Modelling the High Mercury Wet deposition in the Southeastern US by WRF-GC

High mercury wet deposition in southeastern United States has been noticed for many years. Previous studies came up with a theory that it was associated with high-altitude divalent mercury scavenged by convective precipitation. Given the coarse resolution of previous models (e.g. GEOS-Chem), this theory is still not fully tested. Here we employed a newly developed WRF-GEOS-Chem (WRF-GC) model implemented with mercury simulation. We conduct extensive model benchmarking by comparing WRF-GC with different resolutions (from 50 km to 25 km) to GEOS-Chem output (4° × 5°) and data from Mercury Deposition Network (MDN) in July–September 2013. The comparison of mercury wet deposition from two models both present high mercury wet deposition in southeastern United States. We divided simulation results by heights, different types of precipitation and combination of these two variations together and find most of mercury wet deposition concentrates on higher space and caused by convective precipitation. Therefore, we conclude that it is the deep convection caused enhanced mercury wet deposition in the southeastern United States.

Large-eddy simulations of convection initiation over heterogeneous, low terrain

Large-eddy simulations are conducted to investigate and physically interpret the impacts of heterogeneous, low terrain on deep-convection initiation (CI). The simulations are based on a case of shallow-to-deep convective transition over the Amazon River basin, and use idealized terrains with varying levels of ruggedness. The terrain is designed by specifying its power-spectral shape in wavenumber space, inverting to physical space assuming random phases for all wave modes, and scaling the terrain to have a peak height of 200 m. For the case in question, these modest terrain fields expedite CI by up to 2-3 h, largely due to the impacts of the terrain on the size of, and subcloud support for, incipient cumuli. Terrain-induced circulations enhance subcloud kinetic energy on the mesoscale, which is realized as wider and longer-lived subcloud circulations. When the updraft branches of these circulations breach the level of free convection, they initiate wider and more persistent cumuli that subsequently undergo less entrainment-induced cloud dilution and detrainment-induced mass loss. As a result, the clouds become more vigorous and penetrate deeper into the troposphere. Larger-scale terrains are more effective than smaller-scale terrains in promoting CI because they induce larger enhancements in both the width and the persistence of subcloud updrafts.

The Anatomy of a Series of Cloud Bursts that Eclipsed the US Rainfall Record

A series of extreme cloudbursts occurred on 14 April 2018 over the northern slopes of the island of Kaua‘i. The storm inundated some areas with 1262 mm (∼50”) of rainfall in a 24-hr period, eclipsing the previous 24-hr US rainfall record of 1100 mm (42”) set in Texas in 1979. Three periods of intense rainfall are diagnosed through detailed analysis of National Weather Service operational and special data sets. On the synoptic scale, a slowly southeastward propagating trough aloft over a deep layer of low level moisture (>40 mm of total precipitable water) produced prolonged instability over Kaua‘i. Enhanced NE to E low level flow impacted Kaua‘i’s complex terrain, which includes steep north and eastward facing slopes and cirques. The resulting orographic lift initiated deep convection. The wind profile exhibited significant shear in the troposphere and streamwise vorticity within the convective storm inflow. Evidence suggests that large directional shear in the boundary layer, paired with enhanced orographic vertical motion, produced rotating updrafts within the convective storms. Mesoscale rotation is manifest in the radar data during the latter two periods and reflectivity cores are observed to propagate both to the left and to the right of the mean shear, which is characteristic of supercells. The observations suggest that the terrain configuration in combination with the windshear separates the area of updrafts from the downdraft section of the storm, resulting in almost continuous heavy rainfall over Waipā Garden.

Interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region

The climatic interactions among deep convection, sea surface temperature and radiation in the Asian monsoon region have been examined using various satellite-derived data sets of the period 1983-90. Annual average Frequency of Deep Convection (FDC) is maximum over the equatorial east Indian ocean and adjoining west Pacific and Indonesian region. Maximum FDC zone shifts to Bay of Bengal during the monsoon (June-September) season. There is weak relationship between the variations in FDC and SST in the Indian ocean. Deep convective activity was suppressed over most of the tropical Indian ocean during El Nino of 1987 in spite of warmer SSTs. The pattern of inter-annual variation between FDC and SST behaves differently in the Indian ocean basin as compared to the Pacific ocean basin. Deep convective clouds interact with radiation very effectively in the Asian monsoon region to cause large net negative cloud radiative forcing. Variation in FDC explains more than 70% of the variation in surface short-wave cloud radiative forcing (SWCRF) and long wave cloud radiative forcing (LWCRF) in the atmosphere. On inter-annual scale, warmer SSTs may not necessarily increase deep convection in the Indian ocean. However, the inter-annual variation of deep convective clouds influences significantly the radiative budget of the surface-atmosphere system in the Asian monsoon region. The satellite observations suggest that warmer SSTs in the Indian ocean might have resulted from an increase in the absorbed solar radiation at the surface due to a reduction in deep convective cloud cover.

