Stratiform Precipitation Region Research Articles

Aspects of melting layer and fall streaks in stratiform cloud system over the Western Ghats, India from Ka-band polarimetric radar observations

During the Indian summer monsoon, high temporal and spatial resolution observations from Ka-band dual-polarization radar deployed in the Western Ghats (WG), India are utilized to investigate the vertical structure of the stratiform precipitating system. First, the radar reflectivity (Z) measurements are corrected for rain attenuation, based on the relationship between specific attenuation (A) and Z. The disdrometer dataset is used to calculate the expected value of A at Ka-band for a given Z using microwave scattering simulation. The radar data are also corrected for wet radome transmission loss, gaseous, and melting layer (ML) attenuation. Within the ML, a change in Z and other radar polarimetric variables suggest hydrometeors phase change. In addition to Z, polarimetric measurements such as differential reflectivity (ZDR), linear depolarization ratio (LDR), and copolar correlation coefficient between horizontal and vertical channel (ρHV) are used to determine the ML height. Ka-band polarimetric radar signatures near the ML are characterized by a low ρHV (∼0.93), high Z (∼25–30 dBZ), ZDR (∼3 dB), and LDR (∼ −15 dB) values. In the stratiform precipitation region, well-defined fall streaks are observed above ML, which descends through 0 °C isotherm into the rain region. When the fall streaks are present above the ML, Z increases below the ML, indicating a seeder-feeder mechanism. Because there is little observational evidence concerning the vertical distribution of stratiform clouds over the Indian subcontinent, the current study may help to understand stratiform clouds and the ice processes that occur within them.

Impact of Convectively Generated Low-Frequency Gravity Waves on Evolution of Mesoscale Convective Systems

AbstractIdealized numerical simulations of mesoscale convective systems (MCSs) over a range of instabilities and shears were conducted to examine low-frequency gravity waves generated during initial and mature stages of convection. In all simulations, at initial updraft development a first-order wave was generated by heating extending through the depth of the troposphere. Additional first-order wave modes were generated each time the convective updraft reintensified. Each of these waves stabilized the environment in advance of the system. As precipitation descended below cloud base, and as a stratiform precipitation region developed, second-order wave modes were generated by cooling extending from the midlevels to the surface. These waves destabilized the environment ahead of the system but weakened the 0–5 km shear. Third-order wave modes could be generated by midlevel cooling caused by rear inflow intensification; these wave modes cooled the midlevels destabilizing the environment. The developing stage of each MCS was characterized by a cyclical process: developing updraft, generation of n = 1 wave, increase in precipitation, generation of n = 2 wave, and subsequent environmental destabilization reinvigorating the updraft. After rearward expansion of the stratiform region, the MCSs entered their mature stage and the method of updraft reinvigoration shifted to absorbing discrete convective cells produced in advance of each system. Higher-order wave modes destabilized the environment, making it more favorable to development of these cells and maintenance of the MCS. As initial simulation shear or instability increased, the transition from cyclical wave/updraft development to discrete cell/updraft development occurred more quickly.

Sensitivity studies of cloud microphysical schemes of WRF-ARW model in the simulation of trailing stratiform squall lines over the Gangetic West Bengal region

In this paper an attempt has been made to investigate the impact of different microphysical (MP) parameterization schemes in simulation of leading convective line with trailing stratiform (TS) squall lines observed over the Gangetic West Bengal (GWB) region of India during the pre-monsoon season of March–May using Advanced Research Weather Research and Forecasting (WRF-ARW). The model is employed to study the features associated with three TS squall line (May 15, 2009, May 5, 2010 and May 7, 2010) observed over the study region. The model sensitivity is evaluated with four different microphysics schemes of the model (Goddard, WDM6, Thompson and Morrison). Model simulations are carried out with three two-way nested domains with horizontal resolution of 18, 6 and 2 km grid spacing and explicit convection in the innermost domain. The model simulated results are compared with available observations (Surface data, rainfall, hydrometeors and Doppler Weather Radar products). The analysis demonstrates that the simulation of TS squall lines are very sensitive to the parameterization of microphysical processes of different MP schemes, in spite of using similar initial and boundary conditions and model configuration. The model provides better simulations of rainfall, maximum reflectivity and vertical profiles of hydrometeors with the Morrison MP Scheme. The study reveals that snow and graupel particles has significant impacts on the simulation of stratiform precipitation region of the TS squall line. The study suggests that simulation of TS squall lines using a high-resolution mesoscale model with suitable cloud microphysics scheme is useful for the prediction of such severe events.

