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Min Min, Lu Zhang, Peng Zhang, and Zhigang Yao

Abstract

The plane-parallel atmosphere as an underlying assumption in physics is appropriately used in the rigorous numerical simulation of the atmospheric radiative transfer model (RTM) with incident solar light. The solar irradiance is a constant with the plane-parallel assumption, which is attributed to the small difference in the distance between any point on Earth’s surface to the sun. However, at night, atmospheric RTMs use the moon as a unique incident light source in the sky. The Earth–moon distance is approximately 1/400 of the Earth–sun distance. Thus, the varying Earth–moon distance on Earth’s surface can influence the top of atmosphere (TOA) lunar irradiance for the plane-parallel atmosphere assumption. In this investigation, we observe that the maximum biases in Earth–moon distance and day/night band lunar irradiance at the TOA are ±1.7% and ±3.3%, respectively, with the plane-parallel assumption. According to our calculations, this bias effect on the Earth–moon distance and lunar irradiance shows a noticeable spatiotemporal variation on a global scale that can impact the computational accuracy of an RTM at night. In addition, we also developed a fast and portable correction algorithm for the Earth–moon distance within a maximum bias of 18 km or ±0.05%, because of the relatively low computational efficiency and the large storage space necessary for the standard ephemeris computational software. This novel correction algorithm can be easily used or integrated into the atmospheric RTM at night.

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Daniel J. Kirshbaum and Katia Lamer

Abstract

Cumulus entrainment is a complex process that has long challenged conceptual understanding and atmospheric prediction. To investigate this process observationally, two retrievals are used to generate multiyear climatologies of shallow-cumulus bulk entrainment (ϵ) at two Atmospheric Radiation Measurement cloud observatories, one in the U.S. Southern Great Plains (SGP) and the other in the Azores archipelago in the eastern North Atlantic (ENA). The statistical distributions of ϵ thus obtained, as well as certain environmental and cloud-related sensitivities of ϵ, are consistent with previous findings from large-eddy simulations. The retrieved ϵ robustly increases with cloud-layer relative humidity and decreases in wider clouds and cloud ensembles with larger cloud-base mass fluxes. While ϵ also correlates negatively with measures of cloud-layer vigor (e.g., maximum in-cloud vertical velocity and cloud depth), the extent to which these metrics actually regulate ϵ (or vice versa) is unclear. Novel sensitivities of ϵ include a robust decrease of ϵ with increasing subcloud wind speed in oceanic flows, as well as a decrease of ϵ with increasing cloud-base mass flux in individual cumuli. A strong land–ocean contrast in ϵ is also found, with median values of 0.5–0.6 km−1 at the continental SGP site and 1.0–1.1 km−1 at the oceanic ENA site. This trend is associated with drier and deeper cloud layers, along with larger cloud-base mass fluxes, at SGP, all of which favor reduced ϵ. The flow dependence of retrieved ϵ implies that its various sensitivities should be accounted for in cumulus parameterization schemes.

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George Duffy, Greg Mcfarquhar, Stephen W. Nesbitt, and Ralf Bennartz

Abstract

The retrieval of the mass-weighted mean diameter (D m) is a fundamental component of spaceborne precipitation retrievals. The Dual-Frequency Precipitation Radar (DPR) on the Global Precipitation Measurement (GPM) satellite is the first satellite to use dual-wavelength ratio measurements—the quotient of radar reflectivity factors (Z) measured at Ku and Ka wavelengths—to retrieve D m. While it is established that DWR, being theoretically insensitive to changes in ice crystal mass and concentration, can provide a superior retrieval of D m compared to Z-based retrievals, the benefits of this retrieval have yet to be directly observed or quantified. In this study, DWR–D m and ZD m relationships are empirically generated from collocated airborne radar and in situ cloud particle probe measurements. Data are collected during nine intensive observation periods (IOPs) from three experiments representing different locations and times of year. Across IOPs with varying ice crystal concentrations, cloud temperatures, and storm types, ZD m relationships vary considerably while the DWR–D m relationship remains consistent. This study confirms that a DWR–D m relationship can provide a more accurate and consistent D m retrieval than a ZD m relationship, quantified by a reduced overall RMSE (0.19 and 0.25 mm, respectively) and a reduced range of biases between experiments (0.11 and 0.32 mm, respectively).

