Dry Intrusion Research Articles

Convective self‐aggregation feedbacks in near‐global cloud‐resolving simulations of an aquaplanet

AbstractPositive feedbacks between precipitable water, reduced radiative cooling and enhanced surface fluxes promote convective self‐aggregation in limited‐area cloud‐resolving model (CRM) simulations over uniform sea‐surface temperature (SST). Near‐global aquaplanet simulations with 4 km horizontal grid spacing and no cumulus or boundary layer parameterization are used to test the importance of these feedbacks to realistically organized tropical convection. A 20,480 × 10,240 km equatorially centered channel with latitudinally varying SST is used. Realistic midlatitude and tropical cloud structures develop. The natural zonal variability of humidity and convection are studied in a 30 day control simulation. The temporal growth of a small white‐noise humidity perturbation and intrinsic predictability implications are explored. Atmospheric column budgets of moist‐static energy (MSE) quantify its covariation with precipitation, surface heat flux, and radiative energy loss. Zonal Fourier analysis partitions these budgets by length scale. Radiative feedbacks on MSE natural variability and perturbation growth are found to be positive, broadly similar across scales, and comparable to limited‐area CRMs, capable of e‐folding a column MSE perturbation in 6–14 days. Surface fluxes are highest in synoptic‐scale dry intrusions, inhibiting aggregation by damping tropical MSE perturbations. Sub‐4‐day MSE variations are due mainly to advection. Both tropics and midlatitudes have large‐scale intrinsic predictability horizons of 15–30 days. An identical simulation but with 20 km grid spacing has more mesoscale variability and low cloud.

Turbulent mixing and its impact on lower tropospheric moisture over tropical ocean

AbstractThe variability of lower tropospheric humidity is a crucial feature of the tropical climate. Among the processes that impact moisture budget, the vertical transport by turbulent mixing is generally overlooked. Using observations from Cooperative Indian Ocean experiment on intraseasonal variability/Dynamics of the Madden‐Julian Oscillation (MJO), CINDY/DYNAMO, campaign, this is a first attempt to quantify it over the tropical ocean. Turbulent patches of ~100 m depth are observed in relation with large vertical gradients of specific humidity. Intense mixing is diagnosed within these intermittent patches. Three approaches are used to diagnose the overall effect of this intermittent turbulence. Large uncertainties on the corresponding eddy diffusivity coefficient arise from parameters hard to experimentally constrain. However, dry conditions are associated with steep moisture vertical gradients above the boundary layers. Owing to the uncertainties on the eddy diffusivity, these gradients can correspond to negligible or to significant moisture tendencies (~0.5–1 g kg−1 d−1) during the recovery following a dry intrusion or the preconditioning stage of an MJO.

The effects of springtime mid-latitude storms on trace gas composition determined from the MACC reanalysis

Abstract. The relationship between springtime air pollution transport of ozone (O3) and carbon monoxide (CO) and mid-latitude cyclones is explored for the first time using the Monitoring Atmospheric Composition and Climate (MACC) reanalysis for the period 2003–2012. In this study, the most intense spring storms (95th percentile) are selected for two regions, the North Pacific (NP) and the North Atlantic (NA). These storms (∼60 storms over each region) often track over the major emission sources of East Asia and eastern North America. By compositing the storms, the distributions of O3 and CO within a "typical" intense storm are examined. We compare the storm-centered composite to background composites of "average conditions" created by sampling the reanalysis data of the previous year to the storm locations. Mid-latitude storms are found to redistribute concentrations of O3 and CO horizontally and vertically throughout the storm. This is clearly shown to occur through two main mechanisms: (1) vertical lifting of CO-rich and O3-poor air isentropically, from near the surface to the mid- to upper-troposphere in the region of the warm conveyor belt; and (2) descent of O3-rich and CO-poor air isentropically in the vicinity of the dry intrusion, from the stratosphere toward the mid-troposphere. This can be seen in the composite storm's life cycle as the storm intensifies, with area-averaged O3 (CO) increasing (decreasing) between 200 and 500 hPa. The influence of the storm dynamics compared to the background environment on the composition within an area around the storm center at the time of maximum intensity is as follows. Area-averaged O3 at 300 hPa is enhanced by 50 and 36% and by 11 and 7.6% at 500 hPa for the NP and NA regions, respectively. In contrast, area-averaged CO at 300 hPa decreases by 12% for NP and 5.5% for NA, and area-averaged CO at 500 hPa decreases by 2.4% for NP while there is little change over the NA region. From the mid-troposphere, O3-rich air is clearly seen to be transported toward the surface, but the downward transport of CO-poor air is not discernible due to the high levels of CO in the lower troposphere. Area-averaged O3 is slightly higher at 1000 hPa (3.5 and 1.8% for the NP and NA regions, respectively). There is an increase of CO at 1000 hPa for the NP region (3.3%) relative to the background composite and a~slight decrease in area-averaged CO for the NA region at 1000 hPa (-2.7%).

