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Convectively generated cold pools are important to the Earth system as they exert strong controls on deep convective-storm initiation, intensity, and life cycle. Despite their importance, efforts to introduce such cold pool controls into weather and climate models lack guidance and/or physical constraints from cold pool observations. This work presents a detailed, purely observational analysis of a cold pool event that took place on 23–24 May 2011 in north-central Oklahoma. The characteristics of the cold pool, and the spatiotemporal evolution of the hydrometeors and dynamics in the proximity of the cold pool, are studied with high-resolution observations. The unprecedented dataset used in this work to study cold pool characteristics includes an enhanced network of surface weather stations, a high-temporal-frequency sounding array, and the NEXRAD and Atmospheric Radiation Measurement (ARM) Southern Great Plains radar networks. The potential use of NEXRAD surveillance scans to estimate height and propagation speed of the leading edge of the cold pool (LECP) is presented in this work. Manual identification and tracking of the LECP from NEXRAD imagery shows a spatial and temporal heterogeneity of the LECP properties. Surprisingly, over its detected life cycle, the LECP speed remains almost constant, even though the strength of the cold pool diminishes in time and its height varies. Radar analysis shows that pulses of graupel and hail within downdrafts in the convective system generating the cold pool appeared to be related to temporary increases in the LECP height.

When precipitation evaporates in a sub-saturated boundary layer, it cools the air and produces dense downdrafts, which flow towards the surface and can spread horizontally as a gust front. These spreading “cold pools” (CPs) can trigger convection and thus new precipitation events due to dynamical and thermodynamical lifting mechanisms. Due to their role in the local organization of convection, CP properties are currently being studied with the use of high-resolution numerical simulations. Measurement campaigns have been conducted over the ocean to validate the models. However, fewer studies have specifically targeted cold pools over land.We use the observational network of the Netherlands (meteorological stations and radar) to study CPs developing from summer convection and their role in triggering new convective events over land. Detailed information about CP gust fronts in terms of temperature, wind speed, heat fluxes, moisture and pressure at high vertical resolution is obtained from time series, measured at the 213-meter Cabauw tower. We aim to create an algorithm that detects the passage of a CP from the tower time series to automatize the finding of CPs from a point measurement. To confirm the results, we have access to temperature time series from a spatially dense crowdsourcing weather station network (WOW-NL).The properties of the detected CPs are further studied with imagery from the Herwijnen Doppler radar, situated in proximity to the Cabauw tower. We can see clear signatures of spreading CPs in reflectivity plots, probably caused by the upwelling of dust and insects in the gust front. We currently explore how this can serve as a direct way of visualizing the dynamics of CPs and their collisions.With enough observations of CPs, we expect to learn more about the CP spreading velocity and lifetime in dependence of precipitation intensity of the generating precipitation cell and eventually triggered cell. This link will help gain more insight into the role of CPs in organizing convection over land.

Simulations of two leading-line, trailing-stratiform mesoscale convective system (MCS) events that occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E) have been used to understand the relative microphysical impacts of lower- versus midtropospheric aerosol particles (APs) on MCS precipitation. For each MCS event, four simulations were conducted in which the initial vertical location and concentrations of cloud droplet nucleating APs were varied. These simulations were used to determine the precipitation response to AP vertical location. Importantly, the total integrated number and mass of the initial aerosol profiles used in the sensitivity simulations remained constant, such that differences in the simulations could be directly attributable to changes in the vertical location of cloud droplet nucleating APs. These simulations demonstrate that lower-tropospheric APs largely influenced the precipitation response directly rearward of the leading cold pool boundary. However, farther rearward in the MCS, the relative impact of lower- versus midtropospheric APs largely depended on the MCS structure, which varied between the two events because of differences in line-normal wind shear. Midtropospheric APs were able to activate new cloud droplets in the midtropospheric levels of convective updrafts and to enhance mixed-phase precipitation through increased cloud riming, and this microphysical pathway had a more significant impact on mixed-phase precipitation in weaker line-normal wind shear conditions. This result exposes the importance of properly representing midtropospheric APs when assessing aerosol effects on clouds. This study also demonstrates the utility of assessing aerosol effects within the different regions of MCSs.

Cold pools produced by convective storms play an important role in Earth's climate system. However, a common framework does not exist for objectively identifying convective cold pools in observations and models. The present study investigates convective cold pools within a simulation of tropical continental convection that uses a cloud-resolving model with a coupled land-surface model. Multiple variables are assessed for their potential in identifying convective cold pool boundaries, and a novel technique is developed and tested for identifying and tracking cold pools in numerical model simulations. This algorithm is based on surface rainfall rates and radial gradients in the density potential temperature field. The algorithm successfully identifies near-surface cold pool boundaries and is able to distinguish between connected cold pools. Once cold pools have been identified and tracked, composites of cold pool evolution are then constructed, and average cold pool properties are investigated. Wet patches are found to develop within the centers of cold pools where the ground has been soaked with rainwater. These wet patches help to maintain cool surface temperatures and reduce cold pool dissipation, which has implications for the development of subsequent convection. This article is protected by copyright. All rights reserved.

During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer monsoon current carries the upwelled cold waters into the Bay of Bengal and forms the ‘cold pool of the Bay of Bengal’, with its core south of Sri Lanka and over the south-central Bay of Bengal. The present study focuses on the intrusion of these cold waters into the south of the Bay of Bengal, its interannual variability, and its association with the surface wind during the break phase of the summer monsoon, when strong westerly surface winds flow south of 10°N. The authors hypothesize that the enhanced cooling in the cold pool region during monsoon spells is associated with the strong westerly wind stress there during the break spells of the monsoon. Seven cases of long break monsoon spells that occurred during the nine years from 2001 to 2009 are analyzed, and the results confirm our hypothesis.

