How Moist and Dry Intrusions Control the Local Hydrologic Cycle in Present and Future Climates

Models disagree on how much the hydrologic cycle could intensify under climate change. These changes are expected to scale with the Clausius-Clapeyron relation but may locally diverge due in part to the uncertain response of the general circulation, causing the hydrologic cycle to inherit this uncertainty. To identify how the circulation contributes, we link circulation changes to changes in the higher moments of the hydrologic cycle using the novel dynamical framework of the local hydrologic cycle, the portion of the hydrologic cycle driven by moist or dry intrusions. We expand this dynamical framework, developing a closed budget which diagnoses thermodynamic, advective, and overturning contributions to future hydrologic cycle changes. In analyzing these changes for the Community Earth System Model Large Ensemble, we show that overturning is the main dynamic contributor to the tropical and subtropical annual response, consistent with a weakening of this circulation. In the extratropics, we show that advective contributions, likely from storm track changes, dominate the response. We achieve a cleaner separation between dynamic and thermodynamic contributions through a semi-empirical scaling, which reveals the robustness of the Clausius-Clapeyron scaling for the local hydrologic cycle. This scaling also demonstrates the slowing of the local hydrologic cycle and how changing subtropical dynamics asymmetrically impact wave breaking and suppress meridional moisture transport. We conclude that dynamic changes in the subtropics are predominantly responsible for the annual, dynamic response in the extratropics and thus a significant contributor to uncertainty in future projections.

Impact of Dry Intrusions on the Marine Boundary Layer

<p>Intrusions of dry air from the upper troposphere were recently suggested to reach the boundary layer and cause its significant deepening. Dry intrusions (DIs) are synoptic-scale slantwise descending airstreams from the midlatitude upper tropospheric jet towards the boundary layer at lower latitudes, thus acting as a circulation type potentially key for understanding boundary-layer cloud occurrence and regime transition. DIs occur mainly during winter over the mid-latitude oceanic storm track regions behind cold fronts trailing from cyclones. These regions are also home to marine boundary clouds that are an important component of the Earth’s radiation budget as they reflect much higher radiation back to the space compared to the ocean surface thereby cooling the Earth’s surface. Although subsidence is generally an inherent feature of the subtropical marine boundary layer, it is unclear how the marine boundary layer reacts to the transient, dynamically distinct DI, differently from the nominal subtropical subsidence resulting from the descending branch of Hadley circulation.</p><p>In this study we use the observations made at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site (39N, 28W) to characterize the impact of dry intrusions on Marine Boundary Layer (MBL) characteristics such as surface fluxes, thermodynamic stabilities and winds. Our analyses are based on measurements from the campaign: radiosondes, surface station data, polarimetric radar, lidar, radar wind profiler, ceilometer among others. Using all identified DI trajectories during the winters of 2016-2018 based on European Center for Medium-range Weather Forecasts (ECMWF) ERA Interim reanalysis data, we distinguish DI days from those before and following DIs, as well as periods with no DIs at all (with and without the occurrence of cold fronts for comparison). We find that during DI events the well-mixed MBL deepens and its vertical structure changes dramatically. Namely, the lower troposphere cools and dries substantially, inducing strong surface sensible and latent heat fluxes, while a strong inversion builds up at the MBL top, all affecting cloud occurrence. Finally, we used the numerical weather prediction (NWP) model COSMO at 2.2 km horizontal resolution to understand the detailed flows and structure in the MBL during DI events.</p>

The Interaction of Indian Monsoon Depressions with Northwesterly Midlevel Dry Intrusions

Monsoon depressions (MDs) bring substantial monsoon rainfall to northern and central India. These events usually form over the Bay of Bengal and travel across northern India toward Pakistan. Using European Centre for Medium-Range Weather Forecasts interim reanalysis, an MD-tracking algorithm, and an objective identification method, the authors find that about 40% of MDs interact with northerly intrusions of dry desert air masses as the MDs traverse the subcontinent. MD interactions with dry intrusions are often preceded by positive potential vorticity (PV) anomalies on the subtropical jet and low-level anticyclonic anomalies over the north Arabian Sea. Dry intrusions nearly halve the precipitation rate in the southwest quadrant of MDs, where MDs rain the most. However, dry intrusions increase the rainfall rate near the MD center. Similarly, ascent is reduced west of the MD center and enhanced at the MD center, especially in the upper troposphere. The reduced ascent west of MD centers is likely attributable to changes in vertical shear reducing differential cyclonic vorticity advection. Dry intrusions slightly reduce MDs’ propagation speed. For the mid- to upper-level vortex, this can be explained by anomalous westerlies reducing propagation by adiabatic advection. For the lower-tropospheric vortex, it is likely that reduced diabatic generation of PV plays a role in slowing propagation, along with reduced adiabatic advection.

DETECTION OF DRY INTRUSION ON WATER VAPOR IMAGES OVER CENTRAL EUROPE – JUNE 2010 TO SEPTEMBER 2011

The knowledge of evaluation of the intensity of cyclogenesis which could be connected with the weather having a significant impact on Earth’s surface is quite useful. If, as one of the basic assumptions, the existence of connection between dry intrusions, dry bands, tropopause height and warm dark areas distribution on water vapor images (WV images) is considered, it is possible to set up a method of detecting dry intrusions on searching and tracking areas with higher brightness temperature compared with the surrounding environment. This paper covers the period between June 2010 and September 2011 over Central Europe. The ISIS method (<i>Instrument de Suivi dans I’Imagerie satellitaire</i>), originally developed for detection of cold cloud tops, was used as an initial ideological point. Subsequently, this method was modified by Michel and Bouttier for usage on WV images. Some of the applied criteria and parameters were chosen with reference to the results published by Michel and Bouttier as well as by Novotny. The procedure can be divided into two steps: detection of warm areas and their tracking. Cases of detection of areas not evidently connected with dry intrusions can be solved by filtering off based on the connection between detected warm areas to the cyclonic side of jet streams and significant lowering of the tropopause.