Extreme small-scale wind episodes over the Barents Sea: When, where and why?

The Barents Sea is mostly ice-free during winter and therefore prone to severe weather associated with marine cold air outbreaks, such as polar lows. With the increasing marine activity in the region, it is important to study the climatology and variability of episodes with strong winds, as well as to understand their causes. Explosive marine cyclogenesis is usually caused by a combination of several mechanisms: upper-level forcing, stratospheric dry intrusions, latent heat release, surface energy fluxes, low-level baroclinicity. An additional factor that has been linked to extremely strong surface winds, is low static stability in the lower atmosphere, which allows for downward transfer of high-momentum air. Here the most extreme small-scale wind episodes in a high-resolution (5 km) 35-year hindcast were analyzed, and it was found that they were associated with unusually strong low-level baroclinicity and surface heat fluxes. And crucially, the 12 most severe episodes had stronger cold-air advection than 12 slightly less severe cases, suggesting that marine cold air outbreaks are the most important mechanism for extreme winds on small spatial scales over the Barents Sea. Because weather models are often unable to explicitly forecast small-scale developments in data-sparse regions such as the Barents Sea, these results can be used by forecasters as supplements to forecast model data.

Numerical diagnosis of a heavy snowfall event in the center of the Iberian Peninsula

On 4 March 2011, an exceptionally heavy snowfall event affected the Madrid region on the central Iberian Peninsula. At altitudes of 1200 m, snowfall reached a record of 34cm in 24h and produced considerable damage and disruption to electricity distribution and transport systems. Maximum intensity precipitation was identified between 1600 and 1800 UTC. Associated precipitation was particularly intense in the Guadarrama Mountains (at the center of the Peninsula, near Madrid).Analysis of Meteosat Second Generation (MSG) satellite images revealed a dark area, generated by a stratospheric intrusion originating in the Atlantic and reaching the Iberian Peninsula. We studied synoptic conditions and mesoscale factors involved in the event, using the Weather Research and Forecasting (WRF) model. This permitted analysis of the evolution of the dry intrusion caused by a tropopause fold, its movement, and frontogenesis-related mechanisms during its crossing of the Guadarrama Mountains. The blocking of a wet warm mass at altitude owing to a descent of the tropopause but mainly at low levels because of orographic effects, helped concentrate moisture and generate potential instability (PI). This was subsequently released in deep convection, owing to the formation of frontogenesis.

Case Study of a Split Front and Associated Precipitation during the Mei-Yu Season

Abstract This study investigated the relationship between a split front and precipitation over western Japan on 10–11 July 2007 based on data from routine observations and a global objective analysis. The split front formed at the leading edge of a dry intrusion overrunning a mei-yu front in a deep layer from the north. Strong upward motion was present ahead of the frontogenesis associated with the split front. In contrast, upward motion over the mei-yu front was suppressed by downward motion behind the split front. An intense rainband developed along the split front south of the mei-yu front. In contrast, precipitation over the mei-yu front gradually disappeared with the southward advance of the split front. Rainfall as heavy as 99 mm h−1 was observed under the rainband associated with the split front. The strong upward motion that induced heavy rains was attributed to the rising branch of the frontal circulation of the split front and the intense low-level convergence facilitated by the dry intrusion behind the split front. These findings indicate that split fronts have a substantial impact on the development and distribution of precipitation during the mei-yu season.