Radar-Observed Bulk Microphysics of Midlatitude Leading-Line Trailing-Stratiform Mesoscale Convective Systems

AbstractIn 2013, all NEXRAD WSR-88D units in the United States were upgraded to dual polarization. Dual polarization allows for the identification of precipitation particle shape, size, orientation, and concentration. In this study, dual-polarization NEXRAD observations from 34 recent events are used to identify the bulk microphysical characteristics of a specific subset of mesoscale convective systems (MCSs), the leading-line trailing-stratiform (LLTS) MCS. NEXRAD observations are used to examine hydrometeor distributions in relative altitude to the 0°C level and as a function of storm life cycle, precipitation source (convective or stratiform), and storm environment. The analysis reveals that graupel particles are the most frequently classified hydrometeor class in a layer extending from the 0°C-level altitude to approximately 5 km above within the convective region. Below the 0°C level, rain is the most frequently classified hydrometeor, with small hail and graupel concentrations present throughout the LLTS system’s life cycle. The stratiform precipitation region contains small graupel concentrations in a shallow layer above the 0°C level, with pristine ice crystals being classified as the most frequently observed hydrometeor at higher altitudes and snow aggregates being classified as the most frequently observed hydrometeor at lower altitudes above the environmental 0°C level. Variations in most unstable convective available potential energy (MUCAPE) have the largest impact on the vertical distribution of hydrometeors, because more-unstable environments are characterized by a greater production of rimed ice.

Microphysical Characteristics of Squall-Line Stratiform Precipitation and Transition Zones Simulated Using an Ice Particle Property-Evolving Model

Abstract A quasi-idealized 3D squall-line case is simulated using a novel bulk microphysics scheme called the Ice-Spheroids Habit Model with Aspect-ratio Evolution (ISHMAEL). In ISHMAEL, the evolution of ice particle properties (e.g., mass, shape, maximum diameter, density, and fall speed) are predicted during vapor growth, sublimation, riming, and melting, allowing ice properties to evolve from various microphysical processes without needing separate unrimed and rimed ice categories. ISHMAEL produces both a transition zone and an enhanced stratiform precipitation region, and ice particle properties are analyzed to determine the characteristics of ice that lead to the development of these squall-line features. Rimed particles advected rearward from the convective region produce the enhanced stratiform precipitation region. The transition zone results from hydrometeor sorting; the evolution of ice particle properties in the convective region leads to fall speeds that favor ice advecting rearward of the transition zone before reaching the melting level, causing a local minimum in precipitation rate and reflectivity there. Sensitivity studies show that the fall speed of ice particles largely determines the location of the enhanced stratiform precipitation region and whether or not a transition zone forms. The representation of microphysical processes, such as rime splintering and aggregation, and ice size distribution shape can impact the mean ice particle fall speeds enough to significantly impact the location of the enhanced stratiform precipitation region and the existence of the transition zone.

