Deep convection overshooting the lowermost stratosphere is well known for its role in the local stratospheric water vapour (WV) budget. While it is seldom the case, local enhancement of WV associated with stratospheric overshoots is often published. Nevertheless, one debatable topic persists regarding the global impact of this event with respect to the temperature-driven dehydration of air parcels entering the stratosphere. As a first step, it is critical to quantify their role at a cloud-resolving scale before assessing their impact on a large scale in a climate model. It would lead to a nudging scheme for large-scale simulation of overshoots.
This paper reports on the local enhancements of WV linked to stratospheric overshoots, observed during the TRO-Pico campaign conducted in March 2012 in Bauru, Brazil, using the BRAMS (Brazilian version of the Regional Atmospheric Modeling System; RAMS) mesoscale model. Since numerical simulations depend on the choice of several preferred parameters, each having its uncertainties, we vary the microphysics or the vertical resolution while simulating the overshoots. Thus, we produce a set of simulations illustrating the possible variations in representing the stratospheric overshoots. To better resolve the stratospheric hydration, we opt for simulations with the 800 m horizontal-grid-point presentation. Next, we validate these simulations against the Bauru S-band radar echo tops and the TRO-Pico balloon-borne observations of WV and particles. Two of the three simulations' setups yield results compatible with the TRO-Pico observations. From these two simulations, we determine approximately 333–2000 t of WV mass prevailing in the stratosphere due to an overshooting plume depending on the simulation setup. About 70 % of the ice mass remains between the 380 and 385 K isentropic levels. The overshooting top comprises pristine ice and snow, while aggregates only play a role just above the tropopause. Interestingly, the horizontal cross section of the overshooting top is about 450 km
Water vapour (WV) concentrations in the stratosphere impact both chemistry
The tropical tropopause layer serves as a gate where water enters the stratosphere
One identified factor is the deep convection in the tropics, overshooting the stratosphere. It injects ice particles directly above the tropopause, which may experience partial sublimation before falling back to the troposphere. Consequently, the net effect should be hydration that mitigates the large-scale dehydration effect. Recently many case studies, based both on modelling
In recent years, studies suggest that deep convection reaching the tropopause may influence the stratospheric WV budget on a large scale. Subsequently, the deep convection is now a part of trajectory domain-filling studies of stratospheric WV distribution
Another potential strategy is to upscale stratospheric overshooting effects by forcing them into a large-scale simulation, where the overshoots are explicitly resolved in cloud-resolving numerical simulations. However, cloud-resolving simulation studies of several cases must be conducted before proceeding with this phase. The combined study of results corroborated by observations would encourage a stratospheric overshoot nudging strategy in a larger-scale or Brazilian size simulation. Furthermore, utilising the superparameterisation method
Here, we perform three simulations of an observed case of stratospheric overshoots using the BRAMS (Brazilian version of the Regional Atmospheric Modeling System; RAMS) mesoscale model. They are different from each other in terms of the microphysical setup or the vertical grid structure. As a result, this study generates a variety of estimations for ice injection into the stratosphere and water remaining after sublimation. We use the data from a well-documented case on 13 March 2012 in Bauru, São Paulo state, Brazil, during the TRO-Pico, a small balloon campaign
The paper is organised as follows: Sect.