Two cases of dryline convection initiation (CI) over north Texas have been simulated (3 April 2012 and 15 May 2013) from a 50-member WRF-DART ensemble adjustment Kalman filter (EAKF) ensemble. In this study, ensemble sensitivity analysis (ESA) is applied to a convective forecast metric, maximum composite reflectivity (referred to as the response function), as a simple proxy for CI to analyze dynamic mesoscale sensitivities at the surface and aloft. Analysis reveals positional and magnitude sensitivities related to the strength and placement of important dynamic features. Convection initiation is sensitive to the evolving temperature and dewpoint fields upstream of the forecast response region in the near-CI time frame (0–12 h), prior to initiation. The sensitivity to thermodynamics is also manifest in the magnitude of dewpoint gradients along the dryline that triggers the convection. ESA additionally highlights the importance of antecedent precipitation and cold pool generation that modifies the pre-CI environment. Aloft, sensitivity of CI to a weak short-wave trough and capping inversion-level temperature is coherent, consistent, and traceable through the entire forecast period. Notwithstanding the (often) non-Gaussian distribution of ensemble member forecasts of convection, which violate the underpinnings of ESA theory, ESA is demonstrated to sufficiently identify regions that influence dryline CI. These results indicate an application of ESA for severe storm forecasting at operational centers and forecast offices as well as other mesoscale forecasting applications.

In this study, an extreme rainfall-producing quasi-stationary mesoscale convective system (MCS) associated with the Changma front in southeastern South Korea is investigated using numerical simulations and sensitivity tests. A record-breaking rainfall amount was recorded in response to repeated initiation of new cells (i.e., back-building) over the same area for several hours. The aim of this study is to realistically simulate and analyze this extreme rainfall event to better understand an impact of the cold pool that leads to the quasi-stationary MCS over southeastern South Korea by using a convection-allowing-resolution (2 km) nonhydrostatic atmospheric model.The control experiment (CNTL) was successfully performed, yielding the quasi-stationary, back-building MCS at approximately the correct location and time. In the CNTL run, diabatic cooling due to evaporation of raindrops was responsible for the formation of the cold pool. The development of the cold pool was responsible for the deceleration of the propagating convective line, which played a role in the stalling of the MCS over southeastern South Korea. Moreover, new convective cells were repeatedly initiated in the region where an oncoming warm inflow met the leading edge of the cold pool and was uplifted. In an experiment without evaporative cooling (NOEVA), the simulated precipitation pattern was shifted to the northeast because the MCS became nonstationary without the cold pool. The cold pool had an essential role in the stationarity of the MCS, which resulted in extreme rainfall over the Busan metropolitan area.

Observational and numerical investigations have been conducted to explain the formation of a long, quasi-stationary convection band (CB) that occurred over the Yellow Sea between August 1 and 2, 2008, in association with a synoptic-scale trough extending across the sea along the northwestern edge of the western Pacific subtropical high. The observations show that a line of weak echoes begins to appear over the Yellow Sea after 2030 UTC on August 1 and develops into a well-defined CB at approximately 0000 UTC on August 2, 2008. The life of the CB is divided into three stages: 1) initiation, 2) development, and 3) maturity. The initiation of the CB proceeds slowly and simultaneously along a long line over a period of approximately 2 h. The mature CB remains at nearly the same location until approximately 0300 UTC and is 10–20 km wide and greater than 300 km long. Both the observations and a numerical simulation show that the CB is initiated in an area of prevailing southwesterly surface winds to the south of an existing convection line, which is found to occur along or near a surface wind shift line. A cold pool or associated outflow is not observed at or near the existing convection line or the CB. The numerical simulation indicates that the convergence line along which the CB forms develops at the southern edge of a low-level convergence layer within a confluent flow associated with the extended trough. The formation of this convergence line appears to be related mainly to the southeastward movement of the low-level trough and convective circulation associated with an existing linear convective system.

A pool of relatively cooler water, called here as the southern Bay of Bengal cold pool, exists around Sri Lanka and southern tip of India during the summer monsoon. This cold pool is enveloped by the larger Indian Ocean warm pool and is believed to affect the intraseasonal variations of summer monsoon rainfall. In this study, we have investigated the mechanisms responsible for the formation of the cold pool using a combination of both satellite data sets and a general circulation model of the Indian Ocean. Sea surface temperature (SST) within the cold pool, after the steady increase during the February-April period, decreases first during a pre-monsoon spell in April and then with the monsoon onset during May. The onset cooling is stronger (similar to 1.8 ) than the pre-monsoon cooling (similar to 0.8 ) and culminates in the formation of the cold pool. Analysis of the model temperature equation shows that SST decrease during both events is primarily due to a decrease in incoming solar radiation and an increase in latent heat loss. These changes in the net heat flux are brought about by the arrival of cloud bands above the cold pool during both periods. During the pre-monsoon period, a cloud band originates in the western equatorial Indian Ocean and subsequently arrives above the cold pool. Similarly, during the monsoon onset, a band of clouds originating in the eastern equatorial Indian Ocean comes over the cold pool region. A lead-lag correlation calculation between daily SST and rainfall anomalies suggest that cooling in SST occurs in response to rainfall events with a lag of 5 days. These sequence of events occur every year with certain amount of interannual variability.