Subrainband Structure and Dynamic Characteristics in the Principal Rainband of Typhoon Hagupit (2008)

Abstract The principal rainband in tropical cyclones is currently depicted as a solitary and continuous precipitation region. However, the airborne radar observations of the principal rainband in Typhoon Hagupit (2008) reveal multiple subrainband structures. These subbands possess many characteristics of the squall lines with trailing stratiform in the midlatitudes and are different from those documented in previous principal rainband studies. The updraft and reflectivity cores are upright and elevated. The updraft is fed by a low-level radial outflow from the inner side. The tangential wind speed shows a clear midlevel jet on the inner side of the reflectivity core. Except for the structural similarities, the dynamics of the subbands is also similar to the squall lines. The local environment near the subbands shows little convective inhibition, modest instability, and vertical wind shear. The temperature retrieval shows a cold pool structure in the stratiform precipitation region. The estimated vertical wind shear induced by the cold pool is close to that of the local environment. The structural and dynamic similarities to the squall lines imply that the variation of principal rainbands is subjected to convective-scale dynamics related to the local environment in addition to storm-scale dynamics. The subbands show positive impacts to the vortex intensity in terms of potential vorticity redistribution and absolute angular momentum advection. The positive impacts are closely related to specific structural characteristics of the subbands, which suggests the importance of understanding the convective-scale structure and dynamics of the principal rainband.

Numerical simulation of mesoscale surface pressure features with trailing stratiform squall lines using WRF -ARW model over Gangetic West Bengal region

In the present study, an attempt has been made to investigate the simulation of mesoscale surface pressure patterns like pre-squall mesolow, mesohigh and wake low associated with leading convective line-trailing stratiform (TS) squall lines over Gangetic West Bengal (GWB). For this purpose, a two way interactive triple nested domain with high resolution WRF model having2 km grid length in the innermost domain is used. The model simulated results are compared with the available in-situ observations obtained as a part of Severe Thunderstorm: Observations and Regional Modeling (STORM) programme, reflectivity products of Doppler Weather Radar (DWR) Kolkata and TRMM rainfall. Three TS squall lines (15 May 2009, 5 May 2010 and 7 May 2010) are chosen during pre-monsoon thunderstorm season for this study. The model simulated results of diurnal variation of temperature, relative humidity, wind speed and direction at the station Kharagpur in GWB region reveal a sudden fall in temperature, increase in the amount of relative humidity and sudden rise in wind speed during the arrival of the storms. Such results are well comparable with the observations though there are some leading or lagging of time in respect of actual occurrences of such events. The study indicates that the model is able to predict the occurrences of three typical surface pressure features namely: pre-squall mesolow, meso high and wake low. The predicted surface parameters like accumulated rainfall, maximum reflectivity and vertical profiles (temperature, relative humidity and winds) are well accorded with the observations. The convective and stratiform precipitation region of the TS squall lines are well represented by the model. A strong downdraft is observed to be a contributory factor for formation of mesohigh in the convective region of the squall line. Wake low is observed to reside in the stratiform rain region and the descending dry air at this place has triggered the wake low through adiabatic warming. This study has established the usefulness of the high resolution model in predicting trailing stratiform squall lines and its associated features over the study region.

Ensemble Probabilistic Prediction of a Mesoscale Convective System and Associated Polarimetric Radar Variables Using Single-Moment and Double-Moment Microphysics Schemes and EnKF Radar Data Assimilation

Abstract Ensemble-based probabilistic forecasts are performed for a mesoscale convective system (MCS) that occurred over Oklahoma on 8–9 May 2007, initialized from ensemble Kalman filter analyses using multinetwork radar data and different microphysics schemes. Two experiments are conducted, using either a single-moment or double-moment microphysics scheme during the 1-h-long assimilation period and in subsequent 3-h ensemble forecasts. Qualitative and quantitative verifications are performed on the ensemble forecasts, including probabilistic skill scores. The predicted dual-polarization (dual-pol) radar variables and their probabilistic forecasts are also evaluated against available dual-pol radar observations, and discussed in relation to predicted microphysical states and structures. Evaluation of predicted reflectivity (Z) fields shows that the double-moment ensemble predicts the precipitation coverage of the leading convective line and stratiform precipitation regions of the MCS with higher probabilities throughout the forecast period compared to the single-moment ensemble. In terms of the simulated differential reflectivity (ZDR) and specific differential phase (KDP) fields, the double-moment ensemble compares more realistically to the observations and better distinguishes the stratiform and convective precipitation regions. The ZDR from individual ensemble members indicates better raindrop size sorting along the leading convective line in the double-moment ensemble. Various commonly used ensemble forecast verification methods are examined for the prediction of dual-pol variables. The results demonstrate the challenges associated with verifying predicted dual-pol fields that can vary significantly in value over small distances. Several microphysics biases are noted with the help of simulated dual-pol variables, such as substantial overprediction of KDP values in the single-moment ensemble.