Inter-Zone Differences of Convective Development in a Convection Outbreak Event over Southeastern Coast of China: An Observational Analysis

Among the densely-populated coastal areas of China, the southeastern coast has received less attention in convective development despite having been suffering from significantly increasing thunderstorm activities. The convective complexity under such a region with extremely complex underlying and convective conditions deserves in-depth observational surveys. This present study examined a high-impact convection outbreak event with over 40 hail reports in the southeastern coast of China on 6 May 2020 by focusing on contrasting the convective development (from convective initiation to supercell occurrences) among three adjacent convection-active zones (north (N), middle (M), and south (S)). The areas from N to S featured overall flatter terrain, higher levels of free convection, lower relative humidity, larger convective inhibition, more convective available potential energy, and greater vertical wind shears. With these mesoscale environmental variations, distinct inter-zone differences in the convective development were observed with the region’s surveillance radar network and the Himawari-8 geostationary satellite. Convection initiated in succession from N to S and began with more warm-rain processes in N and M and more ice-phase processes in S. The subsequent convection underwent more vigorous vertical growth from N to S. The extremely deep convection in S was characterized by the considerably strong precipitation above the freezing level, echo tops of up to 18 km, and a great amount of deep (even overshooting) and thick convective clouds with significant cloud-top glaciation. Horizontal anvil expansion in convective clouds was uniquely apparent over S. From N to S, more pronounced mesocyclone and weak-echo region signatures indicated high risks of severe supercell hailstorms. These results demonstrate the strong linkage between the occurrence likelihood of severe convection and associated weather (such as supercells and hailstones) and the early-stage convective development that can be well-captured by high-resolution observations and may facilitate fine-scale convection nowcasting.

Diurnal differences in tropical maritime anvil cloud evolution

Satellite observations of tropical maritime convection indicate an afternoon maximum in anvil cloud fraction that cannot be explained by the diurnal cycle of deep convection peaking at night. We use idealized cloud-resolving model simulations of single anvil cloud evolution pathways, initialized at different times of the day, to show that tropical anvil clouds formed during the day are more widespread and longer lasting than those formed at night. This diurnal difference is caused by shortwave radiative heating, which lofts and spreads anvil clouds via a mesoscale circulation that is largely absent at night, when a different, longwave-driven circulation dominates. The nighttime circulation entrains dry environmental air that erodes cloud top and shortens anvil lifetime. Increased ice nucleation in more turbulent nighttime conditions supported by the longwave cloud top cooling and cloud base heating dipole cannot overcompensate for the effect of diurnal shortwave radiative heating. Radiative-convective equilibrium simulations with a realistic diurnal cycle of insolation confirm the crucial role of shortwave heating in lofting and sustaining anvil clouds. The shortwave-driven mesoscale ascent leads to daytime anvils with larger ice crystal size, number concentration, and water content at cloud top than their nighttime counterparts.

The influence of shallow cloud populations on transitions to deep convection in the Amazon

In this study, a pair of convection-permitting (2-km grid spacing), month-long, wet season Weather Research and Forecasting (WRF) simulations with and without the Eddy-Diffusivity Mass-Flux (EDMF) scheme are performed for a portion of the Green Ocean Amazon (GoAmazon) 2014/5 field campaign period. EDMF produces an ensemble of subgrid-scale convective plumes that evolve in response to the boundary layer meteorology and can develop into shallow clouds. The objective of this study is to determine how different treatments of shallow cumulus clouds (i.e., with and without EDMF) impact the total cloud population and precipitation across the Amazonian rainforest, with emphasis on impacts on the likelihood of shallow-to-deep convection transitions. Results indicate that the large-scale synoptic conditions in the EDMF and control simulations are nearly identical, however, on the local scale their rainfall patterns diverge drastically and the biases decrease in EDMF. The EDMF scheme significantly increases the frequency of shallow clouds, but the frequencies of deep clouds are similar between the simulations. Deep convective clouds (DCC) are tracked using a cloud tracking algorithm to examine the impact of shallow cumulus on the surrounding ambient environment where deep convective clouds initiate. Results suggest that a rapid increase of low-level cloudiness acts to cool and moisten the low-to-mid troposphere during the day, favoring the transition to deep convection.

The Interaction between African Easterly Waves and Different Types of Deep Convection and Its Influence on Atlantic Tropical Cyclones

This study revisited the association of African easterly waves (AEWs) to Atlantic tropical cyclone (TC) development using weather states (WSs) from the International Satellite Cloud Climatology Project, National Hurricane Center best track hurricane data (HURDAT2), and reanalysis products. The WS data are used as a proxy for two different types of deep convection. This study covers July–October 1984–2009. Statistical analysis based on HURDAT2 and objectively tracked AEWs has shown that a small fraction (~20%) of the AEWs that propagate from Africa serve as TC precursors. About 80% of the AEWs from the continent were non-developing. As in the past work, our study showed an important difference between developing and non-developing AEWs. Composites based on developing AEWs revealed well-organized large scale deep convection (one type, composed of mesoscale systems and thick anvil clouds) is tightly coupled to the AEW trough, while scattered, less well-organized deep convection (second type, isolated cumulonimbus and cumulus congestus clouds) dominated a large area downstream of the developing AEW trough. Developing AEWs propagate westwards while strengthening. In contrast, non-developing AEWs showed that the peak well-organized deep convection is located either behind (to the east of) or far ahead (to the west) of the AEW trough (peaks values are not in close proximity). Moreover, well-organized deep convections associated with non-developing AEWs were weaker than those associated with developing AEWs. The results indicated that convective activity ahead of the non-developing AEWs is weak. Positive relative humidity (RH) anomalies dominate the area around AEWs and downstream over the main TC development region. In contrast, negative RH dominated the main TC development region ahead of non-developing AEWs, suggesting an unfavorable environment downstream of the AEWs. The results also showed that developing AEWs maintained stronger features in the lower and middle troposphere, while non-developing AEWs exhibited weaker structures, in agreement with past work. (Supplemental information related to this paper is available at the journal’s website of this edition).