TRO-Pico is a French initiative based on a small balloon campaign in Bauru (22.36
Pico-SDLA is an infrared laser hygrometer emitting at 2.61
Before discussing the details of the observations, we summarise the meteorological conditions on 13 March 2012, in the central region of the state of São Paulo. This day was after the peak of the rainy season, with frequent heavy thunderstorms. There was no noticeable deep convective activity around Bauru before local noon (15:00 UT). The synoptic situation during the entire day exhibited an extremely weak pressure gradient across all of São Paulo, with very light westerly winds in the mid-levels of the troposphere. Nonetheless, a vigorous thermodynamic instability prevailed throughout that afternoon. At IPMet in Bauru, convective available potential energy (CAPE) values of 4000 J kg
On 13 March 2012, a flight train comprising Pico-SDLA and LOAC sensors was launched at 20:20 UT under a 500 m
The trajectory study of
This modelling study benefits from the echo tops product of convective systems observed by the Doppler S-band radar, located at IPMet/UNESP in Bauru. It facilitates the validation of our simulations. The echo top measurements depend highly on the technical specifications of the radar, such as wavelength, beam width, pulse width (PW), pulse repetition frequency (PRF), and radial and azimuth resolution. In the case of Bauru S-band radar, the beam width is 2
BRAMS, version 4.2, maintained at Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)
In Eq. (
Furthermore, using a smart grid-nesting system that solves equations simultaneously between computational meshes while applying any number of two-way interactions, the BRAMS/RAMS can solve the fully compressible non-hydrostatic equations
We use the BRAMS model to run three cloud-resolving simulations, including multiple grid-nesting to explicitly address the stratospheric overshoots associated with the case study in Sect.
Following that, we run three simulations with a spatial resolution of 800 m
REF, NU21, and HVR comprise the grid-nesting system of three grids holding the same grid positions and the same horizontal grid-point presentation. The horizontal grid-point resolution increases from 20 km, parent grid, to 4 km in the second grid and 800 m in the third grid. The parent grid encompasses a large part of southern Brazil with a domain of 1840 km
Each simulation begins at 12:00 UT on 12 March 2012, and ends 48 h later. To reduce computing costs, we activated the third grid only at 10:00 UT on 13 March and recorded model outputs every 7.5 min after that. This data record frequency corresponds to the volume scans produced by the IPMet S-band radar. These are used to validate the cloud-top models. To ensure numerical stability, the simulation integration time step varies between 2 and 10 s for the coarsest grid. It is 5 times smaller for the second grid and 25 times lower for the third grid. Invoking the radiation module has a time resolution of 300–500 s. The ECMWF operational analyses with 1.0
REF, NU21, and HVR simulations deviate from each other over the following points.
The shape parameter HVR differs from REF with respect to the vertical grid-point resolution in the TTL. REF has 68 vertical levels with about 300 m resolution within the TTL, whereas HVR has 99 vertical levels with typically 150 m vertical resolution within the TTL, except at the tropopause level where it is 100 m. Unlike REF and NU21, HVR is carried out entirely at the higher vertical resolution starting at 12:00 UT on 12 March 2012. In the BRAMS model, it is unfeasible to change the vertical grid structure in the middle of the integration of simulation unless each layer in REF would correspond to a layer in HVR, which is not the case here.
We validate the three BRAMS simulations using observations from the S-band radar of IPMet, located in Bauru, and the balloon-borne measurements of the TRO-Pico campaign, respectively. Note that the balloon-borne measurements are part of the first IOP phase of the 2-year field campaign.
We examine the BRAMS model's capacity to initiate and describe deep convection activity at an accurate time and location by comparing simulated outputs to S-band radar data. To do so, we estimate the modelled cloud-top layers every 1 km at altitudes ranging from 9 to 20 km, much like the echo top products. We determine the modelled cloud-top height for this altitude range if the concentration of condensed water, i.e. ice plus liquid, exceeds a specified mixing ratio threshold within a specific layer. The cloud-top altitude assignment for a given (
Snapshots of echo tops, observed by the S-band radar and modelled cloud tops from the BRAMS simulations on 13 March 2012, centred at Bauru.
Figure
The full time series of the comparison between the modelled cloud tops and the S-band radar echo tops is in the Supplement (animation of cloud tops) every 7.5 min from 15:01 to 18:52 UT on 13 March 2012. Figure
Count of overshoots above 17 km altitude for the S-band radar (end time UT of the volume scan) and for the REF, NU21, and HVR simulations. Their counts are represented as multiples of
However, during the period 15:00–18:30 UT on 13 March 2012, within a 100 km radius of Bauru, we tabulate (Table
To further understand the situation, one can expect HVR to determine more reliable dynamics across the tropical tropopause than REF and NU21, respectively. Contrary to expectations, it tends to intensify massive deep convection activity. A plausible fact to explain such behaviour in HVR is the ratio between vertical and horizontal grid points, which overestimates vertical motions due to grid cell saturation
In Sect.