Improvements to the snow melting process in a partially double moment microphysics parameterization

AbstractPolarimetric upgrades to the U.S. radar network have allowed new insight into the precipitation processes of tropical cyclones. Previous work by the authors compared the reflectivity at horizontal polarization and differential reflectivity observations from two hurricanes to simulated radar observations from the WRF model, and found that the aerosol‐aware Thompson‐Eidhammer microphysical scheme performed the best of several commonly used bulk microphysical parameterizations. Here we expand our investigation of the Thompson‐Eidhammer scheme, and find that though it provided the most accurate forecast in terms of wind speed and simulated radar signatures, the scheme produces areas in which the differential reflectivity was much higher than observed. We conclude that the Thompson‐Eidhammer scheme produces drop size distributions that have a larger median drop size than observed in regions of light stratiform precipitation. Examination of the vertical structure of simulated differential reflectivity indicates that the source of the discrepancy between the model and radar observations likely originates within the melting layer. The treatment of number production of rain drops from melting snow in the microphysical scheme is shown to be the ultimate source of the enhancement of differential reflectivity. A modification to the scheme is shown to result in better fidelity of the radar variables with the observations without degrading the short‐term intensity forecast. Additional tests with an idealized squall line simulation are consistent with the hurricane results, suggesting the modification is generally applicable. The modifications to the Thompson‐Eidhammer scheme shown here have been incorporated into updates of the WRF model starting with version 3.8.1.

Investigation of riming within mixed-phase stratiform clouds using Weather Research and Forecasting (WRF) model

In this study, we investigated stratiform precipitation associated with an upper-level westerly trough and a cold front over northern China between 30 Apr. and 1 May 2009. We employed the Weather Research and Forecasting (WRF) model (version 3.4.1) to perform high-resolution numerical simulations of rainfall. We also conducted simulations with two microphysics schemes and sensitivity experiments without riming of snow and changing cloud droplet number concentrations (CDNCs) to determine the effect of snow riming on cloud structure and precipitation. Then we compared our results with CloudSat, Doppler radar and rain gauge observations. The comparison with the Doppler radar observations suggested that the WRF model was quite successful in capturing the timing and location of the stratiform precipitation region. Further comparisons with the CloudSat retrievals suggested that both microphysics schemes overestimated ice and liquid water contents. The sensitivity experiments without riming of snow suggested that the presence or absence of riming significantly influenced the precipitation distribution, but only slightly affected total accumulated precipitation. Without riming of snow, the changes of updrafts from the two microphysics schemes were different due to a different consideration of ice particle capacitance and latent heat effect of riming on deposition. While sensitivity experiments with three different CDNC values of 100, 250 and 1000cm−3 suggested variations in snow riming rates, changing CDNC had little impact on precipitation.

Incorporating Diagnosed Intercept Parameters and the Graupel Category within the ARPS Cloud Analysis System for the Initialization of Double-Moment Microphysics: Testing with a Squall Line over South China

Abstract In this study, a new set of reflectivity equations are introduced into the Advanced Regional Prediction System (ARPS) cloud analysis system. This set of equations incorporates double-moment microphysics information in the analysis by adopting a set of diagnostic relationships between the intercept parameters and the corresponding mass mixing ratios. A reflectivity- and temperature-based graupel classification scheme is also implemented according to a hydrometeor identification (HID) diagram. A squall line that occurred on 23 April 2007 over southern China containing a pronounced trailing stratiform precipitation region is used as a test case to evaluate the impacts of the enhanced cloud analysis scheme. The results show that using the enhanced cloud analysis scheme is able to better capture the characteristics of the squall line in the forecast. The predicted squall line exhibits a wider stratiform region and a more clearly defined transition zone between the leading convection and the trailing stratiform precipitation region agreeing better with observations in general, when using the enhanced cloud analysis together with the two-moment microphysics scheme. The quantitative precipitation forecast skill score is also improved.