Structural Characteristics of T-PARC Typhoon Sinlaku during Its Extratropical Transition

Abstract The structure and the environment of Typhoon Sinlaku (2008) were investigated during its life cycle in The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC). On 20 September 2008, during the transformation stage of Sinlaku’s extratropical transition (ET), research aircraft equipped with dual-Doppler radar and dropsondes documented the structure of the convection surrounding Sinlaku and low-level frontogenetical processes. The observational data obtained were assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software tool. The resulting analysis provides detailed insight into the ET system and allows specific features of the system to be identified, including deep convection, a stratiform precipitation region, warm- and cold-frontal structures, and a dry intrusion. The analysis offers valuable information about the interaction of the features identified within the transitioning tropical cyclone. The existence of dry midlatitude air above warm-moist tropical air led to strong potential instability. Quasigeostrophic diagnostics suggest that forced ascent during warm frontogenesis triggered the deep convective development in this potentially unstable environment. The deep convection itself produced a positive potential vorticity anomaly at midlevels that modified the environmental flow. A comparison of the operational ECMWF analysis and the observation-based SAMURAI analysis exhibits important differences. In particular, the ECMWF analysis does not capture the deep convection adequately. The nonexistence of the deep convection has considerable implications on the potential vorticity structure of the remnants of the typhoon at midlevels. An inaccurate representation of the thermodynamic structure of the dry intrusion has considerable implications on the frontogenesis and the quasigeostrophic forcing.

Diabatic processes modifying potential vorticity in a North Atlantic cyclone

AbstractPotential vorticity (PV) succinctly describes the evolution of large‐scale atmospheric flow because of its material conservation and invertibility properties. However, diabatic processes in extratropical cyclones can modify PV and influence both mesoscale weather and the evolution of the synoptic‐scale wave pattern. In this investigation, modification of PV by diabatic processes is diagnosed in a Met Office Unified Model (MetUM) simulation of a North Atlantic cyclone using a set of PV tracers. The structure of diabatic PV within the extratropical cyclone is investigated and linked to the processes responsible for it. On the mesoscale, a tripole of diabatic PV is generated across the tropopause fold extending down to the cold front. The structure results from a dipole in heating across the frontal interface due to condensation in the warm conveyor belt flanking the upper side of the fold and evaporation of precipitation in the dry intrusion and below. On isentropic surfaces intersecting the tropopause, positive diabatic PV is generated on the stratospheric side, while negative diabatic PV is generated on the tropospheric side. The stratospheric diabatic PV is generated primarily by long‐wave cooling which peaks at the tropopause itself due to the sharp gradient in humidity there. The tropospheric diabatic PV originates locally from the long‐wave radiation and non‐locally by advection out of the top of heating associated with the large‐scale cloud, convection and boundary layer schemes. In most locations there is no diabatic modification of PV at the tropopause itself but diabatic PV anomalies would influence the tropopause indirectly through the winds they induce and subsequent advection. The consequences of this diabatic PV dipole for the evolution of synoptic‐scale wave patterns are discussed.

An Extratropical Cyclone Atlas: A Tool for Illustrating Cyclone Structure and Evolution Characteristics

CORRESPONDING AUTHOR: H. F. Dacre, Department of Meteorology, University of Reading, Earley Gate, P.O. Box 243, Reading, RG6 6BB United Kingdom, E-mail: h.f.dacre@reading.ac.uk

