Role of Advection of Parameterized Turbulence Kinetic Energy in Idealized Tropical Cyclone Simulations

Horizontal homogeneity is typically assumed in the design of planetary boundary layer (PBL) parameterizations in weather prediction models. Consistent with this assumption, PBL schemes with predictive equations for subgrid turbulence kinetic energy (TKE) typically neglect advection of TKE. However, tropical cyclone (TC) boundary layers are inhomogeneous, particularly in the eyewall. To gain further insight, this study examines the effect of advection of TKE using the Mellor-Yamada-Nakanishi-Niino (MYNN) PBL scheme in idealized TC simulations. The analysis focuses on two simulations, one that includes TKE advection (CTL) and one that does not (NoADV). Results show that relatively large TKE in the eyewall above 2 km is predominantly attributable to vertical advection of TKE in CTL. Interestingly, buoyancy production of TKE is negative in this region in both simulations; thus, buoyancy effects cannot explain observed columns of TKE in TC eyewalls. Both horizontal and vertical advection of TKE tends to reduce TKE and vertical viscosity (Km) in the near-surface inflow layer, particularly in the eyewall of TCs. Results also show that the simulated TC in CTL has slightly stronger maximum winds, slightly smaller radius of maximum wind (RMW), and ~5% smaller radius of gale-force wind than in NoADV. These differences are consistent with absolute angular momentum being advected to smaller radii in CTL. Sensitivity simulations further reveal that the differences between CTL and NoADV are more attributable to vertical advection (rather than horizontal advection) of TKE. Recommendations for improvements of PBL schemes that use predictive equations for TKE are also discussed.

The Moist Entropy Budget of Terminating Madden–Julian Oscillation Events

Recent studies have examined moist entropy (ME) as a proxy for moist static energy (MSE) and the relative role of the underlying processes responsible for changes in ME that potentially affect MJO propagation. This study presents an analysis of the intraseasonally varying (ISV) ME anomalies throughout the lifetime of observed MJO events. A climatology of continuing and terminating MJO events is created from an event identification algorithm using common tracking indices including the OLR-based MJO index (OMI), filtered OMI (FMO), real-time multivariate MJO (RMM), and velocity potential MJO (VPM) index. ME composites for all indices show a statistically significant break in the wavenumber-1 oscillation at day 0 for terminating events in nearly all domains except RMM phase 6 and phase 7. The ME tendency is decomposed into horizontal and vertical advection, sensible and latent heat fluxes, and shortwave and longwave radiative fluxes using ERA-Interim data. The relative role of each processes toward the eastward propagation is discussed as well as their effects on MJO stabilization. Statistically significant differences occur for all terms by day −10. A domain sensitivity test is performed where eastward propagation is favored for vertical advection given a larger, asymmetric domain for continuing events. A reduced eastward propagation from vertical advection is evident 2–3 days before similar differences in horizontal advection for terminating events. The importance of horizontal advection for the eastward propagation of the MJO is discussed in addition to the relative destabilization from vertical advection in the convectively suppressed region downstream of future terminating MJOs.

Periodic Cycles of Eyewall Convection Limit the Rapid Intensification of Typhoon Hato (2017)

The ability to forecast tropical cyclone (TC) intensity has improved modestly in recent years, partly because of an inadequate understanding of eyewall convection processes. Short-term periodic convection activities (period: 3–5 h) have been identified in a number of TCs, but the effect of these activities on the evolution of TC intensity at the hourly scale is yet to be fully investigated. Using radar observations and a high-resolution numerical simulation based on the Weather Research and Forecasting model, we analyzed the periodic cycles of eyewall convection associated with the intensification of Typhoon Hato (2017). Results indicate the presence of four short-term periodic cycles (period: 3–5 h) in the eyewall convection, which correspond to TC intensification. We further divided each cycle into three stages. The periodic evolution of convection inhibited the rapid intensification of the TC. The highest and lowest intensification rates were associated with the first and third stages according to the virtual potential temperature tendency in the eyewall region, respectively. Heating was dominated by the vertical advection associated with sensible heat and latent heat, which were controlled by the eyewall convection and structure. Of the three stages in each cycle, the vertical transport released the largest amount of latent heat in the first stage; consequently, the highest intensification rate occurred in this stage. In the second stage, heating was reduced because of decreased latent heat and increased cooling of sensible heat associated with vertical advection as the eyewall intensified. Vertical transport was the weakest in the third stage; this resulted in the smallest amount of heating, which limited the rapid intensification of the TC.