Mesoscale Convection and Bimodal Cyclogenesis over the Bay of Bengal

AbstractMesoscale convective systems (MCSs) are an essential component of cyclogenesis, and their structure and characteristics determine the intensity and severity of associated cyclones. Case studies were performed by simulating tropical cyclones that formed during the pre- and postmonsoon periods in 2007 and 2010 over the Bay of Bengal (BoB). The pre- (post) monsoon environment was characterized by the coupling of northwesterly (southwesterly) wind to the early advance southwesterly (northeasterly) monsoonal wind in the BoB. The surges of low-level warm southwesterlies with clockwise-rotating vertical shear in the premonsoon period and moderately cool northeasterlies with anticlockwise-rotating vertical shear in the postmonsoon period transported moisture and triggered MCSs within preexisting disturbances near the monsoon trough over the BoB. Mature MCSs associated with bimodal cyclone formations were quasi linear, and they featured leading-edge deep convection and a trailing stratiform precipitation region, which was very narrow in the postmonsoon cases.In the premonsoon cases, the MCSs became severe bow echoes when intense and moist southwesterlies were imposed along the dryline convergence zone in the northern and northwestern BoB. However, the development formed a nonsevere and nonorganized linear system when the convergence zone was farther south of the dryline. In the postmonsoon cases, cyclogenesis was favored by squall-line MCSs with a north–south orientation over the BoB. All convective systems moved quickly, persisted for a long time, and contained suitable environments for developing low-level cyclonic mesovortices at their leading edges, which played an additional role in forming mesoscale convective vortices during cyclogenesis in the BoB.

Vertical distribution of precipitation particles in Baiu frontal stratiform intense rainfall around Okinawa Island, Japan

AbstractThe vertical distribution of precipitation particles in an intensely precipitating stratiform cloud associated with the Baiu front around Okinawa Island was observed. X‐band polarimetric radar, disdrometer, and hydrometeor videosonde data were used to examine the precipitation processes. The cloud top was approximately 12 km above sea level, as convection was depressed while stratiform regions developed near Okinawa Island. In the rain region below 3 km, the mean median volume diameter of the raindrop size distribution (DSD) estimated from the radar variables was 1.55 mm, and the mean normalized intercept parameter was 104.12 mm−1 m−3 with a mean radar reflectivity of 40.5 dBZe. The DSD indicates that the stratiform precipitation was characterized by higher number concentrations of smaller drops than observed previously in convective cells in a Baiu frontal convective precipitation region around Okinawa Island. The DSD also suggests the presence of larger raindrops than in convective cells embedded in a Baiu frontal stratiform precipitation region around Okinawa Island. In the ice region at 5–6 km, just above the melting layer and 6 km below the cloud top, the differential reflectivity and specific differential phase showed positive values, and videosonde measurements revealed that the number concentration of column‐, plate‐, and capped‐column‐like crystals (maximum dimensions of ≥0.1 mm) was 112 L−1. The high number concentration of these crystals contributed to the intense stratiform rainfall associated with the Baiu front.