영동지역 악기상 사례에 대한 MTSAT 위성 영상의 특징

An unusual autumn storm developed rapidly in the western part of the East sea on the early morning of 23 October 2006. This storm produced a record-breaking heavy rain and strong wind in the northern and middle part of the Yeong-dong region; 24-h rainfall of 304 mm over Gangneung and wind speed exceeding 63.7 m <TEX>$s^{-1}$</TEX> over Sokcho. In this study, MTSAT-1R (Multi-fuctional Transport Satellite) water vapor and infrared channel imagery are examined to find out some features which are dynamically associated with the development of the storm. These features may be the precursor signals of the rapidly developing storm and can be employed for very short range forecast and nowcasting of severe storm. The satellite features are summarized: 1) MTSAT-1R Water Vapor imagery exhibited that distinct dark region develops over the Yellow sea at about 12 hours before the occurrence of maximum rainfall about 1100 KST on 23 October 2006. After then, it changes gradually into dry intrusion. This dark region in the water vapor image is closely related with the positive anomaly in 500 hPa Potential Vorticity field. 2) In the Infrared imagery, low stratus (brightness temperature: <TEX>$0{\sim}5^{\circ}C$</TEX>) develops from near Bo-Hai bay and Shanfung peninsula and then dissipates partially on the western coast of Korean peninsula. These features are found at 10~12 hours before the maximum rainfall occurrence, which are associated with the cold and warm advection in the lower troposphere. 3) The IR imagery reveals that two convective cloud cells (brightness temperature below <TEX>$-50^{\circ}C$</TEX>) merge each other and after merging it grows up rapidly over the western part of East sea at about 5 hours before the maximum rainfall occurrence. These features remind that there must be the upward flow in the upper troposphere and the low-layer convergence over the same region of East sea. The time of maximum growth of the convective cloud agrees well with the time of the maximum rainfall.

Comparative Study of the West African Continental, Coastal, and Marine Atmospheric Profiles during the Summer of 2006

We used sounding data of the Multidisciplinary Analysis of the African Monsoon experience in summer 2006 at continental and coastal sites of West Africa, respectively, to analyze the vertical profiles of relative humidity, temperature, dew point, and speed and wind direction for the JJAS rainy period. The vertical gradient method is applied to the profiles of some thermodynamic parameters estimated from sounding data to do a comparative study of the structure and thermal properties, moisture, and static stability of the atmospheric boundary layer of inland, coastal, and marine sites to show consistent differences related to geographic factors. In vertical profiles of relative humidity, the intensity is higher in Dakar than in Niamey particularly in the core of the season. There are dry intrusions in the low levels at the beginning and end of the season in Dakar, which do not exist in Niamey. The mixing layer on the continent during the day can reach a height greater than 1100 m, and the inversion layer height can exceed 1700 m. Therefore, the maximum thickness of the boundary layer is observed on the continent during the day, while at night the marine boundary layer is the thickest. The diurnal evolution shows that the mixing layer thickness decreases during the night over the continent but increases at the coast and at sea. In the night at the continental site there is a division of the mixing layer with a consistent residual mixing layer. Continental boundary layer is more unstable during the day, while at night it is the marine boundary layer that is more unstable than the coastal and inland ones.

The Coupled Effect of Mid-Tropospheric Moisture and Aerosol Abundance on Deep Convective Cloud Dynamics and Microphysics

The humidity of the mid troposphere has a significant effect on the development of deep convection. Dry layers (dry intrusions) can inhibit deep convection through the effect of a thermal inversion resulting from radiation and due to the reduction in buoyancy resulting from entrainment. Recent observations have shown that the sensitivity of cloud top height to changes in mid-tropospheric humidity can be larger than straightforward “parcel dilution” would lead us to expect. Here, we investigate how aerosol effects on cloud development and microphysics are coupled to the effects of mid-tropospheric dry air. The two effects are coupled because the buoyancy loss through entrainment depends on droplet evaporation, so is controlled both by the environmental humidity and by droplet sizes, which are, in turn, controlled in part by the aerosol size distribution. Previous studies have not taken these microphysical effects into account. Cloud development and microphysics are examined using a 2-D non-hydrostatic cloud model with a detailed treatment of aerosol, drop, and ice-phase hydrometeor size spectra. A moderately deep mixed-phase convective cloud that developed over the High Plains of the United States is simulated. We find that a dry layer in the mid troposphere leads to a reduction in cloud updraft strength, droplet number, liquid water content and ice mass above the layer. The effect of the dry layer on these cloud properties is greatly enhanced under elevated aerosol conditions. In an environment with doubled aerosol number (but still realistic for continental conditions) the dry layer has about a three-times larger effect on cloud drop number and 50% greater effect on ice mass compared to an environment with lower aerosol. In the case with high aerosol loading, the dry layer stops convective development for over 10 min, and the maximum cloud top height reached is lower. However, the effect of the dry layer on cloud vertical development is significantly reduced when aerosol concentrations are lower. The coupled effect of mid-tropospheric dry air and aerosol on convective development is an additional way in which long term changes in aerosol may impact planetary cloud processes and climate.

