Favorable Synoptic Conditions Research Articles

Синоптические условия образования гроз с выпадением ливневых осадков над территорией Западной Сибири

The article presents the study results of thunderstorm activity with rainfall over a large territory of Western Siberia in its three regions: Northern, Central and Southern over a seven-year period from 2017 to 2023. The study revealed the most favorable synoptic conditions for the formation of thunderstorm activity with heavy rainfall over a large area of Western Siberia. Cases are also reviewed and factors that need to be paid attention to when forecasting thunderstorms with squally winds are identified.

The Dynamics and Microphysical Characteristics of the Convection Producing the Record-Breaking Hourly Precipitation on 20 July 2021 in Zhengzhou, China

An hourly rainfall of 201.9 mm fell in Zhengzhou on 20 July 2021, breaking the hourly rainfall record of mainland China and causing severe urban flooding and human casualties. This observation-based study investigates the associated convective-scale and mesoscale dynamics and microphysical processes using disdrometer and polarimetric radar observations aided by retrievals from the Variational Doppler Radar Analysis System. The synoptic flow forcing brought abundant moisture from the oceans and converged at Zhengzhou; then, the extreme rainfall was produced by a slow-moving convective storm that persisted throughout the hour over Zhengzhou. Unusually high concentrations of raindrops of all sizes (showing combined properties of maritime and continental convection) are revealed by the disdrometer data, whereas the polarimetric radar data suggest that both ice-based and warm rain processes were important contributors to the total rainfall. High precipitation efficiency was achieved with an erect updraft at the low levels, whereas enhanced easterly inflows kept the storm moving slowly. The interaction between convective-scale and mesoscale dynamics and microphysical processes within the favorable synoptic conditions led to this extremely heavy rainfall.

On the High Winds in the Tianshan Grand Canyon in Northwest China: General Features, Synoptic Conditions, and Mesoscale Structures

This work studies the characteristics of high winds in the Tianshan Grand Canyon (TGC), Northwest China by using surface wind observations at six automatic weather stations during 2017–2018. Three high wind indices are examined, namely, high wind hour (HWH), station high wind event (SHWE) and regional high wind event (RHWE). SHWE denotes persistent high winds of more than 3 h at an individual station while RHWE is defined for the southern TGC as a whole. High winds are mainly northwesterly/westerly, occurring predominantly in southern TGC. HWHs most often occur in spring and summer, while winter and spring HWHs possess the strongest intensity. The occurrence of HWHs exhibits an apparent diurnal variation, most frequent in the afternoon and evening while least from mid-night to early morning. SHWEs and RHWEs are prone to take place in spring, possessing relatively long lifetime and strong intensity. According to composite analysis of the 0.25° FNL reanalysis, favorable synoptic conditions are obtained for the persistent high winds in spring. Of great importance is the mid-tropospheric trough in the mid-high latitudes located to the north (i.e., upstream) of the TGC. The cold advection behind the trough cools and stabilizes the mid-lower troposphere, while the post-trough sinking motion causes a surface high that accelerates the low-level northwesterly wind. The synoptic thermal and dynamical forcing help set an upstream flow of moderate Froude number such that the mesoscale orographic flow traversing the TGC is in the nonlinear high-drag regime. Large-amplitude gravity waves are excited, with the isentropes descending abruptly over the lee slope which resembles internal hydraulic jump. Downslope windstorms are produced at the expense of flow potential energy as the subcritical upstream flow is transitioned to supercritical at the TGC peak. These findings have important implications on the formation and prediction of high winds in the TGC.

A Synoptic Evolution Comparison of the Smallest and Largest MCSs in Subtropical South America between Spring and Summer

AbstractSubtropical South America (SSA) east of the Andes Mountains is a global hotspot for mesoscale convective systems (MCSs). Wide convective cores (WCCs) are typically embedded within mature MCSs, contribute over 40% of SSA’s warm-season rainfall, and are often associated with severe weather. Prior analysis of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data identified WCCs in SSA and associated synoptic conditions during austral summer. As WCCs also occur during the austral spring, this study uses the 16-year TRMM PR dataset and ERA5 reanalysis to compare anomalies in environmental conditions between austral spring (SON) and summer (DJF) for the largest and smallest WCCs in SSA. During both seasons, large WCCs are associated with an anomalous mid-level trough that slowly crosses the Andes Mountains and a northerly South American low-level jet (SALLJ) over SSA, though the SON trough and SALLJ anomalies are stronger and located farther northeastward than in DJF. A synoptic pattern evolution resembling large WCC environments is illustrated through a multi-day case during the RELAMPAGO field campaign (10-13 November 2018). Unique high-temporal resolution soundings showed strong mid-level vertical wind shear associated with this event, induced by the juxtaposition of the northerly SALLJ and southerly near-surface flow. It is hypothesized that the Andes help create a quasi-stationary trough/ridge pattern such that favorable synoptic conditions for deep convection persist for multiple days. For the smallest WCCs, anomalously weaker synoptic-scale forcing was present compared to the largest events, especially for DJF, pointing to future work exploring MCS formation under weaker synoptic conditions.