Impacts of the Kuroshio Intrusion through the Luzon Strait on the Local Precipitation Anomaly

The Kuroshio Current has its origin in the northwestern Pacific, flowing northward to the east of Taiwan and the northern part of Luzon Island. As the Kuroshio Current flows northward, it quasi-periodically intrudes (hereafter referred to as Kuroshio intrusion (KI)) into the northern South China Sea (SCS) basin through the Luzon Strait. Despite the complex generation mechanisms of KI, the purpose of this study is to improve our understanding of the effects of KI through the Luzon Strait on the regional atmospheric and weather variations. Long-term multiple satellite observations, including absolute dynamic topography, absolute geostrophic currents, sea surface winds by ASCAT, multi-scale ultra-high resolution sea surface temperature (MURSST) level-four analysis, and research-quality three-hourly TRMM multi-satellite precipitation analysis (TMPA), was used to systematically examine the aforementioned scientific problem. Analysis indicates that the KI is interlinked with the consequential anomalous precipitation off southwestern Taiwan. This anomalous precipitation would lead to ~560 million tons of freshwater influx during each KI event. Subsequently, independent moisture budget analysis suggests that moisture, mainly from vertical advection, is the possible source of the precipitation anomaly. Additionally, a bulk formula analysis was applied to understand how KI can trigger the precipitation anomaly through vertical advection of moisture without causing an evident change in the low-level flows. These new research findings might reconcile the divisiveness on why winds are not showing a synchronous response during the KI and consequential anomalous precipitation events.

An evaluation of size-resolved cloud microphysics scheme numerics for use with radar observations Part II: Condensation and evaporation

Accurate numerical modeling of clouds and precipitation is essential for weather forecasting and climate change research. While size-resolved (bin) cloud microphysics models predict particle size distributions without imposing shapes, results are subject to artificial size distribution broadening owing to numerical diffusion associated with various processes. Whereas Part 1 addressed collision-coalescence, here we investigate numerical diffusion that occurs in solving condensation and evaporation. In a parcel model framework, all of the numerical schemes examined converge to one solution of condensation and evaporation as the mass grid is refined, and the advection-based schemes are recommended over the reassigning schemes. Including Eulerian vertical advection in a column limits the convergence to some extent, but that limitation occurs at a sufficiently fine mass grid, and the number of iterations in solving vertical advection should be minimized to reduce numerical diffusion. Insubstantial numerical diffusion in solving condensation can be amplified if collision-coalescence is also active, which in turn can be substantially diminished if turbulence effects on collision are incorporated. Large-eddy simulations of a drizzling stratocumulus field reveal that changes in moments of Doppler spectra obtained using different mass grids are consistent with those obtained from the simplified framework, and that spectral moments obtained using a mass grid designed to effectively reduce numerical diffusion are generally closer to observations. Notable differences between the simulations and observations still exist, and our results suggest a need to consider whether factors other than numerical diffusion in the fundamental process schemes employed can cause such differences.

A new process-based vertical advection/diffusion theoretical model of ocean heat uptake

<p>Ocean heat uptake is a key process for climate change owing to its control of global mean temperature trends. To understand the underlying internal ocean processes and vertical heat transfer controlling it, ocean heat uptake has been often analysed in terms of the simple one-dimensional vertical advection diffusion model. The standard version of this model, formulated in terms of the horizontally-averaged potential temperature is known to poorly capture important effects such as isopycnal mixing, density-compensated temperature anomalies, meso-scale eddy-induced advection and the depth-varying ocean area.</p><p>To overcome this problem a new theoretical model of vertical heat transfer for the ocean heat uptake has been developed in an isopycnal framework that exploits advances achieved in the theory of water masses over the past 30 years or so. The new theoretical model describes the temporal evolution of the isopycnally-averaged thickness-weighted potential temperature in terms of an effective velocity that depends uniquely on the surface heating conditionally integrated in density classes, an effective diapycnal diffusivity controlled by isoneutral and dianeutral mixing, and an additional term linked to the meridional transport of density-compensated temperature anomalies by the diabatic residual overturning circulation. The advantage of the isopycnally-averaged construction over the horizontally-averaged construction is that all the terms that enters it have explicit analytical expressions that are more easily evaluated from observations or model outputs, as well as having clearer physical interpretations.</p><p>As a first step, the terms of this new model of ocean heat uptake are evaluated by using a range of different datasets, net surface heat flux products and temporal averages to evaluate their sensitivity to input fields. One key feature of the new model is that its effective velocity and diffusivity are positive over most of the ocean column depth. This is in contrast to the horizontally-averaged construction, in which downwelling and ant-diffusive behavior were occasionally observed in previous studies. The hope is that this insight can then be used to develop an improved representation of ocean heat uptake in simple climate models.</p>

Contribution of vertical advection to supergradient wind in tropical cyclone boundary layer: A numerical study

The existence of supergradient wind in the interior of the boundary layer is a distinct feature of a tropical cyclone (TC). Although the vertical advection is shown to enhance supergradient wind in TC boundary layer (TCBL), how and to what extent the strength and structure of supergradient wind are modulated by vertical advection are not well understood. In this study, both a TCBL model and an axisymmetric full-physics model are used to quantify the contribution of vertical advection process to the strength and vertical structure of supergradient wind in TCBL. Results from the TCBL model show that the removal of vertical advection of radial wind reduces both the strength and height of supergradient wind by slightly more than 50%. The removal of vertical advection of agradient wind reduces the height of the supergradient wind core by ~30% but increases the strength of supergradient wind by ~10%. Results from the full-physics model show that the removal of vertical advection of radial wind or agradient wind reduces both the strength and height of supergradient wind but the removal of that of radial wind produces a more substantial reduction (52%) than the removal of that of agradient wind (35%). However, both the intensification rate and final intensity of the simulated TCs in terms of maximum 10-m wind speed show little differences in experiments with and without the vertical advection of radial or agradient wind, suggesting that supergradient wind contributes little to either the intensification rate or the steady-state intensity of the simulated TC.