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Donglei Shi and Guanghua Chen

Abstract

The rapid intensification (RI) of Supertyphoon Lekima (2019) is investigated from the perspective of balanced potential vorticity (PV) dynamics using a high-resolution numerical simulation. The PV budget shows that the inner-core PV anomalies (PVAs) formed during the RI mainly comprise an eyewall PV tower generated by diabatic heating, a high-PV bridge extending into the eye resulting from the PV mixing, and an upper-tropospheric high-PV core induced by the PV intrusion from stratosphere. The inversion of the total PVA at the end of the RI captures about 90% of changes in pressure and wind fields, indicating that the storm is quasi-balanced. The piecewise PV inversion further demonstrates that the eyewall and mixed PVAs induce the upper-level and midlevel warm cores in the eye region, respectively. The two warm cores cause nearly all the balanced central pressure decrease and thus dominate the RI, with the contribution of the upper warm core being twice that of the midlevel one. In contrast, the upper-tropospheric PV core induces significant warming near the tropopause and deep-layer cooling beneath, reinforcing the upper-level warm core but causing little surface pressure drop. By comparing the diabatic PV generation due to the convective burst (CB) and non-CB precipitation, we found that the non-CB precipitation accounts for a larger portion for the eyewall PVA and thus the associated upper-level warming, distinct from previous studies that primarily attributed the upper-level warm-core formation to the CB. Nevertheless, CBs act to be more efficient PV generators due to their vigorous latent heat release and are thus favorable for RI.

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Hung-I Lee and Jonathan L. Mitchell

Abstract

A global Hovmöller diagram of column water vapor (CWV) at 30°N from daily ERA-Interim data shows seasonally migrating North Pacific/Atlantic quasi-stationary atmospheric rivers (QSARs) located in the eastern Pacific/Atlantic in winter and propagate to the western Pacific/Atlantic in summer. Simplified general circulation model (GCM) experiments produce QSAR-like features if the boundary conditions include 1) the sea surface temperature contrast from the tropical warm pool–cold tongue and 2) topographic contrast similar to the Tibetan Plateau. Simulated QSARs form downstream of topographic contrast during winter and coincide with it in summer. Two models of baroclinic instability demonstrate that QSARs coincide with the location where the most unstable mode phase speed equals that of the upper-level zonal winds. A consistent interpretation is that the waves become quasi stationary at this location and break. The location of quasi stationarity migrates from the eastern Pacific/Atlantic in the winter, when upper-level winds are strong and extended over the basin, to the western Pacific/Atlantic when winds are weak and contracted. Low-level wind convergence and moist static energy coincide with QSARs, and since the former two are essential ingredients to monsoon formation, this implies an important role for QSARs in monsoon onset. This connection opens a new window into the dynamics of subtropical monsoon extensions.

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Ryan Eastman, Isabel L. McCoy, and Robert Wood

Abstract

Classifications of mesoscale cellular convection (MCC) for marine boundary layer clouds are produced using a supervised neural network algorithm applied to MODIS daytime liquid water path data. The classifier, used in prior studies, distinguishes closed, open, and cellular but disorganized MCC. This work uses trajectories in four eastern subtropical ocean basins to compare meteorological variables and the structures of boundary layers for trajectories that begin as closed cells but evolve into either open cells or disorganized cells or remain closed cells over one afternoon–afternoon cycle. Results show contrasts between the trajectory sets: Trajectories for MCC that remain closed cells are more frequently observed nearer coasts, whereas trajectories that break into open and disorganized cells begin farther offshore. The frequency at which closed cells transition to open cells is seasonally invariant. The fraction of trajectories that stay as closed MCC varies throughout the year in opposition to those that break into disorganized cells, so that their annual cycles are 180° out of phase. Trajectories remain as closed cell more frequently in austral spring and boreal summer when the trade inversion is stronger. The closed–disorganized MCC breakup is associated with weaker subsidence, a weaker inversion, a drier free troposphere, and enhanced nighttime boundary layer deepening, consistent with a warming–drying mechanism. The closed–open transition occurs in meteorological conditions similar to closed–closed trajectories. However, prior to the transition, the closed–open trajectories exhibit stronger surface winds and lower cloud droplet concentrations and rain more heavily overnight. Results suggest that multiple, independent mechanisms drive changes in cloud amount and morphology.