The WV and particle measurements performed in the vicinity of overshoots in the frame of the TRO-Pico campaign establish a well-documented database to validate model simulations. For our study, as the balloon-borne measurements belong to a moment several hours after the overshooting event – this time interval between the overshooting event and the balloon-borne measurements is indicated as
To validate the local WV enhancement at 17.2 km altitude due to the modelled overshoots, we combine the TRO-Pico measurements by FLASH-B at 23:45 UT corresponding to an overshooting event that occurred at 16:46 UT with
BRAMS simulation: REF total water content, ice, liquid, and vapour in g kg
Figure
REF providing total water (ice, liquid, and vapour) enhancement at
Figure
Then, we implement the same strategy to validate the hydration due to overshoot at 17.8 km altitude; see Fig.
The purpose of the investigation is to witness the same order
With the same validation approach, as in REF, we select the overshooting plume that occurred at 16:15 UT in NU21. We study the time evolution of the overshooting plume at 17.2 km altitude from 16:15 UT to (16:15
Like Fig.
In Fig.
In Sect.
Thus, this study brings to the fore that fine-scale simulations using the BRAMS model can reproduce the overshooting convection. Both REF and NU21 can lead now to more insight into the overshooting plumes within unorganised deep convective plumes. Certain standard features like the amount of ice injection, width and surface area of the plume,
We provide the five conceivable combinations of hydrometeors inside an overshooting plume to document the quantitative information collected from the simulations on the structural characteristics of a typical overshooting plume. Its base is at the 380 K isentropic level, which is the stratosphere's lowest layer. At the 380 K isentropic level, the instantaneous mass flux of individual hydrometeors is also estimated. Between the 380 and 430 K isentropic levels, it comprises the estimation of total ice mass and the five types of ice particles. Finally, a table provides the quantities that could lead to a road map of a nudging scheme of the water vapour enhancement in the lower stratosphere due to overshoots in large-scale simulations, which could lead to the quantification of the influence of overshoots on a large scale.
We assess all the five types of ice hydrometeors during an overshooting event. The series of plots in Fig.
Vertical distribution of horizontal cross section of hydrometeors, viz., snow, pristine ice, graupel, and aggregates, within the third grid, spanning over 15–19 km altitude. It is for the ratio of four types of ice hydrometeors against the entire ice content from REF – upper panel, and NU21 – lower panel, shown at 16:15 UT. Hail is not included because of its negligible values within the plume.
Above the tropopause, we find pristine ice and snow to be the primary ice hydrometeors (
Size of the overshooting plumes at the 380 K isentropic level, shown for REF
The contact area or spreading (km
Furthermore, we compare the horizontal spreading between REF and NU21. In Fig.
We estimate each hydrometeor's instantaneous mass-flux rate across the 380 K isentropic level. The rates are the average over the domain that comprises only the third grid of simulation. Please note that it is not representative of a property of any particular overshooting plume but preferably addresses a realistic estimation on the flux rates of ice particles entering the 380 K isentropic layer. Besides, we evaluate the net
Figure
The instantaneous domain-average mass-flux rate (g m
Figure
Water mass budget (ice and water vapour) for
In both simulations, the total
In REF, we explain the peak of total water content at 16:22 UT with the last two overshooting events that occurred at 16:15 UT (refer to Fig.