Climatological impact of the Brewer–Dobson circulation on the N₂O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses

The Brewer–Dobson circulation (BDC) is a stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. As the photochemical losses of nitrous oxide (N2O) are well known, model differences in its rate of change are due to transport processes that can be separated into the mean residual advection and the isentropic mixing terms in the transformed Eulerian mean (TEM) framework. Here, the climatological impact of the stratospheric BDC on the long-lived tracer N2O is evaluated through a comparison of its TEM budget in the Whole Atmosphere Community Climate Model (WACCM), in a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a chemistry transport model (CTM) driven by four modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim; Dee et al., 2011), the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al., 2015), and the Modern-Era Retrospective analysis for Research and Applications version 1 (MERRA; Rienecker et al., 2011) and version 2 (MERRA-2; Gelaro et al., 2017). The effects of stratospheric transport on the N2O rate of change, as depicted in this study, have not been compared before across this variety of datasets and have never been investigated in a modern chemical reanalysis. We focus on the seasonal means and climatological annual cycles of the two main contributions to the N2O TEM budget: the vertical residual advection and the horizontal mixing terms. The N2O mixing ratio in the CTM experiments has a spread of approximately ∼20 % in the middle stratosphere, reflecting the large diversity in the mean age of air obtained with the same CTM experiments in a previous study. In all datasets, the TEM budget is closed well; the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and it is good in the Southern Hemisphere except above the Antarctic region. The datasets do not agree as well with respect to the horizontal mixing term, especially in the Northern Hemisphere where horizontal mixing has a smaller contribution in WACCM than in the reanalyses. WACCM is investigated through three model realizations and a sensitivity test using the previous version of the gravity wave parameterization. The internal variability of the horizontal mixing in WACCM is large in the polar regions and is comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but it significantly changes the vertical advection term and produces a less realistic N2O annual cycle above the Antarctic. In this region, all reanalyses show a large wintertime N2O decrease, which is mainly due to horizontal mixing. This is not seen with WACCM, where the horizontal mixing term barely contributes to the TEM budget. While we must use caution in the interpretation of the differences in this region (where the reanalyses show large residuals of the TEM budget), they could be due to the fact that the polar jet is stronger and is not tilted equatorward in WACCM compared with the reanalyses. We also compare the interannual variability in the horizontal mixing and the vertical advection terms between the different datasets. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions. In the tropics, the interannual variability of the vertical advection term is much smaller in WACCM and JRA-55 than in the other experiments. The large residual in the reanalyses and the disagreement between WACCM and the reanalyses in the Antarctic region highlight the need for further investigations on the modeling of transport in this region of the stratosphere.

Heat Flow Data in an Area of the Eastern Southern Basin and Range in Arizona Contribute to an Analysis of Neogene Lithosphere Thinning Greater than 100 Km

Heat flow data and thermochronologic derived paleotemperature gradient data are examined to calculate heat flow ~25 Ma and, at present, for a southern Basin and Range location north of Tucson, Arizona. An increase in the surface heat flow is estimated from ~25 Ma to the present; changing from ~47 to ~83 mW m-2. Steady-state conduction temperature vs. depth profiles provide estimates of lithosphere thicknesses both for the present and for ~25 Ma. Different heat transfer models for present heat flow predict present LAB depth that agrees with seismic studies. From these temperature profiles, lithosphere thinning from ~184 km to ~70 km is suggested during the Neogene. Mantle lithosphere thinning caused by thermal phenomena is likely a fundamental driving force for southern Basin and Range extension. Because the mantle lithosphere has likely thinned much more than the crust, it is shown that additional vertical advection, such as an asthenosphere plume, delaminating part of the mantle lithosphere, convection cells, and rising magmas along conduits, add to the vertical advection component of upper mantle lithosphere extension. Interestingly, values of heat flow 25 Ma, lithosphere thicknesses 25 Ma, and Neogene lithosphere thinning are somewhat similar for the Four Corners area of the Colorado Plateau and the southern Basin and Range, even though Neogene tectonic development was quite different, i.e., no Neogene extension in the Colorado Plateau vs. ~57% in the southern Basin and Range. Neogene lithosphere thinning phenomena are likely different in the two regions.