly dominated by the Norwegian Atlantic Current. Tidal effects are weak. Observed events of short peak value in current speed along with peak in temperature close to the seabed at the shelf slope may be driven by strong pressure gradients. That is, strong atmospheric low pressures and/or internal pressure fronts between warmer Atlantic Water (AW) and colder Norwegian Sea Water (NSW). Along the shelf slope at OL there may be steepening of the isopycnals, separating AW and NSW, due to wind generated downwelling and strong Ekman veering during storms or approaching internal density fronts. During such events the density surfaces tend to undershoot their equilibrium level, and as the forcing weakens, the suppressed water may run up along the shelf slope. In this run up phase, peak values in the velocities are often found.Since the atmospheric forcing may be a major forcing mechanism behind the observed events, we have performed a numerical study of the sensitivity of near bottom velocities at OL to the pathway of low pressure disturbances and the strength and the radii of them.The results indicate how depth dependent currents at different locations respond to varying atmospheric forcing, with highest currents close to the surface, time delay of response at greater depths and highest impact of storms passing further out in the Nordic Seas than immediately over OL.

First considerations on the structure and development of the Iberian thermal low-pressure system

Abstract. During the summer a thermal low-pressure system is locked over the Iberian Peninsula. We present a first analysis of such a system using the potential vorticity approach. Our results show that its main characteristic is the existence of a negative potential vorticity (PV) dome and a funnel-like structure for potential temperature, both located at the centre of the low. The build-up and evolution of this PV dome can be understood in terms of the dot products of the absolute vorticity and the gradient of diabetic heating vectors and the curl of the friction forces and the gradient of potential temperature vectors. The inhibition of the Algerian Mediterranean cyclogenesis during the summer seems to bear some relation to the existence of this kind of low-pressure disturbance over the Iberian Peninsula.

First considerations on the structure and development of the Iberian thermal low-pressure system

During the summer a thermal low-pressure system is locked over the Iberian Peninsula. We present a first analysis of such a system using the potential vorticity approach. Our results show that its main characteristic is the existence of a negative potential vorticity (PV) dome and a funnel-like structure for potential temperature, both located at the centre of the low. The build-up and evolution of this PV dome can be understood in terms of the dot products of the absolute vorticity and the gradient of diabetic heating vectors and the curl of the friction forces and the gradient of potential temperature vectors. The inhibition of the Algerian Mediterranean cyclogenesis during the summer seems to bear some relation to the existence of this kind of low-pressure disturbance over the Iberian Peninsula.

Heavy winter precipitation in southwest Arizona

During December 1992, according to the Weekly Climate Bulletin of the Climate Analysis Center in Washington, D.C., heavy precipitation inundated parts of Arizona causing more than 400% of normal precipitation to fall in the southwestern part of the state. Heavy precipitation continued to fall during the next 2 months, causing extensive flooding along the Gila River.Phoenix Weather Service Forecast Office monthly storm data reports indicated flooding along the Santa Cruz and San Pedro Rivers on December 29. From January 7 to 20, roads, bridges, homes, businesses, and farmland suffered considerable flood damage from Graham County westward to Yuma County as rivers and streams swelled. Several thousand people were isolated in their homes as flood waters cut off roads. The January storm data report shows that the combination of a northward‐displaced subtropical jet stream, with its abundant moisture supply and associated low pressure disturbances and a southward‐displaced polar jet stream, with its storm track, led to the abnormally wet period from late December to mid‐January. In February, severe flooding was reported in several areas as water rose in the Painted Rock Reservoir; water accumulating behind the dam produced the largest lake in the state. After exceeding the 2.5 million acre‐feet capacity of the reservoir, water began spilling over the dam and damaging homes, crops, farmland, roads, and bridges. About 3,500 residents were evacuated, and the National Guard responded to the flooding with various relief efforts including helicopter support operations. The U.S. and Arizona Departments of Agriculture reported flood damage in excess of $50 million.

