The implication of outflow structure for the rapid intensification of tropical cyclones under vertical wind shear

The implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best-track, reanalysis and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s-1) or strong (> 11 m s-1) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (< 4.5 m s-1) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.

Two Types of Diurnal Variations in Heavy Rainfall during July over Korea

This study examined the characteristics of the diurnal variations of heavy rainfall (⩾110 mm in 12 hours) in Korea and the related atmospheric circulation for July from 1980–2020. During the analysis period, two dominant pattens of diurnal variation of the heavy rainfall emerged: all-day heavy rainfall (AD) and morning only heavy rainfall (MO) types. For the AD-type, the heavy rainfall is caused by abundant moisture content in conjunction with active convection in the morning (0000–1200, LST; LST = UTC + 9) and the afternoon hours (1200–2400 LST). These systems are related to the enhanced moisture inflow and upward motion induced by the strengthening of the western North Pacific subtropical high and upper-tropospheric jet. For the MO-type, heavy rainfall occurs mostly in the morning hours; the associated atmospheric patterns are similar to the climatology. We find that the atmospheric pattern related to severe heavy rainfalls in 2020 corresponds to a typical AD-type and resembles the 1991 heavy-rainfall system in its overall synoptic/mesoscale circulations. The present results imply that extremely heavy rainfall episodes in Korea during the 2020 summer may occur again in the future associated with the recurring atmospheric phenomenon related to the heavy rainfall.

Evaluation of Tropospheric Moisture Characteristics Among COSMIC-2, ERA5 and MERRA-2 in the Tropics and Subtropics

Global navigation satellite system (GNSS) radio occultation (RO) receivers onboard the recently-launched COSMIC-2 (C2) satellite constellation provide an unprecedented number of high vertical resolution moisture profiles throughout the tropical and subtropical atmosphere. In this study, the distribution and variability of water vapor was investigated using specific humidity retrievals from C2 observations and compared to collocated ERA5 and MERRA-2 reanalysis profiles within 40°N to 40°S from September to December 2019, which is prior to the assimilation of C2 in the reanalyses. Negative C2 moisture biases are evident within the boundary layer, so we focused on levels above the boundary layer in this study. Overall, C2 specific humidity shows excellent agreement with that of ERA5 and has larger differences with that of MERRA-2. In the tropical mid-troposphere, C2 shows positive biases compared to ERA5 (6–12%) and larger negative biases with MERRA-2 (15–30%). Strong correlations are observed between C2 and reanalysis specific humidity in the subtropics (>0.8) whereas correlations are slightly weaker in the deep tropics, especially for MERRA-2. Profile pairs with large moisture differences often occur in areas with sharp moisture gradients, highlighting the importance of measurement resolution. Locations which demonstrated weaker humidity correlations in active convection regions show that ERA5 has a negative specific humidity bias at 3 km in higher moisture environments, whereas MERRA-2 displays a large positive bias at 7 km. However, additional explanations for profile pairs with large moisture differences remain unclear and require further study.

Active oxygen transport in tissue by interstitial flow

Oxygen (O2) transport through diffusion from capillary to tissue has long been established by Krogh. However, the interstitial fluid in the interspace between tissue and capillary has a high Prandtl number around 103 and hence its convective mass transport is more efficient than its diffusive transport. The interstitial flow drained by the initial lymphatics contributes to the convective transport of O2 through tissue, which can be modeled as aligned blood capillaries in parallel and the initial lymphatics. It is found that both the O2 concentration distribution and the total O2 flux are sensitive to the flow rate of interstitial fluid. The convection contribution has been evaluated based on the Peclet number, feature flow rate, and convection-diffusion boundary. At the same interstitial flow rate, convection delivers more O2 to type I muscle fibers with a higher concentration of myoglobin than to type IIX muscle fibers. Even with a small external force, tissue with a higher specific hydraulic conductance (permeability) has a larger interstitial flow rate and a higher O2 transport rate than those in healthy tissue. Hence, the overall O2 transport from capillary to tissue includes two components, i.e., active convection transport by interstitial flow due to pressure gradient and passive diffusion transport due to concentration gradient. The active convective O2 transport is crucial for the recovery of damaged tissue where the contribution from passive diffusion transport is constrained by regulation of capillary opening. The convection facilitated O2 transport can be the basis for cell differentiation, morphogenesis, and therapeutic effects of massage and acupuncture.Key pointsInterstitial flow plays a key role in active O2 transport in tissue due to its high Prandtl number v/D~103;O2 transport in tissue is balanced by both active convection and passive diffusion transport.Interstitial flow in form of active convective transport can pump more than hundred times of O2 into tissue than those by passive diffusion transport due to the concentration gradient.Active convection transport can be triggered by external pressure, which is crucial for damage tissue recovery.