The mesoscale structure of a polar low: airborne lidar measurements and simulations

AbstractThe mesoscale structure of a mature polar low was studied on the basis of high‐resolution airborne measurements and numerical modelling. A polar low was measured by light detection and ranging (lidar) and dropsonde observations over the Norwegian Sea on 3 and 4 March 2008. Lidar observations provided cross‐sections of water‐vapour mixing ratio, backscatter ratio and horizontal wind speed around the polar low and through its centre. Mesoscale structures, such as shallow convection in a cold‐air outbreak, a dry intrusion in the eye‐like centre of the cyclone and deep convection surrounding it could be identified. Numerical simulations were performed with the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) and a high‐resolution, polar version of the Weather Research and Forecasting (WRF) model. WRF simulations reproduced these structures and showed that the polar low had a warm, upper‐level core with descending motions. The eye‐like centre had a diameter of about 100–150 km and was characterized by rather stable stratification, horizontally constant potential temperatures and calm winds. Beyond the centre, wind speeds increased rapidly. The observed radial wind and temperature profiles support previous idealized simulations. Several WRF sensitivity tests showed the influence of the initialization time and sensible and latent heat fluxes from the surface on the simulated polar‐low development. The polar‐low simulations were more accurate in runs starting at the mature stage. Heat fluxes from the surface were important for the polar‐low energetics, especially at the final stages. Copyright © 2011 Royal Meteorological Society

Genesis of Typhoon Chanchu (2006) from a Westerly Wind Burst Associated with the MJO. Part II: Roles of Deep Convection in Tropical Transition

Abstract In this study, the life cycles of a series of four major mesoscale convective systems (MCSs) during genesis and their roles in transforming a vertically tilted vortex associated with a westerly wind burst (hereafter the WWB vortex) into Typhoon Chanchu (2006) are examined using 11-day cloud-resolving simulations presented in Part I. It is found that the tilted WWB vortex at early stages is characterized by an elevated cold-core layer (about 200 hPa thick) below and a weak warm column above with large vertical wind shear across the layer, which extends over a horizontal distance of about 450 km between the vortex’s 400- and 900-hPa centers. During the final two days of the genesis process, the upper-level warm column increases in depth and intensity as a result of the absorption of convectively generated vortices (CGVs), including a mesoscale convective vortex (MCV), causing more rapid amplification of cyclonic vorticity in the middle than the lower troposphere. The commencement of sustained intensification of Chanchu occurs when the upper-level warm column is vertically aligned with the surface-based warm-core vortex. Results show that four unique MCSs develop in succession on the downtilt-right side of the WWB vortex. The first MCS develops as a squall line with trailing stratiform precipitation and an MCV; subsequent MCSs include a convective cluster whose shape changes from an inverted U to a question mark and finally a spiral rainband as the WWB vortex decreases its vertical tilt. Strong cold pools are favored behind the leading connective lines during the earlier tilted-vortex stages due primarily to dry intrusion by the midlevel sheared flows, whereas few cold downdrafts occur at later stages as the WWB vortex becomes more upright and sufficiently moist. The authors conclude that the roles of the MCSs during genesis are to (a) generate cyclonic vorticity and then store it mostly in the midtroposphere, after merging CGVs within the WWB vortex; (b) moisten the low- and midlevels; (c) enhance the northward displacement of the WWB vortex; and (d) reduce the vertical tilt of the WWB vortex.