CHARACTERISTIC OF CONDITIONS OF HEAVY RAINS IN PERM REGION USING ATMOSPHERIC INSTABILITY INDICES

Currently, interest to alternative methods of the analysis and forecasting of dangerous meteorological phenomena has considerably increased due to the low density of the observational network, which is insufficient to provide all users with necessary information in a timely and qualitative way. A general analysis of precipitation characteristics for the period from 2004 to 2016 was conducted during the work. The favorable synoptic conditions for heavy rains precipitation have been identified; their dependence on the altitude was discovered. The aim of this research is to analyze the incidence of heavy rains in the Perm region and to assess instability indexes, which are calculated for specific cases of this phenomenon. For this goal, 5 indices (LI, VT, CN, TT, K) were selected. Their values were calculated in terms of heavy rains based on reanalysis data. It was found that the average settings, which were obtained during the calculation, are slightly lower than the values calculated for other regions and given in previously published researches of this field of science. It means that it is necessary to establish specific indices criteria for the Perm region. Developing of convection and heavy rain in Perm region can be expected when the values of instability indices are near to the calculated values, at which at least 90 % of the total number of cases of the studied meteorological phenomenon occurred. A larger sample should be used to calculate the instability indices for analysis and forecast of heavy rains in practice.

The comparison of statistical features and synoptic circulations between the eastward-propagating and quasi-stationary MCSs during the warm season around the second-step terrain along the middle reaches of the Yangtze River

Mesoscale convective systems (MCSs) around the second-step terrain (106°–113°E, 28°–35°N), along the middle reaches of the Yangtze River, were detected, tracked and classified using a black body temperature (TBB) dataset during May to August 2000–2016 (except 2005). The MCSs were divided into eastward-propagating (EP) and quasi-stationary (QS) types, to compare their spatial and temporal distributions and convective intensities, and to identify the favorable synoptic conditions for the formation and evolution of EP MCSs. The results showed that both MCS types occurred most often in July. The EP MCSs were mainly initiated over the eastern regions of the study area, while the QS type mainly originated in the western regions of the study area. Both MCS types mainly formed in the afternoon, but a second peak occurred in the early morning for QS MCSs. The EP MCSs had a larger cloud area at their mature stage and a lower cloud brightness temperature, indicating more intense convection. Additionally, the longer lifetime and further eastward propagation of the EP MCSs meant that they had a great influence on the precipitation over the middle and lower reaches of the Yangtze River. Synoptic circulation analysis demonstrated that the combination of the mid-level low trough east of the Tibetan Plateau (TP), and the western pacific subtropical high (WPSH), favored the formation and eastward propagation of EP MCSs. The positive vertical relative vorticity and stronger vertical wind shear provided dynamic conditions favorable for convective organization and development. Furthermore, a stronger low level jet imported warm and moist air to the eastern edge of, and the regions east of, the second-step terrain. The substantial convergence of water vapor promoted the development and long-lived maintenance of the EP MCSs.

Remote Thermodynamic Impact of the Kuroshio Current on a Developing Tropical Cyclone Over the Western North Pacific in Boreal Fall

AbstractTo examine the remote linkage between tropical cyclone (TC) development and moisture transport from the Kuroshio Current toward the TC, we conducted sensitivity experiments that modified the surface latent heat flux (LHF) over the Kuroshio, applying a cloud‐resolving regional model and Lagrangian diagnostics to Typhoon Chaba as it approached Japan in October 2010. The synoptic environment was characterized by a combination of an anticyclone of continental origin around Japan and the northward‐migrating TC, bringing about low‐level northeasterlies over the Kuroshio through an enhanced meridional pressure gradient. When the LHF over the Kuroshio was artificially reduced or removed under this environmental flow, the intensity of the TC, which was fully away from the domain in which the LHF is modified, appreciably attenuated during its developing stage, implying the importance of moisture influx due to the environmental flow in TC development. Since dry air of midlatitude origin could receive little of the moisture supply from the Kuroshio in the sensitivity experiments, the values of equivalent potential temperature within the inflow layer tend to decrease, thus facilitating the weakening of convective updrafts and downdrafts around the eyewall of the TC. It is suggested that such changes act as an inhibitory factor for TC development. The Kuroshio has the potential to remotely influence the TC intensity over the western North Pacific if favorable synoptic conditions are satisfied.

