Joint Seminar: Mesoscale circulation: Key to understanding mesoscale variability in tropical low-level cloudiness

Recent advances in measuring atmospheric circulation have provided new pathways to study the problem of clouds-circulation coupling at the mesoscale. At the core of my talk lie two questions: (a) What do we know about the meso-scale (~200 km) circulation of the trade-wind environment and (b) What can we say about its association with shallow cloudiness? Here, by circulation, I am interested in the derivatives of the motion field which define quantities such as divergence, vorticity, vertical/pressure velocity, etc. These are obtained from measurements made during field campaigns such as NARVAL2 and EUREC4A in the tropical north Atlantic. An important aspect of both campaigns was to derive the area-averaged estimates of the aforementioned motion-field parameters using dropsonde measurements taken along a circular flight path of around 200 km diameter. Findings from the NARVAL2 measurements suggest that vertical velocity in the moist layer (up to ~2 km) has a more direct control on the low-level cloudiness in these regions. A simplistic method using mass flux shows that vertical velocity at the lifting condensation level can be a useful determinant for explaining the variability in cloud-base cloudiness. EUREC4A measurements are used to characterise the circulation in the trades. I show that vertical velocity above the sub-cloud layer is quite consistent vertically when averaged over time. The value stays around ~1-1.5 hPa/h, which agrees well with the ~1.5 K/day cooling rate of the trades. However, significant intra- and inter-day variability can be found between vertical velocity profiles, in terms of the magnitudes, ranging from -7 hPa/h to 6 hPa/h as well as in terms of the vertical structure of these profiles. The characterisation of the circulation and its association with cloudiness, while improving our understanding of the clouds-circulation coupling, also point toward potential improvements to model the same.

Date

20.07.2021

Time

15:15 h

Place

Virtual Seminar
Hamburg

Speaker

Geet George

Organizers

Diego Jiménez-de-la-Cuesta Otero

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