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Seiya Nishizawa, Tsuyoshi Yamaura, and Yoshiyuki Kajikawa

Abstract

In this study, the effect of submesoscale topography (i.e., topographical features smaller than a few kilometers in size) on precipitation associated with thermally driven local circulations over a mountainous region is examined in the absence of synoptic-scale precipitation systems through a 100-m-mesh large-eddy simulation experiment. The observed effect of topography on precipitation is different than that identified in previous studies; submesoscale topography is observed to induce a weakening effect on precipitation in this study, while previous studies have suggested that submesoscale topography enhances precipitation. This discrepancy between studies is owing to differences in the scale of the topography and the precipitation-inducing system under consideration. Previous studies have focused on precipitation associated with synoptic-scale systems, where mechanical orographic forcing is dominant. The mechanism of the topographic effect where thermal orographic forcing is dominant was clarified in this study. Under thermally driven local circulation, the convergence of upslope flow near large-scale mountain ridges is one of the main causes of precipitation. Submesoscale topographic features promote the detachment of upslope flow from the mountain surface and vertical mixing in the boundary layer. This detachment and mixing result in a weakening of convergence and updraft and reduction of equivalent potential temperature around the ridge that explains the observed weakening effect on precipitation. Cold pools formed by evaporation of rainfall associated with upslope flow enhance the weakening effect. These results confirm the importance of submesoscale topography in orographic precipitation.

Open access
Zeyuan Hu, Fayçal Lamraoui, and Zhiming Kuang

Abstract

It is still debated whether radiative heating observed in the tropical tropopause layer (TTL) is balanced primarily by cooling from convective overshoots, as in an entrainment layer, or by adiabatic cooling from large-scale eddy-driven upwelling. In this study, three-dimensional cloud-resolving model simulations of radiative–convective equilibrium were carried out with three different cloud microphysics schemes and 1-km horizontal resolution. We demonstrate that overshooting cooling in the TTL can be strongly modulated by upper-troposphere stratification. Two of the schemes produce a hard-landing scenario in which convective overshoots reach the TTL with frequent large vertical velocity leading to strong overshooting cooling (~−0.2 K day−1). The third scheme produces a soft-landing scenario in which convective overshoots rarely reach the TTL with large vertical velocity and produce little overshooting cooling (~−0.03 K day−1). The difference between the two scenarios is attributed to changes in the upper-troposphere stratification related to different atmospheric cloud radiative effects (ACRE). The microphysics scheme that produces the soft-landing scenario has much stronger ACRE in the upper troposphere leading to a ~3-K-warmer and more stable layer that acts as a buffer zone to slow down the convective updrafts. The stratification mechanism suggests the possibility for the ozone variation or eddy-driven upwelling in the TTL to modulate convective overshoots. We further test the sensitivity of overshooting cooling to changes in model resolution by increasing the horizontal resolution to 100 m. The corresponding change of overshooting cooling is much smaller compared with the difference between the hard-landing and soft-landing scenarios.

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Subin Thomas, Prasanth Prabhakaran, Will Cantrell, and Raymond A. Shaw

Abstract

Water vapor supersaturation in the atmosphere is produced in a variety of ways, including the lifting of a parcel or via isobaric mixing of parcels. However, irrespective of the mechanism of production, the water vapor supersaturation in the atmosphere has typically been modeled as a Gaussian distribution. In the current theoretical and numerical study, the nature of supersaturation produced by mixing processes is explored. The results from large-eddy simulation and a Gaussian mixing model reveal the distribution of supersaturations produced by mixing to be negatively skewed. Further, the causes of skewness are explored using the models. The correlation in forcing of temperature and water vapor fields is recognized as playing a key role.

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Vishnu Nair, Thijs Heus, and Maarten van Reeuwijk

Abstract

Interfaces at the edge of an idealized, nonprecipitating, warm cloud are studied using direct numerical simulation (DNS) complemented with a Lagrangian particle tracking routine. Once a shell has formed, four zones can be distinguished: the cloud core, visible shell, invisible shell, and the environment. The union of the visible and invisible regions is the shell commonly referred to in literature. The boundary between the invisible shell and the environment is the turbulent–nonturbulent interface (TNTI), which is typically not considered in cloud studies. Three million particles were seeded homogeneously across the domain and properties were recorded along individual trajectories. The results demonstrate that the traditional cloud boundary (separating cloudy and noncloudy regions using thresholds applied on liquid condensate or updraft velocity) are some distance away from the TNTI. Furthermore, there is no dynamic difference between the traditional liquid-condensate boundary and the region extending to the TNTI. However, particles crossing the TNTI exhibit a sharp jump in enstrophy and a smooth increase in buoyancy. The traditional cloud boundary coincides with the location of minimum buoyancy in the shell. The shell premixes the entraining and detraining air and analysis reveals a highly skewed picture of entrainment and detrainment at the traditional cloud boundary. A preferential entrainment of particles with velocity and specific humidity higher than the mean values in the shell is observed. Large-eddy simulation of a more realistic setup detects an interface with similar properties using the same thresholds as in the DNS, indicating that the DNS results extrapolate beyond their idealized conditions.

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