Impact of synoptic patterns and meteorological elements on the wintertime haze in the Beijing-Tianjin-Hebei region, China from 2013 to 2017

Meteorological conditions play a key role in formation of air pollution, determining dispersion or accumulation of air pollutants. Aggressive emission mitigation measures have been taken recently in the Beijing-Tianjin-Hebei region (BTH), China, but pervasive and persistent haze still frequently engulfs this region during wintertime. Occurrence frequency of unfavorable meteorological conditions in winter is anticipated to constitute a significantly important factor in driving the heavy haze formation in BTH. Large scale synoptic patterns influencing BTH during the wintertime from 2013 to 2017 are categorized into six types, including “north-low”, “southwest-trough”, “southeast-high”, “southeast-trough”, “transition”, and “inland-high” using the NCEP reanalysis data. “Southwest-trough” and “southeast-high” are defined as favorable synoptic patterns and the remaining four categories are unfavorable ones based on FLEXPART simulations. Compared to measurements of fine particulate matter (PM2.5) in BTH, favorable synoptic conditions generally correspond to the low level or decreasing trend of PM2.5 concentrations while under unfavorable conditions PM2.5 concentrations are high or increasing. Occurrence of wintertime haze episodes in BTH correlates well with the evolution trend of unfavorable synoptic patterns from 2013 to 2017 although the anthropogenic emissions have substantially decreased. PM2.5 concentrations also exhibit correlations with local meteorological elements, including winds, temperature, and relative humidity, which are ultimately steered by large scale synoptic situations. The WRF-Chem model simulations further reveal the critical role of large-scale synoptic patterns in the heavy haze formation. Overall, under unfavorable synoptic situations, emission mitigation is the best choice to improve the air quality in BTH.

The Climatology of Low‐Level Jet in Beijing and Guangzhou, China

AbstractThe important roles of low‐level jet (LLJ) in transport of heat, moisture, and pollutants have long been recognized. However, partly due to the lack of long‐term observations, the characteristics of LLJs in Beijing and Guangzhou have not been well understood. Using long‐term wind profiler observations and numerical simulations, the basic climatological characteristics of LLJ in Beijing and Guangzhou were documented, and their relationships with large‐ and local‐scale forcings were investigated. The occurrence frequencies of LLJ in both megacities approximately exhibit a bimodal distribution during an annual cycle. In terms of the seasonality, the LLJs in Beijing occur more often in spring and winter, and those in Guangzhou appear more frequently from October to December and from February to April. Pronounced diurnal variations of LLJ are found in Beijing, where most LLJs occurred during the nighttime, and the diurnal variations of LLJ in Guangzhou are less prominent. Favorable synoptic conditions for LLJ formations in Beijing are those having a southwest‐northeast or southeast‐northwest pressure gradient across Beijing. Dominant synoptic patterns associated with LLJs in Guangzhou are those having a low‐pressure or high‐pressure system to the northwest. Overall, the LLJs are primarily induced by synoptic forcings and modulated by the diurnal evolution of planetary boundary layer. The key findings from this study provide us a better understanding of the LLJs in these highly populated regions.

How much does weather control fire size and intensity in the Mediterranean region?

Abstract. This study investigates the synoptic conditions favorable to wildfires in the Mediterranean region, in terms of fire intensity and burnt area. As reported in the literature, Mediterranean large wildfires are associated with a blocking situation. However, this study shows the existence of two types of wildfires controlled by the blocking high intensity: (1) fast build-up of a weak blocking produces intense wildfires associated with strong winds which allow propagation over long distances; (2) longer build-up of strong blocking situation produces less intense wildfires associated with weaker winds which also propagate over long distances. Another major step forward of this study in the understanding of the drivers of those wildfires is the evidence of a perfect match between the period of wildfire activity and the persistence of the favorable synoptic conditions: the wildfire activity starts at the onset of the blocking situation and ends with the transition to a less favorable synoptic weather pattern. Such strong control of the wildfire activity by the concomitant weather is a very promising result regarding fire risk management, especially considering the accidental nature of the Mediterranean wildfires.