Convective cold pool events in (semi) arid areas have significant impacts on their environment. They reach horizontal extents of up to several hundred kilometers and the associated turbulence and shear can cause dust emissions and threaten aviation safety. Furthermore, cold pools play a major role in the organization of deep convection and in horizontal moisture transport. They have even been proposed to have impacts on larger-scale monsoon dynamics. Cold pools are not well represented in models using a convective parameterization. To test and improve these models, it is necessary to reliably detect cold pool occurrence from standard observational data. Former studies, however, focused on single cases or short time periods. Here, an objective and automated method for the generation of multiyear climatologies of cold-pool events is presented. The algorithm combines standard surface observations with satellite microwave data. Representativeness of stations and influence of their spatial density are addressed by comparison to a satellite-only climatology. Applying this algorithm to data from automatic weather stations and manned synoptic stations in and south of the Atlas Mountains in Morocco and Algeria reveals the frequent occurrence of cold pool events in this region. On the order of six cold-pool events per month are detected from May to September when the Saharan heat low is in its northernmost position. The events tend to cluster into several-days-long convectively active periods, often with strong events on consecutive days. The algorithm is flexible enough to be applied in comparable regions around the world.

The Galapagos is a flourishing yet fragile ecosystem whose health is particularly sensitive to regional and global climate variations. The distribution of several species, including the Galapagos Penguin, is intimately tied to upwelling of cold, nutrient–rich water along the western shores of the archipelago. Here we show, using reliable, high–resolution sea surface temperature observations, that the Galapagos cold pool has been intensifying and expanding northward since 1982. The linear cooling trend of 0.8°C per 33 years is likely the result of long–term changes in equatorial ocean circulation previously identified. Moreover, the northward expansion of the cold pool is dynamically consistent with a slackening of the cross–equatorial component of the regional trade winds—leading to an equatorward shift of the mean position of the Equatorial Undercurrent. The implied change in strength and distribution of upwelling has important implications for ongoing and future conservation measures in the Galapagos.

Employing reanalysis datasets, several threshold temperatures at 850 hPa are used to measure the wintertime [December–February (DJF)] areal extent of the lower-tropospheric, Northern Hemisphere, cold-air pool over the past 66 cold seasons. The analysis indicates a systematic contraction of the cold pool at each of the threshold temperatures. Special emphasis is placed on analysis of the trends in the extent of the −5°C air. Composite differences in lower-tropospheric temperature, midtropospheric geopotential height, and tropopause level jet anomalies between the five coldest and five warmest years are considered. Cold years are characterized by an equatorward expansion of the jet in the Pacific and Atlantic sectors of the hemisphere and by invigorated cold-air production in high-latitude Eurasia and North America. Systematic poleward encroachment of the −5°C isotherm in the exit regions of the storm tracks accounts for nearly 50% of the observed contraction of the hemispheric wintertime cold pool since 1948. It is suggested that this trend is linked to displacement of the storm tracks associated with global warming. Correlation analyses suggest that the interannual variability of the areal extent of the 850-hPa cold pool is unrelated to variations in hemispheric snow cover, the Arctic Oscillation, or the phase and intensity of ENSO. A modest statistical connection with the East Asian winter monsoon, however, does appear to exist. Importantly, there is no evidence that a resurgent trend in cold Northern Hemisphere winters is ongoing. In fact, the winter of 2013/14, though desperately cold in North America, was the warmest ever observed in the 66-yr time series.

Numerical weather prediction (NWP) systems operational at many national centers are nowadays used at the kilometer scale. The next generation of NWP models will provide forecasts at the subkilometer scale. Large impacts are expected in mountainous terrain characterized by highly variable orography. This study investigates the ability of the Canadian NWP system to provide an accurate forecast of near-surface variables at the subkilometer scale in the Canadian Rocky Mountains in wintertime when the region is fully covered by snow. Observations collected at valley and high-altitude stations are used to evaluate forecast accuracy at three different grid spacing (2.5, 1, and 0.25 km) over a period of 15 days. Decreasing grid spacing was found to improve temperature forecasts at high-altitude stations because of better orography representation. In contrast, no improvement is obtained at valley stations due to an inability of the model to fully capture at all resolutions the intensity of valley cold pools forming during nighttime. Errors in relative humidity reveal that the model tends to overestimate relative humidity at all resolutions, without improvement with decreasing grid spacing. Wind speed forecasts show large improvements with decreasing grid spacing for high-altitude stations exposed to or sheltered from wind. However, no systematic improvement with decreasing grid spacing is found for all stations, which is similar to previous studies. In addition, the model’s sensitivity at subkilometer grid spacing is investigated by evaluating the effects of (i) accounting for additional drag generated by subgrid orographic features, (ii) considering slope angle and aspect on surface radiation, and (iii) using high-resolution initialization for the surface fields.

This paper reviews the results of recent observations in the Yeonsuri valley of Mt. Youngmun during springtime (March to May) in 2012. Automated weather stations were installed at twelve sites in the valley to measure temperature and 2, 3 dimensional wind. We examined temporal and spatial characteristics of temperatures and wind data. The Yeonsuri valley springtime average temperature lapse rate between the top and bottom of the entire period is -0.44oC/100 m. It can be changed by the synoptic weather conditions, the lapse rates is greatest in order of clear days (-0.48oC/100 m), rainy (-0.41oC/100 m) and cloudy days (-0.40oC/100 m). In the night, the temperature inversion layer (thermal belt) and the cold pool are formed within the valley. In addition, we measured temperature and wind distribution from the bottom to 3.5 m, the cold layers existed up to 1.5 m, which were affected by ground mixed layer. The results will provide useful guidance on agricultural practices as well as model simulations.