Effect of dry large‐scale vertical motions on initial MJO convective onset

AbstractAnomalies of eastward propagating large‐scale vertical motion with ~30 day variability at Addu City, Maldives, move into the Indian Ocean from the west and are implicated in Madden‐Julian Oscillation (MJO) convective onset. Using ground‐based radar and large‐scale forcing data derived from a sounding array, typical profiles of environmental heating, moisture sink, vertical motion, moisture advection, and Eulerian moisture tendency are computed for periods prior to those during which deep convection is prevalent and those during which moderately deep cumulonimbi do not form into deep clouds. Convection with 3–7 km tops is ubiquitous but present in greater numbers when tropospheric moistening occurs below 600 hPa. Vertical eddy convergence of moisture in shallow to moderately deep clouds is likely responsible for moistening during a 3–7 day long transition period between suppressed and active MJO conditions, although moistening via evaporation of cloud condensate detrained into the environment of such clouds may also be important. Reduction in large‐scale subsidence, associated with a vertical velocity structure that travels with a dry eastward propagating zonal wavenumbers 1–1.5 structure in zonal wind, drives a steepening of the lapse rate below 700 hPa, which supports an increase in moderately deep moist convection. As the moderately deep cumulonimbi moisten the lower troposphere, more deep convection develops, which itself moistens the upper troposphere. Reduction in large‐scale subsidence associated with the eastward propagating feature reinforces the upper tropospheric moistening, helping to then rapidly make the environment conducive to formation of large stratiform precipitation regions, whose heating is critical for MJO maintenance.

Hindcast experiments of the derecho in Estonia on 08 August, 2010: Modelling derecho with NWP model HARMONIE

On August 8, 2010, a derecho swept over Northern Europe, causing widespread wind damage and more than 2 million Euros in economic loss in Estonia during its most destructive stage. This paper presents a modelling study of the derecho-producing storm utilising the Hirlam Aladin Research for Mesoscale Operational Numerical Weather Prediction in Europe (HARMONIE) model. The model setup is chosen to mimic near-future, nearly kilometre-scale, operational environments in European national weather services. The model simulations are compared to remote sensing and in situ observations. The HARMONIE model is capable of reproducing the wind gust severity and precipitation intensity. Moreover, 2.5-km grid spacing is shown to be sufficient for producing a reliable signal of the severe convective storm. Storm dynamics are well simulated, including the rear inflow jet. Although the model performance is promising, a strong dependence on the initial data, a weak trailing stratiform precipitation region and an incorrect timing of the storm are identified.

Ensemble Kalman Filter Analyses and Forecasts of a Severe Mesoscale Convective System Using Different Choices of Microphysics Schemes

AbstractA Weather Research and Forecasting Model (WRF)-based ensemble data assimilation system is used to produce storm-scale analyses and forecasts of the 4–5 July 2003 severe mesoscale convective system (MCS) over Indiana and Ohio, which produced numerous high wind reports across the two states. Single-Doppler observations are assimilated into a 36-member, storm-scale ensemble during the developing stage of the MCS with the ensemble Kalman filter (EnKF) approach encoded in the Data Assimilation Research Testbed (DART). The storm-scale ensemble is constructed from mesoscale EnKF analyses produced from the assimilation of routinely available observations from land and marine stations, rawinsondes, and aircraft, in an attempt to better represent the complex mesoscale environment for this event. Three EnKF simulations were performed using the National Severe Storms Laboratory (NSSL) one- and two-moment and Thompson microphysical schemes. All three experiments produce a linear convective segment at the final analysis time, similar to the observed system at 2300 UTC 4 July 2003. The higher-order schemes—in particular, the Thompson scheme—are better able to produce short-range forecasts of both the convective and stratiform components of the observed bowing MCS, and produce the smallest temperature errors when comparing surface observations and dropsonde data to corresponding model data. Only the higher-order microphysical schemes produce any appreciable rear-to-front flow in the stratiform precipitation region that trailed the simulated systems. Forecast performance by the three microphysics schemes is discussed in context of differences in microphysical composition produced in the stratiform precipitation regions of the rearward expanding MCSs.