The relationship between African easterly waves and daily rainfall over West Africa: observations and regional climate simulations

The relationship between summer African easterly waves (AEWs) and daily rainfall is assessed in West Africa for 1998–2008 using various reanalyses, satellite-derived rainfall products, and a regional climate model (RCM) run at 90- and 30-km resolutions. 3–5 and 6–9 day AEWs are extracted by filtering daily 700 hPa meridional wind time series at 1°W and 11.5°N, and 1°W and 17.5°N, respectively. Both observed and simulated rainfall anomalies are of larger magnitude over West Africa during 3–5-d than 6–9-d AEWs. The RCM simulates larger rainfall rates in phase with the 3–5-d wave trough instead of ahead, unlike the observations, and overestimates the intensity and spatial coverage of rainfall associated with 6–9-d AEWs. The observed and simulated co-variability between 3–5-d (6–9-d) AEW activity and daily rainfall is strong (weak) and mostly located south (north) of 15°N. However, the RCM overestimates the spatial coverage of the AEW–rainfall relationship in the longitudinal (latitudinal) direction in the case of 3–5-d (6–9-d) AEWs. Observed and simulated daily intense rainfall events, extracted using a percentile threshold approach, are mostly located south of 15°N during summer. The observed relationship between their frequency of occurrence and active 3–5-d AEWs is maximal west of 8°E, while extends up to southern Chad in both RCM simulations. Their magnitude is also largely overestimated by the RCM, indicating an exaggerated coupling between the wave activity and the convection. Finally, observed and simulated 3–5-d AEWs establish the most favorable synoptic conditions for the development of intense rainfall events over West Africa.

Analysis of a heavy rainfall event over Beijing during 21–22 July 2012 based on high resolution model analyses and forecasts

The heaviest rainfall over 61 yr hit Beijing during 21–22 July 2012. Characterized by great rainfall amount and intensity, wide range, and high impact, this record-breaking heavy rainfall caused dozens of deaths and extensive damage. Despite favorable synoptic conditions, operational forecasts underestimated the precipitation amount and were late at predicting the rainfall start time. To gain a better understanding of the performance of mesoscale models, verification of high-resolution forecasts and analyses from the WRF-based BJ-RUCv2.0 model with a horizontal grid spacing of 3 km is carried out. The results show that water vapor is very rich and a quasi-linear precipitation system produces a rather concentrated rain area. Moreover, model forecasts are first verified statistically using equitable threat score and BIAS score. The BJ-RUCv2.0 forecasts under-predict the rainfall with southwestward displacement error and time delay of the extreme precipitation. Further quantitative analysis based on the contiguous rain area method indicates that major errors for total precipitation (> 5 mm h−1) are due to inaccurate precipitation location and pattern, while forecast errors for heavy rainfall (> 20 mm h−1) mainly come from precipitation intensity. Finally, the possible causes for the poor model performance are discussed through diagnosing large-scale circulation and physical parameters (water vapor flux and instability conditions) of the BJ-RUCv2.0 model output.

Estimating Aerosol Optical Depth Over the Broader Greek Area from MODIS Satellite

A first-to-date comprehensive climatological aspect of the regime of aerosol loading over the entire broader Greek region (19°E–30°E and 34°N–43°N) is obtained using pixel-level (50 km × 50 km) monthly aerosol optical depth (AOD) products derived from raw Level-2 (10 km × 10 km) MODIS-Terra AOD data at 550 nm. The AOD climatology (for the 8-year period from March 2000 to February 2008) indicates that the study region is significantly loaded by aerosols (mean regional AOD equal to 0.196 ± 0.030 on an annual basis, with values ranging from 0.126 up to 0.382 at pixel level). It is also found that our high-resolution AOD data can reveal spatial patterns that are not evident in studies based on lower resolution data. There is a distinct longitudinal gradient of AODs, with higher values in the eastern than western part of the Greek peninsula. There is also a strong latitudinal gradient with decreasing values from south to north attributed to the presence of the Sahara Desert in northern Africa. The annual AOD cycle presents double maximum values, in spring and summer, and minimum ones in winter. There is also a remarkable year-by-year variability of AOD levels, especially as to their maxima, influenced by varying transport of desert dust, from the south, or biomass burning aerosols, from the north, to the region under prevailing favorable synoptic conditions. In this work, the processing, consisting in averaging over space and time, is done applying/testing five different criteria varying in their flexibility/severity in both spatial and temporal data availability. The criteria selection affects the magnitude of computed regional mean AOD value modifying it by up to 19 %, although the patterns of geographical distribution of AOD and its intra-annual variability do not change drastically.

