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

During the Dynamics of the MJO (DYNAMO) field campaign, radiosonde launches were regularly conducted from three small islands/atolls (Male and Gan, Maldives, and Diego Garcia, British Indian Ocean Territory) as part of a large-scale sounding network. Comparison of island upsondes with 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-December 2011 period of DYNAMO. As a result of this procedure, the adjusted diurnal cycle amplitude of surface temperature is reduced fivefold, resembling that over the ocean, and low-level wind speeds are increased in similar to 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, whereas convective parameters are sensitive to the adjustments in thermodynamic fields. Although the impact of the adjustments is generally small (on the order of a few percent), intraseasonal wind regime changes result in some systematic variations in divergence and vertical motion over the sounding arrays.

Automated summary

During the DYNAMO field campaign, radiosonde launches from small islands were compared with dropsondes over the ocean, revealing the islands' influence on the atmosphere and their impact on upsonde profiles, especially in the lowest 200 meters. Adjusting the upsonde profiles to resemble atmospheric structures over the open ocean led to significant changes in temperature diurnal cycle amplitude and low-level wind speeds. These adjustments primarily affected dynamical and budget fields through wind adjustments, while convective parameters were sensitive to changes in thermodynamic fields. The adjustments, albeit small, resulted in systematic variations in divergence and vertical motion over the sounding arrays due to intraseasonal wind regime changes.