Cold pools are fundamental ingredients of deep convection. They contribute to organizing the sub-cloud layer and are considered key elements in triggering convective cells. It was long known that this could happen mechanically, through lifting by the cold pools' fronts. More recently, it has been suggested that convection could also be triggered thermodynamically, by accumulation of moisture around the edges of cold pools. A method based on Lagrangian tracking is here proposed to disentangle the signatures of both forcings and quantify their importance in a given environment. Results from a simulation of radiative-convective equilibrium over the ocean show that parcels reach their level of free convection through a combination of both forcings, each being dominant at different stages of the ascent. Mechanical forcing is an important player in lifting parcels from the surface, whereas thermodynamic forcing reduces the inhibition encountered by parcels before they reach their level of free convection.

Mechanisms for convection triggering by cold pools

Cold‐pool‐driven convective initiation is investigated in high‐resolution, convection‐permitting simulations with a focus on the diurnal cycle and organization of convection and the sensitivity to grid size. Simulations of four different days over Germany were performed using the ICON‐LEM model with grid sizes from 156 to 625 m. In these simulations, we identify cold pools, cold‐pool boundaries and initiated convection. Convection is triggered much more efficiently in the vicinity of cold pools than in other regions and can provide as much as 50% of total convective initiation, in particular in the late afternoon. By comparing different model resolutions, we find that cold pools are more frequent, smaller and less intense in lower‐resolution simulations. Furthermore, their gust fronts are weaker and less likely to trigger new convection. To identify how model resolution affects this triggering probability, we use a linear causal graph analysis. In doing so, we postulate a graph structure with potential causal pathways and then apply multi‐linear regression accordingly. We find a dominant, systematic effect: reducing grid sizes directly reduces upward mass flux at the gust front, which causes weaker triggering probabilities. These findings are expected to be even more relevant for km‐scale, numerical weather prediction models. We thus expect that a better representation of cold‐pool‐driven convective initiation will improve forecasts of convective precipitation.

/pdf/cold-pool-driven-convective-initiation-using-causal-graph-zwk2y5eqby.pdf

Cold-pool-driven convective initiation: using causal graph analysis to determine what convection-permitting models are missing

This study has addressed the spatiotemporal distribution of the daily rainfall concentration and its relation to the teleconnection patterns across the Mediterranean (MR). Daily concentration index (CI) and the ordered n index (nor) are used at annual time scale to reveal the statistical structure of precipitation across the MR based on 233 daily rainfall series for the period 1975–2015. Eight teleconnection patterns, North Atlantic Oscillation (NAO), Mediterranean Oscillation (MO), Western Mediterranean Oscillation (WeMO), Upper-Level Mediterranean Oscillation index (ULMO), East Atlantic (EA) pattern, East Atlantic/West Russia (EATL/WRUS) pattern, Scandinavia (SCAND) pattern and Southern Oscillation (SO) at annual time scale are selected. The spatiotemporal patterns in precipitation concentration indices, annual precipitation and their teleconnections with previous large-scale circulations are investigated. Results show a strong connection between the CI and the nor (r = 0.70, p <.05) which present the same relative areas of high and low concentration. The annual values range from 0.57 to 0.70 for CI and 0.49 to 0.71 for nor index which show a high daily precipitation concentration across the MR. Trend analysis demonstrated mostly significant increasing trends for both indices. This increase is mainly found in south France, northern coastlands of the Iberian Peninsula (IP), Greece and Tunisia. An inverse relationship between the number of rainy days and concentration indices is evident. Both of WeMO and MO can play an important role in modulating rainfall in the northwest Mediterranean. The positive EATL/WRUS phase is mainly connected with positive precipitation mean anomalies in the eastern Mediterranean and vice versa in the west. The high daily precipitation concentration values over south France, northeast Spain, Croatia and Tunisia are linked to the low values of WeMO and high values of EA. These results could pave the way for new possibilities regarding the projection of precipitation concentration and precipitation irregularity in downscaling techniques.