Northern Adriatic meteorological tsunamis: Observations, link to the atmosphere, and predictability

A total of 16 events of tsunami‐like sea level oscillations are documented in the northern Adriatic between 1955 and 2010. These oscillations, recorded at the long‐term operating Rovinj tide gauge, are characterized by wave heights of up to 60 cm, periods of 20 to 150 min, and duration of 1 to 48 h. The sea level oscillations are found to be coincident with pronounced atmospheric pressure disturbances characterized by a 2–4 hPa air pressure change over 10 min. Convective activity is recognized as the most likely source of atmospheric pressure disturbances. Analysis of propagation speed and direction of the atmospheric pressure disturbances indicates that the sea level oscillations were generated and enhanced via the Proudman resonance over a wide and shallow northern Adriatic shelf. Typical conditions under which pronounced air pressure disturbances occur include an air pressure surface minimum centered over the northern Adriatic, a temperature front at a height of approximately 850 hPa, and a strong southwesterly jet stream with wind speeds reaching 20–30 m/s at a height of approximately 500 hPa over the northern Adriatic. Based on these parameters, a possibility for forecasting tsunami‐like sea level oscillations from synoptic conditions is discussed. It appears that under favorable synoptic conditions sea level oscillations are more likely to occur than to not. However, no reliable conclusion on strength of an event can be reached from synoptic conditions only.

Analysis of the convective activity and its synoptic background over Croatia

In this study, deep convective activity, identified by lightning measurements and associated favorable synoptic conditions, is analyzed. The focus was on the warm part of the year (April to October) during the period 2006–2009 over six sub-regions in Croatia. A convectively active day is defined as a day with at least ten lightning strikes over the target sub-region. The obtained dataset is used to determine the climatology of the frequency and regional distribution of convective days and their inter-month variability. The analysis shows that 56% of all examined days, i.e., every second day during the warm part of the year, are days with convective activity. The most convectively active sub-region is the North Adriatic, with 62.4% of all days in the analyzed sample. The areas eastward of 16°E exhibit a peak in convective activity in June, contrary to the more western sub-regions, which show a maximum later in the summer, from July to August. The average temporal characteristics, such as typical duration, onset and cessation of convection, are also estimated. In all sub-regions, the convective activity begins in the early afternoon and ends mostly in the evening. Nocturnal convection occurs more frequently along the Adriatic coast.In the second part of this study, the dominant large-scale weather types and upper-level flow regimes, corresponding to the convectively active days, were determined using surface and upper-levels pressure fields. The lightning flashes are frequently detected in the non-gradient pressure field (23%); in the center (18%), in the leading (15%) and in the rear (12%) parts of the cyclone; and in the front part of a trough (11%). The southwesterly upper-level flow represents the most common flow regime (38%) in the days with convective activity. Slightly less is the northeasterly flow, which occurs with a frequency of approximately 23%, and the northwesterly flow, with a frequency of 18% of all selected days.

Thermally Driven Flows at an Asymmetric Valley Exit: Observations and Model Studies at the Lech Valley Exit

Abstract The summertime thermal circulation in the region of an asymmetric valley exit is investigated by means of observations and high-resolution model simulations. The northeastward-oriented Alpine Lech Valley opening into the Bavarian Alpine foreland has an eastern slope exceeding the western slope by about 15 km. Northerly winds along the eastern slope are frequently observed, reaching substantial strength during fair weather conditions. A field experiment has been conducted to explore this phenomenon and to pinpoint the connection of the northeasterly flow to the Lech Valley wind circulation. Numerical simulations have also been carried out to support the interpretation of the observations. It is found that the northerlies owe their existence to the dominantly easterly flow along the foothills of the Alps, which is partly induced by the Alpine heat low but may be strengthened by favorable synoptic conditions. Examples for both situations will be discussed. The diurnal flow in the Lech Valley has little obvious impact on these northeasterlies. On days with moderate synoptic easterly flow, a wake is present on the lee of the eastern slope of the exit region, accompanied by a shear zone along the edge of the wake. This shear zone is forced southward during the daytime because of thermally initiated pressure gradients between the Alpine foreland and the Alps, leading to sudden wind changes in the exit area at the time of its passage.