Impact of an atmospheric-oceanic disturbance on phytoplankton community dynamics in the North Pacific Central Gyre

During the ADIOS I (Asian Dust Input to the Oceanic System) expedition to 26°N, 155°W in the North Pacific Central Gyre (NPCG) atmospheric-oceanic disturbances caused significant changes in phytoplankton community dynamics. A low pressure disturbance and several major dust deposition events resulted in a 37 and 25% increase in autotrophic carbon and nitrogen assimilation rates, respectively, relative to the pre-storm rates. The integrated submicron autotrophic N-assimilation rate increased by 72%. This submicron autotrophic community was responsible for 63 ± 7% of total autotrophic N-assimilation. New production increased by up to a factor of 4 as a result of the disturbances. The areal Chl a concentration doubled with the most significant increase observed near the base of the photic zone. Chemotaxonomic identification by HPLC pigment analyses indicated that the characteristic pigment concentrations of all major phytoplankton classes (except prokaryotic algae) increased as a result of the disturbances. Water column stability estimates and NO 3 − gradient measurements indicated that turbulent vertical transport of NO 3 − across the pycnocline could not explain the observed increases in photosynthetic parameters. Near surface NO 3 − concentrations increased following a dust deposition event. In addition, a positive temporal correlation was observed between the atmospheric Al concentration and the near-surface Chl a concentration. The data suggested that atmospheric deposition of NO 3 − and Fe stimulated autotrophic growth. Atmospheric deposition of NO 3 − and Fe may significantly enhance new production rates in the NPCG, especially during episodic dust events.

A relationship between planetary waves and persistent rain- and thunderstorms in China

A spectral study of long planetary waves indicates that major and persistent heavy rains and severe thunderstorms in China during the past 50 years were closely related to specific anomalous large-scale circulation patterns, such as blocking highs over East Asia. As the heavy rains of Meiyu occur, such blocking develops at high latitudes, in particular over the Sea of Okhotsk and over Lake Baikal. A southward shift of the westerlies allows upper troughs to pass over the Tibetan Plateau and low pressure disturbances, generated over the plateau area, successively move along the Yangzi River valley, causing prolonged and heavy rainfall. The establishment of an upper trough over the Bay of Bengal also provides an important condition for heavy rainfall in South China. In North and Northwest China, large-scale circulation patterns responsible for heavy rainfall events are characterized by the establishment and slow regression of a high-pressure system over the Seas of Japan and Okhotsk, or by the rapid amplification of long-wave ridges into blocking ridges near the position of seasonal-mean troughs. Owing to the blocking action of these ridges, the rain-bearing synoptic systems (typhoons, upper troughs and low-level vortices) often undergo sudden changes in their tracks or tend to slow down, becoming nearly stationary, thus enhancing and prolonging the heavy rainfall events. At the same time, a well-developed low-pressure zone in the tropics and subtropics helps to establish a strong, low-level easterly wind, which often assumes the intensity of a low-level jet. This flow configuration offers a major moisture supply for heavy rains. The study also reveals that ultralong planetary waves during heavy rainfall are more stationary than under climatological mean conditions. The departures from normal of their phase angles appear to be an important factor in generating heavy rain and severe thunderstorm episodes.

Electrostatic Field Changes Produced by Florida Lightning

The electrical behavior of thunderstorms triggered by local heating and sea-breeze convergence, a low pressure disturbance, and a weak frontal passage has been studied at the NASA Kennedy Space Center, Florida. A nonlinear least-squares minimization procedure has been developed to describe changes in the total electrostatic field produced by lightning in terms of point charge models for the cloud charge distributions. The results indicate that discharges to ground usually neutralize cloud charges in the range from -10 to -40 C. The computed charge altitudes for Florida are somewhat higher than for other geographical locations, 6 to 9.5 km, but the corresponding ambient air temperatures, -10 to -34 C, are similar. A large fraction of the discharges to ground show total field changes which are small or even reversed in polarity within 3 km of the discharges. It is suggested that ground discharges often neutralize a small positive charge, 0.5 to 4 C at altitudes of 1 to 3 km, in addition to the larger negative charge higher in the cloud.