Spatiotemporal variability of daily precipitation concentration and its relationship to teleconnection patterns over the Mediterranean during 1975–2015

Cold pools (CPs) contribute to convective organization. However, it is unclear by which mechanisms organization occurs. By using a particle method to track CP gust fronts in large eddy simulations, we characterize the basic collision modes between CPs. Our results show that CP interactions, where three expanding gust fronts force an updraft, are key at triggering new convection. Using this, we conceptualize CP dynamics into a parameter‐free mathematical model: circles expand from initially random points in space. Where two expanding circles collide, a stationary front is formed. However, where three expanding circles enclose a single point, a new expanding circle is seeded. This simple model supports three fundamental features of CP dynamics: precipitation cells constitute a spatially interacting system; CPs come in generations; and scales steadily increase throughout the diurnal cycle. Finally, this model provides a framework for how CPs act to cause convective self‐organization, clustering, and extremes.

/pdf/circling-in-on-convective-organization-3zegxe21jj.pdf

Circling in on Convective Organization

Forced mechanical lifting through cold pool gust fronts can trigger new convection and, as previous work highlights, is enhanced when cold pools collide. However, as shown by conceptual models, the organization of the convective cloud field emerging from two versus three colliding cold pools differs strongly. In idealized dry large-eddy simulations we therefore compare collisions between two and three cold pools. The triggering likelihood is quantified in terms of the cumulative vertical mass flux of boundary layer air and the instantaneous updraft strength, generated at the cold pool gust fronts. We find that cold pool expansion can be well described by initial potential energy alone. Cold pool expansion monotonically slows but shows an abrupt transition between an axisymmetric and a broken-symmetric state mirrored by a sudden drop in expansion speed. We characterize these two dynamic regimes by two distinct power law exponents and explain the transition by the onset of "lobe-and-cleft" instabilities at the cold pool head. Two-cold pool collisions produce the strongest instantaneous updrafts in the lower boundary layer, which we expect to be important in environments with strong convective inhibition. Three-cold pool collisions generate weaker but deeper updrafts and the strongest cumulative mass flux and are thus predicted to induce the largest midlevel moistening, which has been identified as a precursor for the transition from shallow to deep convection. Combined, our findings may help decipher the role of cold pools in spatially organizing convection and precipitation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020MS002281

Mechanical Forcing of Convection by Cold Pools: Collisions and Energy Scaling.

This paper presents a new set of numerical simulations of
two colliding density currents in a idealized framework, integrating
the Boussinesq vorticity equation in a rectangular
bounded domain. These simulations are used to examine the
dynamical features of the collision, in the light of recent laboratory
experiments. The collision dynamics present various
interesting features. Here we have focused on the interface
slope at the front of the two unequal density currents and on
the maximum height reached by the
fluid after the collision.
For the secondary triggering of atmospheric convection by
colliding cold pools from previous convective events, these
may affect the positioning and the momentum of the collision
uplift, respectively.
The interface slope has been shown to be dependent
on the currents’ buoyancy ratio (i.e. the ratio between the
density differences of the two
fluids with the ambient
fluid)
,
whereas the maximum height has no strong dependence,
for a given initial current depth. A theoretical model, based
on an analogy with a vortex pair, has been proposed for the
interface-slope dependence, taking as input the buoyancy
ratio or the propagating speeds. This model agrees reasonably
well with the observed numerical values.

/pdf/characteristics-of-colliding-density-currents-a-numerical-4ome3o8wwy.pdf

Characteristics of colliding density currents: a numerical and theoretical study

Dynamical downscaling of ensemble forecasts to convectionpermitting resolutions aims to improve forecast skill by explicitly
resolving mesoscale dynamical features. The success of this approach is dependent on the ability of the model to spin up smaller
features embedded in the larger scale flow and provide more local information than could be inferred from knowledge of the
climatological response to the large scale flow alone. Here we
test whether such additional information is obtained from the
Met Office Global and Regional Ensemble Prediction Systems
(MOGREPS) for the sea-breeze phenomena which is resolved
properly only at convection-permitting resolutions but is driven
by large-sale conditions that are well represented in the global
driving model. The sea breeze is detected using a new automatic
tracking algorithm suitable for use in convective-scale forecast
data. The skill of probabilistic forecasts of sea-breeze occurrence from the high resolution ensemble is compared to that of a
Bayesian forecast trained on paired high/low resolution ensemble
members. This creates a statistical forecast of the high-resolution
ensemble member given knowledge of the global forecast ensemble alone. The aim of this paper is twofold: -firstly to assess
whether the information about sea breeze occurrence is encoded
in a few large-scale parameters and can be forecast by a statistical
method ; secondly to estimate what information is gained by running the high resolution forecast beyond that which is contained
in these large-scale flow parameters. Comparison of the two
forecasting methods using a variety of verification methods all
lead to the same conclusion: although both the Bayesian forecast
and convection-permitting ensemble provide information about
sea-breeze occurrence, the convection-permitting ensemble is
significantly more able to discriminate between sea-breeze events
and non-events for all lead times.

