Wintertime precipitation over the Australian Snowy Mountains: Observations from an Intensive Field Campaign 2018

Understanding the key dynamical and microphysical mechanisms driving precipitation in the Snowy Mountains region of southeast Australia, including the role of orography, can help improve precipitation forecasts, which is of great value for efficient water management. An intensive observation campaign was carried out during the 2018 austral winter, providing a comprehensive range of ground-based observations across the Snowy Mountains. We used data from three vertically pointing rain radars, cloud radar, a PARSIVEL disdrometer, and a network of 76 pluviometers. The observations reveal that all of the precipitation events were associated with cold front passages. About half accumulated during the frontal passage associated with deep, fully glaciated cloud tops; while the rest occurred in the post-frontal environment and was associated with clouds with supercooled liquid water (SLW) tops. About three quarters of the accumulated precipitation were observed under blocked conditions, likely associated with blocked stratiform orographic enhancement. Specifically, more than a third of the precipitation resulted from moist cloudless air being lifted over stagnant air, upwind from the barrier, creating SLW-top clouds. These SLW-clouds then produced stratiform precipitation mostly over the upwind slopes and mountain tops, with hydrometeors reaching the mountain tops mostly as rimed snow. Two precipitation events were studied in detail, which showed that during unblocked conditions, orographic convection invigoration and unblocked stratiform enhancement were the two main mechanisms driving the precipitation; with the latter being more prevalent after the frontal passage. During these events, ice particle growth was likely dominated by vapor deposition and aggregation during the frontal periods, while riming dominated during the post-frontal periods.

First Doppler lidar and cloud radar measurements of orographic convection initiation from a mountain observatory in the Al Hajar Mountains of the United Arab Emirates.

<p>We present exciting Doppler lidar and cloud radar measurements from a high-vantage mountain observatory in the hyper-arid United Arab Emirates (UAE) - initiated as part of the UAE Research Program for Rain Enhancement Science (UAEREP). The observatory was designed to study the clear-air pre-convective environment and subsequent convective events in the arid Al Hajar Mountains, with the overarching goal of improving understanding and nowcasting of seedable orographic clouds. During summer in the Al Hajar Mountains (June to September), weather processes are often complex, with summer convection being initiated by several phenomena acting in concert, e.g., interaction between sea breeze and horizontal convective rolls. These interactions can combine to initiate sporadic convective storms and these can be intense enough to cause flash floods and erosion. Such events here are influenced by mesoscale phenomena like the low-level jet and local sea breeze, and are constrained by larger-scale synoptic conditions.</p><p>The Doppler lidar and cloud radar were employed for approximately two years at a high vantage-point to capture valley wind flows and observe convective cells. The instruments were configured to run synchronized polar (PPI) scans at 0°, 5°, and 45° elevation angles and vertical cross-section (RHI) scans at 0°, 30°, 60, 90°, 120°, and 150° azimuth angles. Using this imagery, along with local C-band radar and satellite data, we were able to identify and analyze several convective cases. To illustrate our results, we have selected two cases under unstable conditions - the 5 and 6 September 2018. In both cases, we observed areas of low-level convergence/divergence, particularly associated with wind flow around a peak 2 km to the south-west of the observatory. The extension of these deformations are visible in the atmosphere to a height of 3 km above sea level. Subsequently, we observed convective cells developing at those approximate locations – apparently initiated because of these phenomena. The cloud radar images provided detailed observations of cloud structure, evolution, and precipitation. In both convective cases, pre-convective signatures were apparent before CI, in the form of convergence, wind shear structures, and updrafts.</p><p>These results have demonstrated the value of synergetic observations for understanding orographic convection initiation, improvement of forecast models, and cloud seeding guidance. The manuscript based on these results is now the subject of a peer review (Branch et al., 2021).</p><p> </p><p>Branch, O., Behrendt, Andreas Alnayef, O., Späth, F., Schwitalla, Thomas, Temimi, M., Weston, M., Farrah, S., Al Yazeedi, O., Tampi, S., Waal, K. de and Wulfmeyer, V.: The new Mountain Observatory of the Project “Optimizing Cloud Seeding by Advanced Remote Sensing and Land Cover Modification (OCAL)” in the United Arab Emirates: First results on Convection Initiation, J. Geophys. Res.  Atmos., 2021. In review (submitted 23.11.2020).</p>