Origin and Maintenance of the Long-Lasting, Outer Mesoscale Convective System in Typhoon Fengshen (2008)

Abstract Outer mesoscale convective systems (OMCSs) are long-lasting, heavy rainfall events separate from the inner-core rainfall that have previously been shown to occur in 22% of western North Pacific tropical cyclones (TCs). Environmental conditions accompanying the development of 62 OMCSs are contrasted with the conditions in TCs that do not include an OMCS. The development, kinematic structure, and maintenance mechanisms of an OMCS that occurred to the southwest of Typhoon Fengshen (2008) are studied with Weather Research and Forecasting Model simulations. Quick Scatterometer (QuikSCAT) observations and the simulations indicate the low-level TC circulation was deflected around the Luzon terrain and caused an elongated, north–south moisture band to be displaced to the west such that the OMCS develops in the outer region of Fengshen rather than spiraling into the center. Strong northeasterly vertical wind shear contributed to frictional convergence in the boundary layer, and then the large moisture flux convergence in this moisture band led to the downstream development of the OMCS when the band interacted with the monsoon flow. As the OMCS developed in the region of low-level monsoon westerlies and midlevel northerlies associated with the outer circulation of Fengshen, the characteristic structure of a rear-fed inflow with a leading stratiform rain area in the cross-line direction (toward the south) was established. A cold pool (Δθ < −3 K) associated with the large stratiform precipitation region led to continuous formation of new cells at the leading edge of the cold pool, which contributed to the long duration of the OMCS.

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.

Surface mesoscale features associated with leading convective line-trailing stratiform squall lines over the Gangetic West Bengal

In this paper an attempt is made to identify the mesoscale features in surface pressure pattern, if any, associated with thunderstorm over the Gangetic West Bengal region in India. The study was conducted over Kharagpur and the adjoining area in the Gangetic West Bengal, frequently affected by thunderstorms during the pre-monsoon seasons of April–May. Observations recorded at 50 m instrumented micro-meteorological tower and upper air sounding at Kharagpur under nationally coordinated Severe Thunderstorm Observations and Regional Modeling (STORM) Programme are used to study the variation in surface pressure, wind speed and direction, temperature and relative humidity associated with the squall lines with trailing stratiform precipitation region. In the surface pressure variation, pre-squall mesolow, mesohigh and wake low are identified with the passage of the squall line at Kharagpur. It is observed that in the squall line with trailing stratiform precipitation shield, the mesohigh is associated with convective line and wake low exists at the rear of the storms. The position of the mesohigh is typically found in the vicinity of the heavy rain directly beneath the downdraft. The mesohigh seems to be initiated by the cooling due to evaporation of precipitation in the downdraft and intensified due to the non-hydrostatic effect because of the rainfall directly beneath the downdraft. It is also observed that the passage of trailing edges of the stratiform precipitation coincided with the wake low. Upper air sounding shows mid-tropospheric cooling and lower tropospheric warming. It may be possible due to the dominance of evaporative cooling in the mid-levels and dynamically forced descending motion leading to adiabatic warming in the low levels which may lead to the formation of the wake low.

Correction of Radar QPE Errors for Nonuniform VPRs in Mesoscale Convective Systems Using TRMM Observations

Abstract Mesoscale convective systems (MCSs) contain both regions of convective and stratiform precipitation, and a bright band (BB) is often found in the stratiform region. Inflated reflectivity intensities in the BB often cause positive biases in radar quantitative precipitation estimation (QPE). A vertical profile of reflectivity (VPR) correction is necessary to reduce such biases. However, existing VPR correction methods for ground-based radars often perform poorly for MCSs owing to their coarse resolution and poor coverage in the vertical direction, especially at far ranges. Spaceborne radars such as the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR), on the other hand, can provide high resolution VPRs. The current study explores a new approach of incorporating the TRMM VPRs into the VPR correction for the Weather Surveillance Radar-1988 Doppler (WSR-88D) radar QPE. High-resolution VPRs derived from the Ku-band TRMM PR data are converted into equivalent S-band VPRs using an empirical technique. The equivalent S-band TRMM VPRs are resampled according to the WSR-88D beam resolution, and the resampled (apparent) VPRs are then used to correct for BB effects in the WSR-88D QPE when the ground radar VPR cannot accurately capture the BB bottom. The new scheme was tested on six MCSs from different regions in the United States and it was shown to provide effective mitigation of the radar QPE errors due to BB contamination.