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) model are performed to examine the impact of a thermally driven mountain–plains solenoid (MPS) on the diurnal variation of warm-season precipitation over northern China. The focus of the analyses is a 15-day simulation that uses the 8-day average of the NCEP GFS gridded analyses at 00:00 UT between 17 and 24 June 2004 for the initial conditions and the 8-day averages at 00:00, 06:00, 12:00, and 18:00 UT for the lateral boundary conditions. Despite differences in rainfall intensity and location, the control experiment captures the essence of the observed diurnal variation of warm-season precipitation in northern China. Consistent with observations, the simulated local precipitation peak initiates in the afternoon on the eastern edge and the immediate lee of the mountain ranges due to the upward branch of the MPS. The peak subsequently propagates downslope and southeastward along the steering-level mean flow, reaching the central North China Plain around midnight and early morning hours, resulting in a broad area of nocturnal precipitation maxima over the central plains. Sensitivity experiments show that, besides the impact of the MPS, cold pool dynamics play an essential role in the propagation and maintenance of the precipitation peak over the plains.

Observations from the recent COLd-air Pooling EXperiment (COLPEX) field campaign are used to relate the intensity of cold pools within two small valleys (O(100 m) depth and O(1 km) width) to flow conditions within the airmass passing over the valley. The temperature structure of the cold pools is examined using transects of ground-based sensors through the valleys and radiosonde releases from within the valley, in order to evaluate simple two-dimensional concepts of cold-pool formation due to sheltering. The cold-pool spatial and temporal structure is found to be consistent between different nights when strong cold pools occurred. Initially a sharp surface inversion occurs similarly at all points within the valley, with isentropes roughly terrain-parallel. The inversion layer at the bottom of the valley then grows deeper, spreading over the bottom portion of the valley, and cross-valley isentropes eventually become horizontal. By defining appropriate measures of the relative temperature drop within the cold pool, and non-dimensional valley depth diagnosed with respect to the speed and stability of the approaching flow, the measurements are shown to be consistent with previous relationships determined for simple two-dimensional valleys.

Saharan/Sahelian haboobs are potentially a very large source of atmospheric dust from the African continent. They are usually produced by convective systems that develop into MCSs and propagate westwards along the southern side of the ITD during the summer monsoon season. They are capable of reaching sizes far in excess of haboobs seen elsewhere in the world. Haboobs are partly responsible for various health and social impacts on the local human populace while dust deflated by them has the potential to impact on the global climate and the biogeochemical systems. In order to fully understand the behaviour of Saharan haboobs it is necessary to explore various different areas of meteorological science, including: (1) understanding West African meteorology and climatology, for example by increasing the number of observations in the Sahel and Sahara; (2) understanding the dynamics and thermodynamics of gravity currents and haboobs; (3) understanding the microphysics of convective clouds and how this affects downdraught and cold pool production and (4) making improvements to NWP models such as new parameterizations and modelling at finer resolutions.

This study investigates a long-duration mesoscale system with extremely heavy rainfall over southwest Taiwan during the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). This mesoscale convective system develops offshore and stays quasi-stationary over the upstream ocean and southwest coast of Taiwan. New convection keeps developing upstream offshore but decays or dies after moving into the island, dropping the heaviest rain over the upstream ocean and coastal regions. Warm, moist, unstable conditions and a low-level jet (LLJ) are found only over the upstream ocean, while the island of Taiwan is under the control of a weak cold pool. The LLJ is lifted upward at the boundary between the cold pool and LLJ. Most convective clusters supporting the long-lived rainy mesoscale system are initiated and develop along that boundary. The initiation and maintenance is thought to be a “back-building–quasi-stationary” process. The cold pool forms from previous persistent precipitation with a temperature depression of 2°–4°C in the lowest 500 m, while the high terrain in Taiwan is thought to trap the cold pool from spreading or moving. As a result, the orography of Taiwan is “extended” to the upstream ocean and plays an indirect effect on the long-duration mesoscale system.

On 22–23 June 2003 a broad region of convection, comprising a squall line and discrete supercells, evolved throughout the evening and nighttime hours. These storms were studied by analyzing both observations and idealized numerical simulations. The squall line originated from a group of supercells that were initiated in a north–south line along a preexisting outflow boundary in eastern Nebraska. These supercells anchored to the boundary, facilitating cell mergers. The cell mergers and subsequent enhanced rainfall increased the depth and strength of the surface cold pool, which became the forcing mechanism for reorientation into an east–west, southward-propagating squall line. While this was occurring, isolated supercells developed along the dryline in north-central Kansas. These supercells moved northeastward throughout the night. These two modes of convection developed and evolved in a similar nocturnal environment, suggesting that there were two distinct forcing mechanisms responsible for their maintenance. The nocturnal boundary layer was stable; however, CAPE existed and convection would have been able to remain surface based so long as near-surface air was being lifted to its level of free convection (LFC). In this study, both cold pool dynamics and supercell dynamics appear to have played an important roll in lifting low-level air to its LFC throughout the night.

Through convection-permitting simulations, this study examines a large bowing structure within a squall line that occurred during the rainy season in South China. The bowing structure is closely associated with a local enhancement of (and balance between) the cold pool and the line-normal environmental low-level vertical shear. Rear inflow plays an essential role in the formation and evolution of this large bowing structure. It is found that the low-level rear inflow is largely a natural consequence of the baroclinically generated horizontal vorticity near the surface, while the midtropospheric rear inflow is forced by several pairs of bookend vortices. Vorticity budget and vortex-line analyses show that the bookend vortices form mainly through the tilting of horizontal vorticity. Consolidation of these pairs of bookend vortices forms a broad zone of contiguous rear inflow. The environmental flow and horizontal pressure gradient force associated with the midlevel pressure deficit induced by the rearward-tilting buoyant updrafts, on the other hand, are not primarily responsible for the formation of the rear inflow.