Error Growth Dynamics within Convection-Allowing Ensemble Forecasts over Central U.S. Regions for Days of Active Convection

Error growth is investigated based on convection-allowing ensemble forecasts starting from 0000 UTC for 14 active convection events over central to eastern U.S. regions from spring 2018. The analysis domain is divided into the NW, NE, SE and SW quadrants (subregions). Total difference energy and its decompositions are used to measure and analyze error growth at and across scales. Special attention is paid to the dominant types of convection with respect to their forcing mechanisms in the four subregions and the associated difference in precipitation diurnal cycles. The discussions on the average behaviors of error growth in each region are supplemented by 4 representative cases. Results show that the meso-γ-scale error growth is directly linked to precipitation diurnal cycle while meso-α-scale error growth has strong link to large scale forcing. Upscale error growth is evident in all regions/cases but up-amplitude growth within own scale plays different roles in different regions/cases.When large-scale flow is important (as in the NE region), precipitation is strongly modulated by the large-scale forcing and becomes more organized with time, and upscale transfer of forecast error is stronger. On the other hand, when local instability plays more dominant roles (as in the SE region), precipitation is overall least organized and has the weakest diurnal variations. Its associated errors at the γ– and β-scale can reach their peaks sooner and meso-α-scale error tends to rely more on growth of error with its own scale. Small-scale forecast errors are directly impacted by convective activities and have short response time to convection while increasingly larger scale errors have longer response times and delayed phase within the diurnal cycle.

The Detailed Dynamics of the Hadley Cell. Part II: December–February

This paper complements an earlier paper on the June–August Hadley cell by giving a detailed analysis of the December–February Hadley cell as seen in a 30-yr climatology of ERA-Interim data. The focus is on the dynamics of the upper branch of the Hadley cell. There are significant differences between the Hadley cells in the two solsticial seasons. These are particularly associated with the ITCZs staying north of the equator and with mean westerlies in the equatorial regions of the east Pacific and Atlantic in December–February. The latter enables westward-moving mixed Rossby–gravity waves to be slow moving in those regions and therefore respond strongly to upstream off-equatorial active convection. However, the main result is that in both seasons it is the regions and times of active convection that predominantly lead to upper-tropospheric outflows and structures that average to give the mean flow toward the winter pole, and the steady and transient fluxes of momentum and vorticity that balance the Coriolis terms. The response to active convection in preferred regions is shown by means of regressions on the data from the climatology and by synoptic examples from one season. Eddies with tropical origin are seen to be important in their own right and also in their interaction with higher-latitude systems. There is support for the relevance of a new conceptual model of the Hadley cell based on the sporadic nature of active tropical convection in time and space.

