Predicting the timing and location of convective cloud development is a fundamental challenge in the study of meteorology. Heterogeneities in the atmospheric boundary layer (ABL) particularly in ABL water vapor content and depth, lead to preferred locations for convective initiation. Land surface heterogeneity is an important cause of heterogeneity in the ABL. Hence, land-surface fluxes play an important role in ABL development and convective initiation. The minimum scale of land surface forcing that causes heterogeneity in ABL properties such as depth and water vapor content and consequently leads to preferred locations for cloud formation remains uncertain. Observations and models suggest scales ranging from a few to nearly 100 km.
This project proposes an observational plan as part of the International H2O Project (IHOP). The IHOP is a large multi-agency, multi-investigator project that focuses on the measurement of water vapor and water vapor variability. The goal of this project is to improve understanding of convective initiation, increase short-term precipitation forecast skills and test the capabilities of various instruments to measure the four dimensional characteristics of water vapor.
The critical observations for the research to be performed under this award are maps of surface fluxes of latent and sensible heat over a region of at least 300 x 300 km in Oklahoma and Kansas, and repeated airborne water vapor DIfferential Absorption Lidar (DIAL) observations of ABL depth and lower tropospheric water vapor. The flux maps will be created from a network of surface flux towers, airborne measurements of surface fluxes over repeated flight tracks about 50 km in length, satellite remote sensing of land surface temperature and vegetation cover, and a land surface model. One airborne DIAL will be coupled with an airborne Doppler Lidar, yielding the ability to observe ABL flux profiles via remote eddy covariance. Flights will be focused on the midday hours of 10-15 relatively fair weather days in order to capture the preconvective atmosphere with DIAL.
Observations will be analyzed to determine the degree of spatial heterogeneity in ABL water vapor depth and water budgets and the causes of this heterogeneity, focusing especially on determining the spatial scales at which land surface heterogeneity is an important factor. Ten to fifteen days of data will be analyzed in an attempt to move beyond a case study approach.
Data assimilation will be used to ingest dense, mesoscale observations into a high-resolution mesoscale atmospheric model that includes a sophisticated land-surface scheme. The model will be used as an analysis tool to study mesoscale surface-ABL-cloud interactions captured in the observations. Further the model will be used to assess the impacts of the DIAL observations, detailed land surface flux maps and a new shallow cumulus parameterization on forecasts of ABL heterogeneity and convective initiation.
Products will include ABL depth maps, ABL water budget estimates and model post-analysis fields incorporating all available IHOP observations for the 10-15 days of DIAL observations. Expected results include: an improved understanding of the role of land surface heterogeneity in convective ABL development and convective initiation; the degree to which model prediction of ABL development and moist convection can be improved via dense observations of ABL water vapor content and surface fluxes; and the impact of a shallow convection parameterization on model performance.