Idealized three-dimensional numerical simulations are used to investigate the influence of sea breezes on the evolution of multicell thunderstorms in the Tropics. The study is motivated by a desire to understand a particular type of severe storm system that occurs occasionally over the ‘Top End’ region of northern Australia, but the results should have wider applicability. The calculations are carried out using Bryan’s cloud model. The simulations reproduce the main features of the evolution of an observed storm system. New cells develop on the gust front of the initial updraught, behind the sea-breeze front, and subsequently merge to form a multicell thunderstorm. The propagation speed and direction, the orientation, and length of the line of updraught cells are all similar to those observed. The sea breezes in the model play an important role in the evolution of the storm. In general, the low-level convergence at the gust front produced by the initial cell needs to be strong enough and to persist for a sufficiently long time to allow new cells to develop near this front. A strong cold pool occurs if the initial updraught is sufficiently tilted so that the downdraught does not fall into it, but supplies cold air close to the gust front. A generalized form of the Rotunno–Klemp–Weisman criterion was used to test whether the new updraughts developed at locations where the cold pool circulation was largely opposed by that of the environmental shear. The comparison of the shear components normal to the gust front showed that new cells develop even though this criterion is not fulfilled, suggesting that the criterion is not applicable to the more complex flow configurations studied here. Copyright c

Valley cold pools (VCPs), which are trapped, cold layers of air at the bottoms of basins or valleys, pose a significant problem for forecasters because they can lead to several forms of difficult-to-forecast and hazardous weather such as fog, freezing rain, or poor air quality. Numerical models have historically failed to routinely provide accurate guidance on the formation and demise of VCPs, making the forecast problem more challenging. In some case studies of persistent wintertime VCPs, there is a connection between the movement of upper-level waves and the timing of VCP formation and decay. Herein, a 3-yr climatology of persistent wintertime VCPs for five valleys and basins in the western United States is performed to see how often VCP formation and decay coincides with synoptic-scale (∼200–2000 km) wave motions. Valley cold pools are found to form most frequently as an upper-level ridge approaches the western United States and in response to strong midlevel warming. The VCPs usually last as long as the ridge is over the area and usually only end when a trough, and its associated midlevel cooling, move over the western United States. In fact, VCP strength appears to be almost entirely dictated by midlevel temperature changes, which suggests large-scale forcing is dominant for this type of VCP most of the time.

The main object of the presented study was the creation of a high-resolution monthly temperature climatology for the greater Alpine region (GAR). This climatology, which is determined from observational averages for the period 1961-1990, necessitated a multinational, high-quality temperature dataset, in which especially inhomogeneities due to different methods of means estimation had to be regarded. Based on multilinear regression techniques and regionalisation, significant model improvements could be reached by adjusting for mesoscale effects in cold air pools, coastal and lakeshore belts, urban areas and slopes. The final 1 x 1 km grids allowing temperature description of the orographically complex Alpine terrain with an accuracy of I degrees C have been made available for further applications at the web pages of the Central Institute for Meteorology and Geodynamics.

Supplementary material included in separate file. This article was originally published in International Journal of Climatology and is copyrighted by the Royal Meteorological Society

Serial upstream-propagating mesoscale convective system (MCS) events over southeastern South America are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation. However, the mechanisms of upstream propagation for these events have not been explored. To remedy this situation, a numerical simulation of the composite environmental conditions from 10 observed serial MCS events is conducted. Results indicate that the 3-day simulation from the composite yields a reasonable evolution of the large-scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations. Upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves. However, as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation, the cold pool and its interaction with the environmental low-level flow also begins to play a role in convective evolution. While the internal gravity waves and cold pool interact over a several hour period to control the convective development, the cold pool eventually dominates and determines the propagation of the convective region by the end of the simulation. This upstream propagation of a South American convective region resembles the southward burst convective events described over the United States and highlights the complex interactions and feedbacks that challenge accurate forecasts of convective system evolution.

A 2.5-layer thermodynamic ocean model has been used to understand the processes associated with the formation of cold pool off southern tip of India. The mean model simulations from the 13 year integration for the period 1992–2004 renders a cold region off southern tip of India during the months of June to September which is recognized as mini cold pool (MCP). The inter-annual and intra-seasonal variability of the MCP for the 5 years from 2000 to 2004 is explored in detail in the present paper. The analysis of model SST shows intense and widespread MCP during the long breaks occurred in the months of July and August of the bad monsoon years 2002 and 2004. The intense cooling is in response to the wind induced upwelling and the resultant entrainment of cold water from the bottom layer. The presence of cyclonic gyre in the model currents also supports the intense MCP which might modulate the monsoon during break phase.

An intense rainfall system that developed on the west coast of India caused heavy rain in excess of 900 mm at Santa Cruz, a suburb of Mumbai, on 26 July 2005. This system was simulated by the non-hydrostatic model (NHM) of the Japan Meteorological Agency (JMA), using the global analysis data of JMA as the initial condition. A maximum rainfall amount exceeding 1,100 mm in 17 hours was obtained by the simulation with a horizontal resolution of 5 km. The detailed structure of the rainfall system at the intense rain stage was investigated by NHM with a horizontal resolution of 1 km. The rain was mainly caused by a humid westerly flow near the surface, which overrode a cold pool near the mountain range. A dry southwesterly flow at the height of 2.51 km enhanced the cold pool. Another low-level humid westerly flow entered the rainfall system from the north and formed a thick humid northerly flow that reinforced the rainfall. Sensitivity experiments were conducted to investigate the influences of the mountain range, water vapor of the airflow north of the system, and the dry westerly flow. The results supported the proposed structure of the heavy rainfall system.

