Atmosphere–Land/Ocean Interactions

The atmosphere constantly exchanges heat, moisture and momentum with the land and water beneath it. I study this coupling at the convective scale: how surface heat fluxes shape the cold pools that spread beneath thunderstorms, how cold pools in turn cool the ground and the soil, and how large lakes change the precipitation climate around them. The cold pool work was done at the Max Planck Institute for Meteorology, with storm-resolving simulations and observation network data from the FESSTVaL campaign.

Figure. Cold pools and the surface: thermal and moisture fluxes between the cold pool and the surface can weaken its strength and propagation.

Cold pools in storm-resolving simulations

Using the DYAMOND global storm-resolving simulations over Germany (1 August–10 September 2016), I detected cold pool clusters with a buoyancy threshold and classified them by terrain height and by the size of the accompanying cloud. Cold pools occur more often over high terrain, and the size of cold pools under small clouds is closely tied to convective intensity and to surface sensible and latent heat fluxes. An idealized model that treats a cold pool cluster as cold air released continuously by convective precipitation, weakened by surface heat fluxes and entrainment, reproduces the mean cold pool radius in ICON.

Figure. Animation: cold pool clusters (colored contours) detected in the DYAMOND simulations over Germany, with precipitation (shading) and cloud cover (dots): (a) cold pools under a large precipitating system, (b) small, thermally induced cold pools and (c) cold pools tied to orographic precipitation over the Alps (top), and example cold pool clusters classified by terrain height and cloud size in ICON and NICAM (bottom).

FESSTVaL

The Field Experiment on Submesoscale Spatio-Temporal Variability in Lindenberg (17 May–27 Aug 2021) deployed more than 100 near-surface stations, X-band radar, Doppler lidars and energy-balance instruments to observe cold pools and gust fronts.

Figure. FESSTVaL measurement network around Lindenberg, Germany.

A cold pool over the station network

On 29 June 2021 clustered convective cells produced a strong cold pool over the FESSTVaL network, with a 2-m temperature drop of up to 11 K. As the cold pool passes, 2-m and skin temperatures fall across the network; the soil temperature drops clearly where the soil water content jumps with the rain, but much less where the soil stays dry.

Figure. Animation: (a) 2-m temperature, (b) skin temperature, (c) soil temperature and (d) soil water content at the FESSTVaL stations, with rain rate (shading), 29 June 2021.

Soil temperature cooling

Many soil temperatures decreased during cold pool events when the soil water content also jumped. A conceptual thermal diffusion model shows that one possible reason is the increase of thermal diffusivity as soil water content rises from very low (about 5%) to threshold level (about 20%).

Figure. Soil water content (top) and 2-m and soil temperatures (bottom) at the Falkenberg supersite for the Jogi case (29 June 2021, no jump in soil water content) and the Robinson & Freitag cases (9 July 2021, with a jump).

Lake Victoria: the diurnal cycle of precipitation

Lake Victoria, the second-largest freshwater lake in the world, supplies moisture to East Africa, yet its lake effect had received little attention. Using 1-km WRF simulations for October 2015 with the real lake (LAKE), with the lake replaced by savanna (NOLAKE) and with a lake of constant depth (CONSTANT), we examined why precipitation over the lake peaks in the early morning. After sunset the lake stays warmer than the surrounding land, so the land breeze converges over the western part of the lake; strong ascent there lets convection grow up to the tropopause, and melting snow and graupel produce the 06–10 LT precipitation maximum. After sunrise the land becomes warmer, air diverges from the lake, convection over the lake is suppressed and storms develop around its shores. Without the lake, the strong western precipitation and its clear diurnal cycle disappear, and the lake bathymetry also affects the response.

Figure. Diurnal cycle of precipitation rate (shading) and 10-m wind over Lake Victoria in October 2015, with the real lake (LAKE, left) and with the lake replaced by savanna (NOLAKE, right), from 1-km WRF simulations (Bok, Seo and Baik, KMS 2018).

Lake Baikal: summertime precipitation

Large lakes also reshape the atmosphere above them. With colleagues, I examined Lake Baikal using satellite observations, reanalysis and two 15-year cloud-resolving simulations, one with the lake replaced by forest. The lake cools the daytime near-surface air and stabilizes the atmosphere, weakening convection, latent heat flux and moisture convergence, and reduces summertime precipitation over it by 15% (Lee et al. 2023, GRL).

Figure. Summertime mean precipitation around Lake Baikal from satellite products, their ensemble, ERA5 and WRF (Lee et al. 2023).