Small Island Effects in DYNAMO and their Impact on Large-Scale Budget Analyses

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80524
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Abstract

During the DYNAMO (Dynamics of the MJO) field campaign, radiosonde launches were regularly conducted from three small islands/atolls (Malé, Gan, and Diego Garcia) as part of a large-scale sounding network. Comparison of island upsondes to nearby and near-contemporaneous dropsondes over the ocean provides evidence for the magnitude and scope of the islands’ influence on the surrounding atmosphere, and on the island upsonde profiles. The island’s impact on the upsonde data is most prominent in the lowest 200 m. Noting that the vertical gradients of temperature, moisture, and winds over the ocean are generally constant in the lowest 0.5 km of dropsondes, a simple procedure was constructed to adjust the upsonde profiles in the lowest few hundred meters to resemble the atmospheric structures over the open ocean. This procedure was applied to the soundings from the three islands mentioned above for the October to December 2011 period of DYNAMO. As a result of this procedure, the adjusted diurnal cycle amplitude of surface temperature is reduced five-fold, resembling that over the ocean, and low-level wind speeds are increased in ~90% of the island soundings. Examination of the impact of these sounding adjustments shows that dynamical and budget fields are primarily affected by adjustments to the wind field, while convective parameters are sensitive to the adjustments in thermodynamic fields. Although the impact of the adjustments is generally small (order of a few percent), intraseasonal wind regime changes result in some systematic variations in divergence and vertical motion over the sounding arrays.

This article is included in the DYNAMO/CINDY/AMIE/LASP: Processes, Dynamics, and Prediction of MJO Initiation special collection.

Corresponding author address: Paul E. Ciesielski, Department of Atmospheric Science, 1371 Campus Delivery, Colorado State University, Fort Collins, CO 80523. Email: paulc@atmos.colostate.edu

Abstract

During the DYNAMO (Dynamics of the MJO) field campaign, radiosonde launches were regularly conducted from three small islands/atolls (Malé, Gan, and Diego Garcia) as part of a large-scale sounding network. Comparison of island upsondes to nearby and near-contemporaneous dropsondes over the ocean provides evidence for the magnitude and scope of the islands’ influence on the surrounding atmosphere, and on the island upsonde profiles. The island’s impact on the upsonde data is most prominent in the lowest 200 m. Noting that the vertical gradients of temperature, moisture, and winds over the ocean are generally constant in the lowest 0.5 km of dropsondes, a simple procedure was constructed to adjust the upsonde profiles in the lowest few hundred meters to resemble the atmospheric structures over the open ocean. This procedure was applied to the soundings from the three islands mentioned above for the October to December 2011 period of DYNAMO. As a result of this procedure, the adjusted diurnal cycle amplitude of surface temperature is reduced five-fold, resembling that over the ocean, and low-level wind speeds are increased in ~90% of the island soundings. Examination of the impact of these sounding adjustments shows that dynamical and budget fields are primarily affected by adjustments to the wind field, while convective parameters are sensitive to the adjustments in thermodynamic fields. Although the impact of the adjustments is generally small (order of a few percent), intraseasonal wind regime changes result in some systematic variations in divergence and vertical motion over the sounding arrays.

This article is included in the DYNAMO/CINDY/AMIE/LASP: Processes, Dynamics, and Prediction of MJO Initiation special collection.

Corresponding author address: Paul E. Ciesielski, Department of Atmospheric Science, 1371 Campus Delivery, Colorado State University, Fort Collins, CO 80523. Email: paulc@atmos.colostate.edu
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