© 2015 Elsevier B.V. A methodology is proposed to identify the coldest areas of the island of Mallorca through satellite-derived surface temperatures. Land-surface temperatures from the Meteosat Second Generation (MSG-LST) just before sunrise for 1 year starting on September 1, 2007, were used. Fields for situations with clear skies and weak wind nights were selected, corresponding to 173 days of the year under study. Under these conditions, cold pools were generated in the centre of the three main basins of the island, in agreement with previous numerical and climatological studies. Maps of minimum air temperature and chilling hours, averaged over a season or annually, were obtained from the MSG-LST fields. These maps were then associated with the requirements of actual crop distributions., This work was funded through the projects CGL2009-12797-C03-01 and CGL2012-37416-C04-01 of the Spanish Government, supplied by the European Regional Development Fund (FEDER), and with a JAE-Doc contract from the Junta para la Ampliación de Estudios program from CSIC supplied by the European Social Fund 2007-2013

The föhn/cold-pool interactions in the lower Alpine Rhine valley documented in the framework of the Intensive Observing Period (IOP) 15 of the Mesoscale Alpine Programme (MAP) on 5 November 1999 are analysed. The present study focuses on the water vapour mixing ratio measurements acquired with the airborne differential absorption lidar LEANDRE 2 which enabled detailed documentation of the along-valley structure of the cold pool. LEANDRE 2 and microbarograph measurements revealed the presence of Kelvin-Helmholtz waves (KHW) at the top of the cold pool. The characteristics of the waves were different in the region of the cold- pool leading edge (the southernmost part of the cold pool) and in the vicinity of the Bodensee (Lake Constance), further to the north. Gravity waves were also observed above the cold pool in the in situ aircraft data acquired in the vicinity of the Bodensee. The gravity waves are suspected to be triggered by the KHW at the top of the cold pool. We also investigate the respective role of the three known processes likely to control the structure of the cold pool and its erosion along the Rhine valley, namely (i) convection within the cold pool, (ii) turbulent erosion at the top of the cold pool due to the presence of KHW, and (iii) dynamic displacement of the cold pool by föhn air. The former two processes are likely not to play a role in the erosion of the cold pool observed in the course of this IOP. Finally, the temporal evolution of the heat budget advection term in the lower Rhine valley was investigated using temperature profiles derived from balloon soundings acquired at two sites which were overpassed by the cold-pool edge in the course of its displacement northwards during the early afternoon as the result of the action of the föhn, and then southwards in the late afternoon as the föhn weakened and cold air from the Bodensee area was filling the lower Rhine Valley. Copyright © 2006 Royal Meteorological Society

This paper investigates wintertime cold-air pools in a basinlike part of the Danube Valley, located in the German state of Bavaria. Specifically, the focus is on cold-pool events restricted to the basin area, that is, not extending to the more elevated parts of the Alpine foreland. An analysis of observational data indicates that the delay of warm-air advection in the basin area relative to the Alpine foreland plays a major role in these events. However, the relationship between warming in the Alpine foreland and a temperature deficit in the northeast–southwest-oriented basin appears to depend sensitively on the ambient wind direction. A statistically significant correlation is found only for westerly and southerly wind directions but not for easterly directions. To examine the dynamical reasons for this phenomenon, idealized numerical simulations have been conducted. They are initialized with a pronounced cold pool in the basin area and examine the response of the cold pool to the dynamical forcing imposed by a geostrophically balanced large-scale wind field of various directions and strengths. Sensitivity tests consider the effects of the surrounding mountain ranges and of turbulent vertical mixing. The model results indicate that the most important dynamical processes capable of dissolving cold-air pools in a large basin are (i) ageostrophic advection of the cold air toward lower ambient pressure and (ii) downstream advection by the ambient flow. The former might also be interpreted as an adjustment of the cold air to the external pressure gradient, which can be balanced by the development of a spatial gradient in cold-pool depth. In principle, both advection processes are most effective in the along-basin direction because the advected air does not have to surmount significant topographic obstacles. However, a combination of several effects induced by the surrounding mountain ranges—for example, upstream flow deceleration and wake formation—modifies the dependence of the cold-pool persistence on the ambient wind direction. In agreement with observational data, the simulations with full topography predict a higher tendency for cold-pool persistence in the Danube basin for westerly and southerly flow than for easterly flow. Turbulent vertical mixing is found to make a significant contribution to the erosion of cold pools, but its effect is smaller than the sensitivity to the ambient wind direction.

On 16 January 2001 during the field experiment named WMO-01, a meso-β-scale snowband extending eastward from Toyama Bay was observed in the coastal area of eastern Hokuriku district, middle part of Japan. This snowband stagnated for about half a day and brought a snowfall of about 50 cm in this region.Numerical simulations of this snowband are performed using a nonhydrostatic cloud resolving model (NHM), with a horizontal grid size of 1 km. The NHM well reproduces many characteristics of the observed snowband. The snowband forms over the convergence zone between a cold southerly land breeze and a northwesterly winter monsoon, to which heat and moisture is supplied from relatively warm sea surface. Convective snow cells with the horizontal scale of a few km successively form at the northern part of the snowband and propagate east-southeastward. Meso-γ-scale convective snow systems are organized, consisting of developed snow cells in the snowband. A cold pool forms under the snowband. The diabatic cooling due to the sublimation of snow is responsible for the formation of the cold pool. After the land breeze became weak, the cold pool contributes to the maintenance of the horizontal convergence with the northwesterly winter monsoon at the northern edge of the snowband by compensating for the weakened land breeze. Therefore, the formation of the cold pool is significant for the maintenance of thesnowband.