Displacing Potential Vorticity Structures by the Assimilation of Pseudo-Observations

Abstract Classic formulations of variational data assimilation in amplitude space are not able to directly handle observations that measure the geographical positions of meteorological features like fronts and vortices. These observations can be derived from satellite images, as is already the case for tropical cyclones. Although some advanced data assimilation algorithms have been specifically designed to tackle the problem, a widespread way of dealing with this information is to use so-called bogussing pseudo-observations: user-specified artificial observations are inserted in a traditional data assimilation scheme. At the midlatitudes, there is a relationship between dry intrusions in water vapor images and upper-level potential vorticity structures. Some prior work has also shown that it was possible to automatically identify dry intrusions with tracking algorithms. The difference of positions between model and image dry intrusions could therefore be used as observations of the misplacement of potential vorticity structures. One strategy to achieve the displacement of potential vorticity anomalies is to sample them, and assimilate the values at displaced locations. The uncertainty of these pseudo-observations is left as a tuning parameter to try to make the displacement both effective and robust. A simple one-dimensional assimilation model is used to study the displacement of curves defined by Gaussian humps. The concept is then illustrated in realistic examples from real synoptic systems, where pseudo-observations of potential vorticity are incorporated in a global variational data assimilation scheme. Overall and despite reasonable optimization, the results contain artifacts. This suggests that the use of pseudo-observations to displace identifiable structures is not an effective strategy.

Generation processes of mesoscale convective systems following midlatitude troughs around the Sichuan Basin

[1] The generation processes of mesoscale convective systems (MCSs) that occurred over the Sichuan Basin in China were revealed in the early 2008 monsoon season. MCS occurrences detected in METEOSAT geostational satellite images are associated with the traveling of midlatitude troughs after the onset of the Indian monsoon. Three episodes of large-scale MCSs were generated under synoptic conditions of merging southwesterly low-level monsoon flows with a northerly midlatitude air mass following the trough in the leeward of the Tibetan Plateau without direct migration of the vortex generated from the plateau surface. Numerical simulations using a weather research and forecast (WRF) model showed that the MCS was triggered in the evening by strengthening of the low-level wind convergences with horizontal shear between the southerly monsoon flow, with large convective available potential energy, and the northerly dry intrusion. A sudden increase in the northerly winds was confirmed by sonde observation data in the western basin, and the winds were simulated as intrusions passing over the Qinling Mountains when the daytime clouds over the mountain were diminished. Sensitivity experiments by a WRF simulation revealed that the topography of the Sichuan Basin was able to capture the dry intrusion at the bottom to prevent the propagation of disturbances from the plateau and to cause the sudden onset of MCSs apart from the plateau with a heavy precipitation zone.

Can Climate Models Capture the Structure of Extratropical Cyclones?

Abstract Composites of wind speeds, equivalent potential temperature, mean sea level pressure, vertical velocity, and relative humidity have been produced for the 100 most intense extratropical cyclones in the Northern Hemisphere winter for the 40-yr ECMWF Re-Analysis (ERA-40) and the high resolution global environment model (HiGEM). Features of conceptual models of cyclone structure—the warm conveyor belt, cold conveyor belt, and dry intrusion—have been identified in the composites from ERA-40 and compared to HiGEM. Such features can be identified in the composite fields despite the smoothing that occurs in the compositing process. The surface features and the three-dimensional structure of the cyclones in HiGEM compare very well with those from ERA-40. The warm conveyor belt is identified in the temperature and wind fields as a mass of warm air undergoing moist isentropic uplift and is very similar in ERA-40 and HiGEM. The rate of ascent is lower in HiGEM, associated with a shallower slope of the moist isentropes in the warm sector. There are also differences in the relative humidity fields in the warm conveyor belt. In ERA-40, the high values of relative humidity are strongly associated with the moist isentropic uplift, whereas in HiGEM these are not so strongly associated. The cold conveyor belt is identified as rearward flowing air that undercuts the warm conveyor belt and produces a low-level jet, and is very similar in HiGEM and ERA-40. The dry intrusion is identified in the 500-hPa vertical velocity and relative humidity. The structure of the dry intrusion compares well between HiGEM and ERA-40 but the descent is weaker in HiGEM because of weaker along-isentrope flow behind the composite cyclone. HiGEM’s ability to represent the key features of extratropical cyclone structure can give confidence in future predictions from this model.