Numerical Simulations of Orographic Convection across Multiple Gray Zones

Idealized simulations are used to determine the sensitivity of moist orographic convection to horizontal grid spacing Δh. In simulated mechanically (MECH) and thermally (THERM) forced convection over an isolated ridge, Δh is varied systematically over both the deep-convection (Δh ~ 10–1 km) and turbulence (Δh ~ 1 km–100 m) gray zones. To aid physical interpretation, a new parcel-based bulk entrainment/detrainment diagnosis for horizontally heterogeneous flows is developed. Within the deep-convection gray zone, the Δh sensitivity is dominated by differences in parameterized versus explicit convection; the former initiates convection too far upstream of the ridge (MECH) and too early in the diurnal heating cycle (THERM). These errors stem in part from a large underprediction of parameterized entrainment and detrainment. Within the turbulence gray zone, sensitivities to Δh arise from the representation of both subcloud- and cloud-layer turbulence. As Δh is decreased, MECH exhibits stronger cloud-layer entrainment to enhance the convective mass flux Mco, while THERM exhibits stronger detrainment to suppress Mco and delay convection initiation. The latter is reinforced by increased subcloud turbulence at smaller Δh, which leads to drying and diffusion of the central updraft responsible for initiating moist convection. Numerical convergence to a robust solution occurs only in THERM, which develops a fully turbulent flow with a resolved inertial subrange (for Δh ≤ 250 m). In MECH, by contrast, turbulent transition occurs within the orographic cloud, the details of which depend on both physical location and Δh.

DISTRIBUTION OF CLOUDINESS ON THE CENTRAL PLATEAU OF MOLDAVIA

The distribution of cloudiness on the Central Plateau of Moldavia is almost similar with that of other hilly areas of Romania, although the mentioned area are located in the east of Romania, where continental influences are present. In the Central Moldovan Plateau. The values of the cloudiness are higher than in the south-eastern areas of Romania, due to specific conditions. In winter the values are raised by the lower temperatures, sometimes generated by the presence of the Euro-Asian Anticyclone and by the high frequency of the thermal inversions. In addition, there is an intensification of the passages of Mediterranean cyclones, often above the anticyclonic cold air. At the end of spring and in the first half of summer, the western circulation intensifies, to the north of the Carpathians, as it enters the studied area. The Central Plateau of Moldavia is a relatively high area, located in the front of the predominant north-western local circulation, a fact that produces slight orographic convection in the case of north component circulations. The annual average of cloudiness is heavily influenced by the genesis of stratiform clouds. The multiannual regime shows decreases of the values. The annual regime is similar for entire Central Plateau of Moldova. In the Central Plateau of Moldavia, both the total and low cloudiness recorded has the highest average in December and the lowest average in August. Total hourly cloudiness recorded two main classes of values between 4-6, with presence in the months of warm semester and 6-8 tenths, characteristic of the cold semester. The low cloudiness presents, in the territory, small differencies for the annual averages and higher differencies than the total cloudiness regime. Diurnal, the low cloudiness exceeds 6 tenths only in the mornings of December, in Vaslui and values below 2 tenths occur only on the nights of June, July, August and September in whole area. The values in the territory are similar, only in the high areas we can see a slight increase in the number of clear sky days, with a corresponding decrease in the number of cloudy and covered days.

Dual-Polarization Radar Data Analysis of the Impact of Ground-Based Glaciogenic Seeding on Winter Orographic Clouds. Part II: Convective Clouds

This second paper of a two-part series aims to explore the ground-based glaciogenic seeding impact on wintertime orographic clouds using an X-band dual-polarization radar. It focuses on three cases with shallow to moderately deep orographic convection that were observed in January–February of 2012 as part of the AgI Seeding Cloud Impact Investigation (ASCII) project over the Sierra Madre in Wyoming. In each of the storms the bulk upstream Froude number exceeded 1, suggesting unblocked flow. Low-level potential instability was present, explaining orographic convection. The clouds contained little supercooled liquid water on account of the low cloud-base temperature. Ice-crystal photography shows that snow mainly grew by diffusion and aggregation. To examine the seeding effect of silver iodide (AgI), five study areas are defined: two target areas and three control areas. Comparisons are made between the control and target areas as well as between a treated, or seeded, period and an untreated period. Low-level reflectivity tends to increase in the target areas relative to the control. This increase is larger in the lee target area than in the upwind target area, suggesting that precipitation enhancement is delayed in the presence of convection. The echo tops of the convective cells are not higher during seeding, relative to simultaneous changes in the control regions. This result suggests that the dynamic-seeding mechanism does not apply for the cold-base convective clouds that are studied here. An analysis of differential reflectivity and snow photography suggests that static seeding is the more likely snow-enhancement mechanism in these clouds.

Wind Speed Control of Tropical Orographic Convection

This study compares observations from the Dominica Experiment (DOMEX) field campaign with 3D and 2D Weather Research and Forecasting Model (WRF) simulations to understand how ambient upstream wind speed controls the transition from thermally to mechanically forced moist orographic convection. The environment is a conditionally unstable, tropical atmosphere with shallow trade wind cumulus clouds. Three flow indices are defined to quantify the convective transition: horizontal divergence aloft, cloud location, and island surface temperature. As wind speed increases, horizontal airflow divergence from plume detrainment above the mountain changes to convergence associated with plunging flow, convective clouds relocate from the leeward to the windward side of the mountain as mechanically triggered convection takes over, and the daytime mountaintop temperature decreases because of increased ventilation and cloud shading. Possible mechanisms by which wind speed controls island precipitation are also discussed. The result is a clearer understanding of orographic convection in the tropics.