One of the scientific objectives of MAP (Mesoscale Alpine Programme) addressed the four-dimensional variability of the foehn flow in the Rhine Valley. Within the framework of the field program FORM (Foehn in the Rhine Valley during MAP) a unique observing network was deployed for 2.5 months. One of the ten intensive observing periods has been selected to illustrate the life cycle of a special foehn event, during which the descending of the warm foehn air and the erosion of the cold air pool was very pronounced. Thanks to the excellent resolution in time as well as in height of temperature and wind measured by a boundary layer wind profiler-RASS system it was possible to get a detailed picture of the evolution of this foehn event. Model simulations performed with the non-hydrostatic model Meso-NH are in good agreement with observations. It allows us to show the time evolution of the cold pool area along a distance of 50 km in the Alpenrhein Valley.

A diagnostic climatological study of winter cold temperature extremes in the Metropolitan Area of Sao Paulo (MASP) is presented. This diagnosis is based on temperature data collected at the Meteorological Station of Parque Estadual das Fontes do Ipiranga (IAG/USP) from 1950 to 2000. The persistence of synoptic and climatological patterns has been studied through principal component (PC) analysis and the results are compared with monthly anomalies in sea-surface temperature (SST) of the eastern Pacific and South Atlantic. The extreme cold air temperatures, on a monthly basis, have shown no significant change since 1950. On the other hand, the mean monthly air temperatures have shown a slight warming trend, in agreement with the South Atlantic Ocean warming trend. The PC indicates significant loadings of two SST anomaly types: a cold anomaly of the South Atlantic Ocean, and a warm anomaly off the southern Brazilian coast. The latter could also be responsible for some extreme cold events (for daily minimum temperatures) in the MASP, and to a dominant westerly wind direction (southwest to northwest). Both the cold events and the westerly wind direction were evident in such winters as 1953, 1975, 1978, 1981, and 1994. On the other hand, the cold mean monthly temperatures are very well correlated to a broad cold pool anomaly in the South Atlantic at around 25 to 35° S and 15 to 55° W—sometimes narrower (such as in 1979 and 1988), and sometimes broader (such as 1964 and 1990). Where there was a narrowing or a widening, the prevailing wind direction was from the south–southeast. Therefore, the conclusion of these results is that the SST anomalies in the South Atlantic Ocean have a dominant effect on the Sao Paulo winter temperature climatology. Copyright © 2002 Royal Meteorological Society.

Data collected during the Maritime Continent Thunderstorm Experiment (MCTEX) (10 November–10 December 1995) have been used to analyze boundary layer development and circulations over two almost flat, tropical islands. The two adjacent islands have a combined length of about 170 km from east to west and 70 km from north to south. Intense thunderstorms formed over these islands every day of the field campaign. The boundary layer depth, temperature, and circulation over the island have been measured over the full diurnal cycle using a multiple radar analysis combined with surface and radiosonde measurements. On average, the island boundary layer depth reaches 1.5 km by early to midafternoon coinciding with the development of the deep convection. Thus, the island boundary layer is significantly deeper than the typical tropical oceanic boundary layer. In the midafternoon, thunderstorm outflows and their associated cold pool stabilize the lower boundary layer, suppressing late convection. This is follo...

The occurrence and evolution of an oceanic tropical squall line observed on 22 February 1993 during TOGA—COARE over the equatorial Pacific Ocean were simulated by use of a three—dimensional, nonhydrostatic storm—scale numerical model ARPS. The capacity of ARPS to simulate such tropical squall line was verified. The structure and dynamic mechanism of the squall line were discussed in details as well. The impacts of the different microphysical process that including the ice phase and warm rain schemes on structure and evolution of the squall line were investigated by the sensitive experiment.

A case study of vigorous drizzle development in marine boundary layer clouds is presented. The clouds were observed using a research aircraft west of Tasmania during the Southern Ocean Cloud Experiment. A clean marine boundary layer contained both cumulus clouds (some oriented in bands) and an upper level stratus deck organised in a closed cell circulation. “Cold pools,” that is regions of evaporatively cooled air under mature cumulus clouds, were observed on several occasions. Air in the cold pool centre showed a divergent component of the wind perpendicular to the mature cumulus bands. Ahead of this spreading precipitation downdraft, the wind component perpendicular to the cumulus band was convergent, and new cumulus clouds were subsequently formed over these regions. This dynamical situation is likened to an actively developing squall line of the propagating type. The development of drizzle, and its evaporation in the mixed-layer below the base of mature cumulus bands, is critical to the cloud evolution. It is demonstrated here, that processes resembling “deep convection” may also be responsible for the cloud development in a marine boundary layer of only 1500 m depth.

An extensive cruise program during 1994, 1995 and 1996 provided observations that describe the seasonal evolution of the three-dimensional density field in the western Irish Sea. A cold, dense pool flanked by strong nearbed density gradients was present from May until October. In spring, salinity had a dominant influence on the density structure but from June until October temperature controlled the density stratification. The trajectories of 55 satellite-tracked Argos drifters demonstrated the existence of the cyclonic circulation pattern that constitutes the western Irish Sea gyre and defined the gyre's spatial extent. Several distinct recirculation paths were observed and the implications for planktonic organisms of the seasonal variability in the recirculation are discussed. Drifter speeds were in good agreement with geostrophic calculations based on the observed density field and with de-tided acoustic Doppler current profiler (ADCP) measurements. The dynamical significance of strong nearbed gradients (bottom fronts) is highlighted. The location of the dynamically significant baroclinicity below the level of wind mixing explains the persistence of the cold pool and cyclonic circulation until late October. The dataset described here is valuable for environmental management purposes and it facilitates testing of the prognostic capabilities of the present generation of density-advecting numerical models.

