The Influence of Boundary Layer Mixing Strength on the Evolution of a Baroclinic Cyclone

Sub-grid scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of the 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0–0.25 within the YSU PBL scheme, as a way of directly altering the depth and magnitude of sub-grid scale turbulent mixing. Results suggest varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified ow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. Additionally, trajectory analysis indicates ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.

Addition of Multilayer Urban Canopy Models to a Nonlocal Planetary Boundary Layer Parameterization and Evaluation Using Ideal and Real Cases

The multilayer urban canopy models (UCMs) building effect parameterization (BEP) and BEP + building energy model (BEM; a building energy model integrated in BEP) are added to the Yonsei University (YSU) planetary boundary layer (PBL) parameterization in the Weather Research and Forecasting (WRF) Model. The additions allow for the first analysis of the detailed effects of buildings on the urban boundary layer in a nonlocal closure scheme. The modified YSU PBL parameterization is compared with the other 1.5-order local PBL parameterizations that predict turbulent kinetic energy (TKE), Mellor–Yamada–Janjić and Bougeault–Lacarerre, using both ideal and real cases. The ideal-case evaluation confirms that BEP and BEP+BEM produce the expected results in the YSU PBL parameterization because the simulations are qualitatively similar to the TKE-based PBL parameterizations in which the multilayer UCMs have long existed. The modified YSU PBL parameterization is further evaluated for a real case. Similar to the ideal case, there are larger differences among the different UCMs (simple bulk scheme, BEP, and BEP+BEM) than across the PBL parameterizations when the UCM is held fixed. Based on evaluation against urban near-surface wind and temperature observations for this case, the BEP and BEP+BEM simulations are superior to the simple bulk scheme for each PBL parameterization.

Sensitivity of the WRF – HAILCAST model to microphysics and PBL parameterization shemes

<p>Hail is a significant convective storm hazard in Croatia, often causing property and crop damage. The existing analysis, based on hailpad network data, shows that western and central regions of Croatia have a significant frequency of high-intensity hail events.</p><p>Advances in computational power and recent developments in atmospheric modeling have enabled the use of convection-permitting models (CPM) that can partially resolve deep convective events such as thunderstorms and rain showers. However, hail remains a difficult phenomenon to model or forecast since CPMs are still not able to fully resolve processes involved in producing hail. One way to address this issue is by embedding a physically-based one-dimensional hail model called HAILCAST within a CPM. Here, the HAILCAST model is embedded within the Weather Research and Forecasting (WRF) model.</p><p>The selected hail event is analyzed using WRF-HAILCAST model simulations. HAILCAST forecasts the maximum expected hail diameter using a profile of the vertical updraft, temperature, liquid and ice water content from a given WRF timestep and grid columns. Here, a set of numerical convection-permitting experiments are performed to assess the sensitivity of the results to different microphysics and planetary boundary layer (PBL) parameterization schemes and to provide guidance for WRF-HAILCAST tuning. The results are verified by observational (hailpad, hail observations) data as well as with radar, lightning and satellite measurements where available.</p>

The improvement of the planetary boundary layer simulation over the central Tibetan Plateau

<p><span lang="EN-US">The planetary boundary layer (PBL) parameterization schemes play a critical role in the weather and climate models, while they describe physical processes associated with the momentum, heat and humidity exchange between land surface and atmosphere. The sensitivity of boundary layer variables to eight PBL parameterization schemes (three non-local and five local closure schemes), available in the Weather Research and Forecasting(WRF) model, is evaluated over the central Tibetan Plateau with field measurements of the Third Tibetan Plateau atmospheric scientific experiment (TIPEX III) in July. Model results showed acceptable behavior, but no particular scheme produced the best performances for all observation stations and meteorological parameters. All PBL schemes underestimated the surface temperature over the central Tibetan Plateau. The BouLac scheme showed the minimum cold bias of the surface temperature. For the surface energy budget components, it was found that the sensible heat flux and the downward longwave radiation were the main factors causing the lower surface temperature. The sub-grid scale gravity wave drag was added to reduce biases result from unresolved topography over the central Tibetan Plateau. It led to smaller cold bias, causing warmer lower-tropospheric temperature, smaller water vapor content and higher PBL height. The modified model results show more close to the observation.</span></p> <p> </p>

Evaluation of an E–ε and Three Other Boundary Layer Parameterization Schemes in the WRF Model over the Southeast Pacific and the Southern Great Plains

To accurately simulate the atmospheric state within the planetary boundary layer (PBL) by PBL parameterization scheme in different regions with their dominant weather/climate regimes is important for global/regional atmospheric models. In this study, we introduce the turbulence kinetic energy (TKE) and TKE dissipation rate (ε) based 1.5-order closure PBL parameterization (E–ε, EEPS) in the Weather Research and Forecasting (WRF) Model. The performances of the newly implemented EEPS scheme and the existing Yonsei University (YSU) scheme, the University of Washington (UW) scheme, and Mellor–Yamada–Nakanishi–Niino (MYNN) scheme are evaluated over the stratocumulus dominated southeast Pacific (SEP) and over the Southern Great Plains (SGP) where strong PBL diurnal variation is common. The simulations by these PBL parameterizations are compared with various observations from two field campaigns: the Variability of American Monsoon Systems Project (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS) in 2008 over the SEP and the Land–Atmosphere Feedback Experiment (LAFE) in 2017 over the SGP. Results show that the EEPS and YSU schemes perform comparably over both regions, while the MYNN scheme performs differently in many aspects, especially over the SEP. The EEPS (MYNN) scheme slightly (significantly) underestimates liquid water path over the SEP. Compared with observations, the UW scheme produces the best PBL height over the SEP. The MYNN produces too high PBL height over the western part of the SEP while both the YSU and EEPS schemes produce too low PBL and cloud-top heights. The differences among the PBL schemes in simulating the PBL features over the SGP are relatively small.