Realistic Simulation of Tropical Atmospheric Gravity Waves Using Radar-Observed Precipitation Rate and Echo Top Height

Gravity waves (GWs) generated by tropical convection are important for the simulation of large-scale atmospheric circulations, for example, the quasi-biennial oscillation (QBO), and small-scale phenomena like clear-air turbulence. However, the simulation of these waves still poses a challenge due to the inaccurate representation of convection, and the high computational costs of global, cloud-resolving models. Methods combining models with observations are needed to gain the necessary knowledge on GW generation, propagation, and dissipation so that we may encode this knowledge into fast parameterized physics for global weather and climate simulation or turbulence forecasting. We present a new method suitable for rapid simulation of realistic convective GWs. Here, we associate the profile of latent heating with two parameters: precipitation rate and cloud top height. Full-physics cloud-resolving WRF simulations are used to develop a lookup table for converting instantaneous radar precipitation rates and echo top measurements into a high-resolution, time-dependent latent heating field. The heating field from these simulations is then used to force an idealized dry version of the WRF model. We validate the method by comparing simulated precipitation rates and cloud tops with scanning radar observations and by comparing the GW field in the idealized simulations to satellite measurements. Our results suggest that including variable cloud top height in the derivation of the latent heating profiles leads to better representation of the GWs compared to using only the precipitation rate. The improvement is especially noticeable with respect to wave amplitudes. This improved representation also affects the forcing of GWs on large-scale circulation.

Elliptical Structures of Gravity Waves Produced by Typhoon Soudelor in 2015 near Taiwan

Tropical cyclones (TCs) are complex sources of atmospheric gravity waves (GWs). In this study, the Weather Research and Forecasting Model was used to model TC Soudelor (2015) and the induced elliptical structures of GWs in the upper troposphere (UT) and lower stratosphere (LS) prior to its landfall over Taiwan. Conventional, spectral and wavelet analyses exhibit dominant GWs with horizontal and vertical wavelengths, and periods of 16–700 km, 1.5–5 km, and 1–20 h, respectively. The wave number one (WN1) wind asymmetry generated mesoscale inertia GWs with dominant horizontal wavelengths of 100–300 km, vertical wavelengths of 1.5–2.5 km (3.5 km) and westward (eastward) propagation at the rear of the TC in the UT (LS). It was also revealed to be an active source of GWs. The two warm anomalies of the TC core induced two quasi-diurnal GWs and an intermediate GW mode with a 10-h period. The time evolution of dominant periods could be indicative of changes in TC dynamics. The FormoSat-3/COSMIC (Formosa Satellite Mission-3/Constellation Observing System for Meteorology, Ionosphere, and Climate) dataset confirmed the presence of GWs with dominant vertical wavelengths of about 3.5 km in the UT and LS.

A molecular perspective for global modeling of upper atmospheric NH3 from freezing clouds

Ammonia plays a key role in the neutralization of atmospheric acids such as sulfate and nitrates. A few in situ observations have supported the theory that gas-phase NH₃ concentrations should decrease sharply with altitude and be extremely low in the upper troposphere and lower stratosphere (UTLS). This theory, however, seems inconsistent with recent satellite measurements and is also not supported by the aircraft data showing highly or fully neutralized sulfate aerosol particles by ammonium in the UTLS in many parts of the world. Here we reveal the contributions of deep convective clouds to NH₃ in the UTLS by using integrated cross-scale modeling, which includes molecular dynamic simulations, a global chemistry transport model, and satellite and aircraft measurements. We show that the NH₃ dissolved in liquid cloud droplets is prone to being released into the UTLS upon freezing during deep convection. Because NH₃ emission is not regulated in most countries and its future increase is likely persistent from agricultural growth and the warmer climate, the effect of NH₃ on composition and phase of aerosol particles in the UTLS can be significant, which in turn can affect cirrus cloud formation, radiation, and the budgets of NOx and O₃.