Doppler Aerosol WiNd (DAWN) Lidar during CPEX 2017: Instrument Performance and Data Utility

During 25 May–24 June 2017, NASA’s Doppler Aerosol WiNd (DAWN) lidar was flown on board a NASA DC-8 aircraft as part of the Convective Processes EXperiment (CPEX) airborne campaign based out of Ft. Lauderdale, FL. Central to DAWN’s deployment was the goal of obtaining high time and spatial resolution wind velocity measurements, particularly with respect to the convective life cycle. We describe the processes involved in deriving wind profiles from DAWN observations and evaluate the performance of DAWN in terms of data coverage, resolution and frequency. Comparisons with dropsonde wind measurements show an overall low bias of <0.20 m/s with a RMSD of ~1.6 and R2 > 0.92 for both u and v components for the data set as a whole (over 160 comparisons). From this CPEX experience, we find that the DAWN wind profiles are of high precision, ~30 m vertical resolution and with horizontal spacing as fine as 3–7 km, and rival dropsondes for horizontal wind coverage (aerosols and clouds permitting). Case studies illustrate the benefit of using the DAWN to investigate and characterize the dynamics of the tropical atmosphere over open ocean waters in conditions ranging from undisturbed to active convection.

Inertia-Gravity Wave Generation Near Upper Jets in the Australian Region

&lt;p&gt;During rapid extratropical cyclogenesis in the Australian region, distinctive striated, triangular-shaped clouds commonly form on the poleward side of the jet exit near the axis of inflection between an upstream trough and downstream ridge. These clouds are called striated deltas and the striations are shown here to be caused by radiating inertia-gravity waves. An analysis of 28 striated delta clouds shows that the striated deltas have a mean length of 1034 km and width of 537 km, and a striation wavelength of 74 km. Large parcel accelerations, nonlinear flow imbalance and active convection within the striated delta are features of the composite upper-tropospheric environment. Patterns of Q-vector illustrates the unique shape of striated delta clouds to be coincident with a delta-shaped forcing of adiabatic ascent in the poleward jet exit. One of the extratropical cyclones analysed and its associated striated delta is simulated with WRF-ARW both with and without diabatic heating. Both simulations produce pronounced gravity wave packets along the surface cold front, along the downstream upper jet axis and a delta-shaped packet in the lower stratosphere above the jet exit. Ray tracing identifies the source region of the waves in the stratosphere to be the upper jet. Vertically-radiating gravity waves originating in the vicinity of the jet during rapid extratropical cyclogenesis, propagate both upwards and downwards, imprinting striations onto the cloud in the jet exit and setting the spacing between the convective bands.&lt;/p&gt;

Confinement of air in the Asian monsoon anticyclone and pathways of convective air to the stratosphere during summer season

&lt;p&gt;We study the confinement of the air inside the Asian monsoon anticyclone during summer using both kinematic and diabatic Lagrangian trajectories with ERA5 and ERA-Interim reanalysis, and observed clouds. The improved consistency of ERA5 is demonstrated. It is shown that the escape time from the anticyclone estimated to be 13 days is of the same order as the circulation time which implies weak confinement. Parcels found inside the anticyclone have been mostly detrained by convection above &amp;#952; =364 K, by about 2.6%&amp;#160; of the high clouds over Asia, with a prevalence of continental sources which are located beneath. The Tibetan plateau is found to be the most efficient provider with 10% of its high clouds but this is entirely due to the higher level of cloud tops in this region, and not to any preferred path above.&amp;#160; Actually,&amp;#160; most parcels escape the plateau to rise. The mean trapping is shown to be described by a 1D model that combines a simple mean ascent and a constant erosion loss, without any need of a &amp;#8220;chimney effect&amp;#8221;. The vertical dilution is exponential with a e-folding scale of 15 K in potential temperature from 370 K onward. The mean age of parcels with respect to convection exhibits a minimum at the centre of the Asian monsoon anticyclone due to the permanent renewal by fresh convective air and largest values on the periphery as air spirals out.&lt;/p&gt;&lt;p&gt;The variability of the the confinement is strongly linked with the oscillations of the anticyclone between its Tibetan mode and its Iranian mode, and to break and active periods of monsoon rain. We show that this variability modulates also the moisture in the lower stratosphere with wet events following active convection and dry events following the breaks.&lt;/p&gt;