Moreover, we determine the standard amount of hydration for each overshoot, providing both the upper and lower limits by reflecting the two extreme cases on the fate of ice. As such, (1) the upper limit would assume all the remaining ice sublimates in the stratosphere, and (2) the lower limit would indicate all the remaining ice is falling back to the troposphere without sublimating at all. The upper limit is about 8 kt
Figure
To get quantitative information on the mass distribution of five different types of ice hydrometeors within the overshooting plumes constrained within the thin layer of 380 to 430 K isentropes (see Fig.
Mass (%) of individual ice hydrometeors within the 380 to 385 K isentropic layer
One of the major inferences drawn from Table
This paper describes several cloud-resolving simulations of convective overshoots penetrating the lower stratosphere using the BRAMS mesoscale model, corresponding to an observed case on 13 March 2012, during the TRO-Pico field campaign in Bauru, Brazil. During this series of overshooting convection events, several plumes reached the stratosphere. As a result, it accounts for the hydration heterogeneity produced by overshoots of variable intensity, even when they occur under similar circumstances (e.g. stratospheric humidity). The S-band radar stationed at Bauru, as well as the balloon-borne measurements from this campaign, allow the simulation results to be validated. These simulations, which have been validated as realistic when compared to TRO-Pico measurements, are then used to obtain the main physical characteristics of overshooting plumes.
The main results are as follows.
Primarily, the simulated overshooting plume reaching the lower stratosphere comprises pristine ice and snow, and to some degree aggregates but only at the base, the 380 K isentropic level. The cross section of the overshoots at the 380 K isentropic level is about 450 km Within the limited layer of 380 to 385 K, 68 % of the overall ice mass exists. It also suggests that the remaining 32 % of ice (mostly pristine ice and snow) moves higher in the stratosphere. Because of the very slow fall speed at altitudes above 385 K and the subsaturated conditions with respect to ice, that 32 %, which is pristine ice and snow, is anticipated to stay in the stratosphere and sublimate. A single overshooting plume injects around 4.3 kt of ice in REF and 4.0 kt of ice in NU21 over the 380 K level in this given scenario in Bauru, with NU21 injecting slightly less ice than REF as expected. The stratospheric WV enhancement due to one overshooting event is estimated to range between 1.34–2 kt as the upper limit and 0.34–0.75 kt as the lower limit after sublimation and (or) sedimentation of the stratospheric ice. If we consider complete sublimation of ice, as in REF, it confirms our estimate that the 32 % of 4.3 kt of ice irreversibly travelling further up to the stratosphere results in the stratosphere having the lowest hydration in the upper limit range.
These data can be utilised to develop a nudging method that quantifies the influence of overshooting convection on the stratospheric water vapour using a low-cost large-scale simulation. Though the findings are limited to a case study in Brazil and may not be generalisable, more similar case studies should be conducted in order to gain a better knowledge of the events, and this work is keeping with that goal. This instance would be the next stage in the current research, offering a road map for extending the impact of overshooting convection on stratospheric water vapour on a continental (Brazilian) scale.
All TRO-Pico measurements are publicly available at
Two videos are provided for the time series analysis made every 7.5 min in the Supplement: one for the modelled cloud tops and corresponding S-band radar echo tops; the second one for the vertical distribution of horizontal cross section of different hydrometeors within the overshooting plume. The supplement related to this article is available online at:
EDR and AKB conceptualised the study design, methodology, validation, and analysis. JB provided the support to run BRAMS in different high-performance computing (HPC) machines, and EDR provided the resources to achieve the simulations. AKB and EDR wrote the original draft, and all authors reviewed the paper. EDR and VM received the funding for this research. SMK provided the FLASH measurements, and MG provided the Pico-SDLA measurements. GH provided the meteorology and interpretation of S-band radar data. All authors have read and agreed on the published version of the paper.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This study is based on a case observation of the TRO-Pico campaign. TRO-Pico is a French ANR-funded project (
The article processing charges for this open-access publication were covered by the Centre National de la Recherche Scientifique (CNRS)'s LEFE-IMAGO project “Parashoots”.
This paper was edited by Timothy J. Dunkerton and reviewed by three anonymous referees.