Propagation and Diurnal Evolution of Warm Season Cloudiness in the Australian and Maritime Continent Region

Abstract Warm season cold cloud-top climatology in the Austral–Indonesian region is examined for evidence of propagating modes of precipitation that originate from elevated heat sources and the diurnal heating cycle. Using satellite-inferred cloudiness from the period 1996–2001 as a proxy for rainfall, this coherent regeneration process and subsequent event propagation is found to consistently occur from the midlatitudes (30°–40°S) to the tropics (10°–20°S) in the Austral region. Given favorable environmental shear at midlatitudes, long-lived eastward-propagating events are observed to occur regularly with a span and duration typically larger than observed by Carbone et al. The genesis of these events, while intermittent, is directly related to elevated heat sources and the diurnal cycle, similar to the United States. However, given the relatively flat terrain of Australia, an elevated heat source is often insufficient, thus increasing the relative influence of transient synoptic forcing. In the tropics, the thermal forcing associated with elevated terrain found over the islands of the Maritime Continent and the land–sea interface is increasingly dominant on daily basis. While eastward- and westward-propagating events are found in the more varied environment of the monsoon regime, evidence for meridionally propagating modes is also found. In this manner, complex interactions occur that modify the location and timing of clouds that develop over neighboring oceanic and continental locations. The impact of convection initially linked to the New Guinea highlands and subsequently impacting the Java Sea region is particularly evident affecting the observed diurnal cycle. The subtropics show characteristics intermediate between the above extremes. With the seasonal cycle, the spring environment favors eastward-propagating events but in summer there is an increasing frequency of diurnally forced quasi-stationary development over elevated terrain enhanced by favorable synoptic conditions. Overall the subtropical summer events have a shorter duration and span than their spring counterparts. The increased environmental steering winds and shear in spring are thought to be the primary reason.

A Numerical Study on the Effects of Taiwan Topography on a Convective Line during the Mei-Yu Season

Abstract During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m2 s−2. Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast–southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s−1. In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and cold rain explicit microphysics realistically reproduced the evolution of the convective line and the associated weather with many fine details. Two low-level convergence zones were found to be crucial in the development of this convective line and the subsequent rainfall distribution over Taiwan. The first was along the mei-yu front and forced mainly by the front, but was terrain enhanced off the northwestern coast of Taiwan due to the blocking of air on the windward side of the Central Mountain Range (CMR). After formation, convective cells along this zone propagated southeastward and produced rainfall over the northwestern coast. As the front moved closer to Taiwan, a second arc-shaped convergence zone with a nearly north–south orientation along about 120°E formed ahead of the front between the prevailing flow and near-surface offshore flow induced by the blocking. This second zone was terrain induced, and convection initiated near its northern end was found to be responsible for the rainfall maximum observed near the coast of central Taiwan. Its intensity and position were highly sensitive to terrain height. In the HTRN run where the terrain was reduced by half, a weaker zone closer to the CMR (by about 50 km) was produced, and the rain fell mostly over the windward slope of the terrain instead of over the coastal plain. When the terrain was removed in the NTRN run, no such zone with the correct orientation formed. It was also found that the frontal movement near northern Taiwan was slightly delayed with the presence of terrain, and this affected the timing and distribution of local rainfall during the later stages of this event.

Diurnal Circulation of the Bolivian Altiplano. Part I: Observations

In July and August 2003 a field campaign was conducted to explore the diurnal circulation of the Bolivian Altiplano. Vertical soundings by remote-controlled aircraft yielded profiles of temperature, pressure, and humidity at six passes and in a valley. Pilot balloon observations provided wind profiles. Two permanent stations collected additional data. Typically, inflow toward the Altiplano commences a few hours after sunrise at about the time when the stable nocturnal layer near the ground is transformed by the solar heating into an almost neutrally stratified convective boundary layer. The depth of the inflow layer is comparable to but normally less than that of this boundary layer. There are indications of return flow aloft. The inflow continues at least until sunset. Moisture is imported at the passes leading to the Yungas in the east. Strong upvalley flows were found in the valley of the Rio de La Paz, which connects the wide canyon of La Paz with the tropical lowlands to the east. Inflow was absent at one of the passes despite favorable synoptic conditions. Cases of synoptically forced flows are presented as well where the diurnal signal is difficult to separate. A simple flow scheme is presented that fits the observations reasonably well.

A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic. Part I: Cyclone Structure and Evolution

General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.58 resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland‐Iceland region is higher than previously thought, and some of the events can be extremely violent. Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle. For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a ‘‘parent cyclone’’ with appreciable strength exists to the north/northeast of the incipient system.