/pdf/the-added-value-of-convection-permitting-ensemble-forecasts-34zhqqhzsv.pdf

The added value of convection-permitting ensemble forecasts of sea breeze compared to a Bayesian forecast driven by the global ensemble

Convection-permitting ensemble prediction systems (CP-ENS) have been implemented in the mid-latitudes for weather forecasting timescales over the past decade, enabled by the increase in computational resources. Recently, efforts are being made to study the benefits of CP-ENS for tropical regions. This study examines CP-ENS forecasts produced by the UK Met Office over tropical East Africa, for 24 cases in the period April-May 2019. The CP-ENS, an ensemble with parametrized convection (Glob-ENS), and their deterministic counterparts are evaluated against rainfall estimates derived from satellite observations (GPM-IMERG). The CP configurations have the best representation of the diurnal cycle, although heavy rainfall amounts are overestimated compared to observations. Pairwise comparisons between the different configurations reveal that the CP-ENS is generally the most skilful forecast for both 3-h and 24-h accumulations of heavy rainfall (97th percentile), followed by the CP deterministic forecast. More precisely, probabilistic forecasts of heavy rainfall, verified using a neighbourhood approach, show that the CP-ENS is skilful at scales greater than 100 km, significantly better than the Glob-ENS, although not as good as found in the mid-latitudes. Skill decreases with lead time and varies diurnally, especially for CP forecasts. The CP-ENS is under-spread both in terms of forecasting the locations of heavy rainfall and in terms of domain-averaged rainfall. This study demonstrates potential benefits in using CP-ENS for operational forecasting of heavy rainfall over tropical Africa and gives specific suggestions for further research and development, including probabilistic forecast guidance.

/pdf/do-convection-permitting-ensembles-lead-to-more-skillful-1xzbpgy8ds.pdf

Do Convection-Permitting Ensembles Lead to More Skillful Short-Range Probabilistic Rainfall Forecasts over Tropical East Africa?

Forecasting rainfall in the tropics is a major challenge for numerical weather prediction. Convection-permitting (CP) models are intended to enable forecasts of high-impact weather events. Development and operation of these models in the tropics has only just been realised. This study describes and evaluates a suite of recently developed Met Office Unified Model CP ensemble forecasts over three domains in Southeast Asia, covering Malaysia, Indonesia and the Philippines.

Fractions Skill Score is used to assess the spatial scale-dependence of skill in forecasts of precipitation during October 2018 - March 2019. CP forecasts are skilful for 3-hour precipitation accumulations at spatial scales greater than 200 km in all domains during the first day of forecasts. Skill decreases with lead time but varies depending on time of day over Malaysia and Indonesia, due to the importance of the diurnal cycle in driving rainfall in those regions. Skill is largest during daytime when precipitation is over land and is constrained by orography. Comparison of CP ensembles using 2.2, 4.5 and 8.8 km grid spacing and an 8.8km ensemble with parameterised convection reveals that varying resolution has much less effect on ensemble skill and spread than the representation of convection. The parameterised ensemble is less skilful than CP ensembles over Malaysia and Indonesia and more skilful over the Philippines; however, the parameterised ensemble has large drops in skill and spread related to deficiencies in its diurnal cycle representation. All ensembles are under-spread indicating that future model development should focus on this issue.

Evaluating Convection-Permitting Ensemble Forecasts of Precipitation over Southeast Asia

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Papers

Characteristics of colliding density currents: a numerical and theoretical study

The added value of convection-permitting ensemble forecasts of sea breeze compared to a Bayesian forecast driven by the global ensemble

Do Convection-Permitting Ensembles Lead to More Skillful Short-Range Probabilistic Rainfall Forecasts over Tropical East Africa?

Evaluating Convection-Permitting Ensemble Forecasts of Precipitation over Southeast Asia