Initial Development and Genesis of Hurricane Dolly (2008)

Abstract Based on a successful cloud-resolving simulation with the Weather Research and Forecasting Model, this study examines key processes that led to the early development of Hurricane Dolly (2008). The initial development of Dolly consisted of three stages: (i) an initial burst of convection; (ii) stratiform development, dry intrusion, and thermodynamic recovery; and (iii) reinvigoration of moist convection and rapid intensification. Advanced diagnosis of the simulation—including the use of vorticity budget analysis, contour frequency analysis diagrams, and two-dimensional spectral decomposition and filtering—suggests that the genesis of Dolly is essentially a “bottom-up” process. The enhancement of the low-level vorticity is mainly ascribed to the stretching effect, which converges the ambient vorticity through stretching enhanced by moist convection. In the rapid intensification stage, smaller-scale positive vorticity anomalies resulting from moist convection are wrapped into the storm center area under the influence of background convergent flow. The convergence and accompanying aggregation of vorticity anomalies project the vorticity into larger scales and finally lead to the spinup of the system-scale vortex. On the other hand, although there is apparent stratiform development in the inner-core areas of incipient storm after the initial burst of convection, little evidence is found to support the genesis of Dolly through downward extension of the midlevel vorticity, a key process in the “top-down” thinking.

Diagnostic analyses of dry intrusion and nonuniformly saturated instability during a rainfall event

Using the one by one degree National Centers for Environmental Prediction final data (details of data set can be found in http://dss.ucar.edu/datasets/ds083.2/) and numerical mesoscale results simulated by the WRF model, the characteristics of dry intrusion during a heavy rainfall event in northern China are studied. Since dry intrusion is a Lagrangian feature and should be identified with the aid of trajectory calculations, in this paper this process is simulated by the Lagrangian Flexpart particle dispersion model, and the trajectories of moist and dry air parcels are depicted and traced using a Lagrangian method. It is found that the cold, dry air with high potential vorticity shows up prior to the start of precipitation and persists and extends downward to lower levels during the raining periods. As the dry, cold air from upper levels intrudes into the warm, moist sector in the lower troposphere, the two air masses with different characteristics mix causing nonuniform saturation to occur in the atmosphere. On the basis of this nonuniform saturation concept, a new Brunt‐Väisälä frequency for nonuniformly saturated moist atmospheres is derived and applied to the diagnostic study of precipitation cases. It is found to be more appropriate for depicting the instability of rainy regions than those stability indexes used in dry and moist saturated atmosphere.

Wind profiler analysis of the African Easterly Jet in relation with the boundary layer and the Saharan heat‐low

AbstractWind profiler measurements in Niger and Benin during the African Monsoon Multidisciplinary Analysis project are used to study the intraseasonal variability of the low and mid troposphere at several time‐scales. We focus on the African Easterly Jet (AEJ) and its interaction with the Saharan Heat‐Low (HL) and the planetary boundary layer (PBL).We find a pronounced diurnal cycle of the AEJ, characterised by a decrease of wind speed during the afternoon, reaching a minimum at 1800 UTC of about 15–20% of the daily average during the pre‐onset period. This decrease is out of phase with the HL intensity, but in phase with the daytime turbulent mixing associated with the PBL. The interaction between the PBL and mid‐troposphere is likely responsible for this daily decrease of the AEJ.During the transition periods (dry to wet or wet to dry), the HL seems to govern the AEJ; however, slightly before the monsoon onset, it has no direct influence on the jet. During that time, we find smaller AEJ wind speed for deeper PBL, as found at the diurnal time‐scale. This is consistent with the still large surface heating at that time, which favours deep PBL growth, with a top inversion often higher than the shear layer between the monsoon and the easterlies. After the monsoon onset, deep convection, African Easterly Waves (AEWs) and dry intrusions make the synoptic environment complex and blur the interaction between AEJ and PBL. We still find weaker AEJ for deeper PBL, but likely without a direct connection between them. Copyright © 2009 Royal Meteorological Society