This chapter focuses on the history, physics, climatology, forecasting and the broad effects of Alpine foehn on human populations. In the European Alps, foehn winds have been studied since the mid-1800s. The main focus of the investigations was the question of why foehn winds are so warm. While it soon became clear that adiabatic processes provide an explanation, the role of wet adiabatic rising on the upwind side of the Alps continued to be strongly debated. The so-called textbook theory for foehn – heat gain by wet adiabatic, forced lifting on the upwind side followed by dry adiabatic descent in the lee – represents only an extreme situation. Foehn occurs also with partial or complete blocking of the upwind air mass, i.e., with limited or no heat gain by wet adiabatic expansion. The second focus is on processes which lead to descending air masses after passing the mountain ridge. A discussion of the most important processes shows that there seems to be no theory which is applicable in all situations. Forecasting foehn is still a challenge to meteorologists. While the general foehn situation can be predicted reliably, today’s numerical models still often poorly simulate the sudden break in of potentially devastating foehn air in the lee. Efforts to improve this must continue because foehn storms have a significant societal impact (threat to transportation systems and massive increase of fire danger) as several recent incidents show.

[1] The effects of radiative cooling on the tropical convective response to sea surface temperature (SST) are investigated using a series of two-dimensional large domain cloud-resolving simulations. The experiments are designed with an imposed warm-pool SST at 2.5°C, 3.5°C, and 4.5°C higher than the cold-pool SST, which is specified at 26°C. The area ratio of the model cold pool to the warm pool is set to 2.2. It is observed that the warm-pool convection intensifies when the SST difference between the warm and cold pools increases from 2.5°C to 3.5°C, whereas the warm-pool convection strength does not significantly change when the SST difference increases from 3.5°C to 4.5°C, which is qualitatively consistent with the observations. The analysis of the atmospheric heat budgets shows that the cap on the development of convection over the warm pool is due to a radiative cooling over the cold pool that has a small variability. The cloud-resolving model simulations indicate that the area of the radiatively driven subsidence expands to produce an enhanced mass exchange between the warm and cold pools in response to the enhanced SST difference between them. This acts against the tendency for increased clouds by the enhanced upward mass flux in response to the warmer SST over the warm pool; significantly stronger SST differences between the two pools (e.g., ≥3.5°C in the cloud-resolving model) do not proportionally produce further intensive convection activities over the warm pool.

[1] The observed evolution of a mini-cold pool (MCP) off the southern tip of India (STI) and its intrusion into the south central Bay of Bengal (BoB) during the summer monsoon season shows pronounced cooling episodes on intraseasonal time scale with differences in their number, intensity, duration and spatial extent. This variability that changes from year to year appears to be primarily determined by the corresponding variability in the upwelling driven by the divergence in the near-surface (Ekman + geostrophic) circulation and wind induced mixing. The signature of this cooling carried by the Summer Monsoon Current (SMC) is seen with reduction in intensity in the south central BoB limited mostly only to south of about 10°N. In the background of slow cooling caused by SMC, the cooling episodes of different amplitudes occur in the south central BoB suggesting that spatially variable wind forcing is responsible for producing these episodes simultaneously on intraseasonal time scale.

A synthesis of 9 years of hydrographic measurements, acquired during approximately 25 cruises (1992-2000) on the French continental shelf of the Bay of Biscay, is presented. The main focus is upon salinity distribution, as it is little known in this area. The whole of the data set covers I I months of the year, with some sampling fields repeated several times a year, for multiple years. This time-series allows temperature and salinity distributions to be studied together with their seasonal, inter-annual and mesoscale variability. The seasonal trend in temperature can be described in three stages, which are in agreement with past studies. Thermal stratification occurs between May and mid-September in a layer similar to 50 m in thickness, whereas the water column remains homogenous from January to the beginning of April. The mid-September-December period corresponds to the progressive destruction of the thermocline. In contrast, the salinity distribution displays two main seasonal patterns. From March to June, the haline stratification is strong;, this is in response to high river runoff and relatively low vertical mixing. In autumn, stratification decreases because of low river discharge and wind-induced vertical mixing. Surface salinity increases by up to I psu inshore of the 100-m isobath, from spring to end of summer. The salinity gradient runs cross-shore in spring and along-shore in summer: this is due to the mean wind direction during the previous 6 months (SW from September to March. NW from March to September). Nevertheless, this seasonal signal could be biased by the high inter-annual variability. Indeed. the monthly extent of low surface salinity (S < 35) varied between years, this is driven by river runoff over the previous 3-6 months and short-term wind events (similar to 10 days), particularly when they are upwelling-favourable. Forcing at these time-scales are responsible also for mesoscale structures, such as lower salinity lenses and wind-induced coastal upwelling off Southern Brittany. In the deeper layers the inter-annual variability of a denser water structure named (the "Cold Pool") is also investigated. Consequently, the inter-annual, seasonal and mesoscale variability in salinity patterns are caused by (i) river runoff from the Loire and Gironde, that reduces surface salinity locally; and (ii) the wind that influences the location of low salinity water masses. Mesoscale structures and the observed links between inter-annual and seasonal variability, raise problems in relation to the interpretation of in situ data, making it impossible to establish the actual mean distribution. There is a pressing need within the Bay of Biscay, for sampling at higher spatial and temporal resolution.