skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


  • NSF Public Access
  • Search Results
  • Aircraft observations of gravity wave activity and turbulence in the tropical tropopause layer: prevalence, influence on cirrus clouds, and comparison with global storm-resolving models
Title: Aircraft observations of gravity wave activity and turbulence in the tropical tropopause layer: prevalence, influence on cirrus clouds, and comparison with global storm-resolving models
Abstract. The tropical tropopause layer (TTL) is a sea of vertical motions. Convectively generated gravity waves create vertical winds on scales of a few to thousands of kilometers as they propagate in a stable atmosphere. Turbulence from gravity wave breaking, radiatively driven convection, and Kelvin–Helmholtz instabilities stirs up the TTL on the kilometer scale. TTL cirrus clouds, which moderate the water vapor concentration in the TTL and stratosphere, form in the cold phases of large-scale (> 100 km) wave activity. It has been proposed in several modeling studies that small-scale (< 100 km) vertical motions control the ice crystal number concentration and the dehydration efficiency of TTL cirrus clouds. Here, we present the first observational evidence for this. High-rate vertical winds measured by aircraft are a valuable and underutilized tool for constraining small-scale TTL vertical wind variability, examining its impacts on TTL cirrus clouds, and evaluating atmospheric models. We use 20 Hz data from five National Aeronautics and Space Administration (NASA) campaigns to quantify small-scale vertical wind variability in the TTL and to see how it varies with ice water content, distance from deep convective cores, and height in the TTL. We find that 1 Hz vertical winds are well represented by a normal distribution, with a standard deviation of 0.2–0.4 m s−1. Consistent with a previous observational study that analyzed two out of the five aircraft campaigns that we analyze here, we find that turbulence is enhanced over the tropical west Pacific and within 100 km of convection and is most common in the lower TTL (14–15.5 km), closer to deep convection, and in the upper TTL (15.5–17 km), further from deep convection. An algorithm to classify turbulence and long-wavelength (5 km < λ < 100 km) and short-wavelength (λ < 5 km) gravity wave activity during level flight legs is applied to data from the Airborne Tropical TRopopause EXperiment (ATTREX). The most commonly sampled conditions are (1) a quiescent atmosphere with negligible small-scale vertical wind variability, (2) long-wavelength gravity wave activity (LW GWA), and (3) LW GWA with turbulence. Turbulence rarely occurs in the absence of gravity wave activity. Cirrus clouds with ice crystal number concentrations exceeding 20 L−1 and ice water content exceeding 1 mg m−3 are rare in a quiescent atmosphere but about 20 times more likely when there is gravity wave activity and 50 times more likely when there is also turbulence, confirming the results of the aforementioned modeling studies. Our observational analysis shows that small-scale gravity waves strongly influence the ice crystal number concentration and ice water content within TTL cirrus clouds. Global storm-resolving models have recently been run with horizontal grid spacing between 1 and 10 km, which is sufficient to resolve some small-scale gravity wave activity. We evaluate simulated vertical wind spectra (10–100 km) from four global storm-resolving simulations that have horizontal grid spacing of 3–5 km with aircraft observations from ATTREX. We find that all four models have too little resolved vertical wind at horizontal wavelengths between 10 and 100 km and thus too little small-scale gravity wave activity, although the bias is much less pronounced in global SAM than in the other models. We expect that deficient small-scale gravity wave activity significantly limits the realism of simulated ice microphysics in these models and that improved representation requires moving to finer horizontal and vertical grid spacing.  more » « less
Award ID(s):
1743753
PAR ID:
10466373
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Atmospheric Chemistry and Physics
Volume:
23
Issue:
7
ISSN:
1680-7324
Page Range / eLocation ID:
4009 to 4030
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; (, Journal of Geophysical Research: Atmospheres)
    Abstract Observations from the third campaign of the National Aeronautics and Space Administration Airborne Tropical Tropopause Experiment (ATTREX 3) field mission and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations satellite mission are used to evaluate simulations of tropical tropopause layer (TTL) cirrus clouds in the Community Earth System Model's (CESM) Community Atmosphere Model, CAM5. In this study, CAM5 is coupled with a sectional ice cloud model, the Community Aerosol and Radiation Model for Atmospheres (CARMA). We find that both model variants underrepresent cloud frequency along the ATTREX 3 flight path and both poorly represent relative humidity in the TTL. Furthermore, simulated in‐cloud ice size distributions contained erroneous amounts of ice crystals throughout the distribution. In response, we present a modified ice cloud fraction scheme that boosts the cloud fraction within the TTL. Due to coarse vertical model resolution in the TTL, we also prescribe a 2‐K decrease in cold point tropopause temperatures to better align with observed temperatures. Our modifications improve both CAM5 and CAM5/CARMA's in‐cloud ice size and mass distributions. However, only CAM5/CARMA has a significant improvement in cloud frequency and relative humidity. An investigation of cloud extinction in the ATTREX 3 region found that each model variant struggles to reproduce observed extinctions. As a first‐order approximation, we introduce randomly generated temperature perturbations to simulate the effect of gravity waves into the CAM5/CARMA simulation. These gravity waves significantly increase the incidence of low extinction (<0.02 km−1) values, ice cloud fraction between 16 and 18 km, and ice crystal smaller than 100‐μm concentrations but provided only small changes to high extinction values. 
    more » « less
  2. ; ; ; ; ; ; ; ; (, Geophysical Research Letters)
    Abstract Atmospheric waves in the tropical tropopause layer are recognized as a significant influence on processes that impact global climate. For example, waves drive the quasi‐biennial oscillation (QBO) in equatorial stratospheric winds and modulate occurrences of cirrus clouds. However, the QBO in the lower stratosphere and thin cirrus have continued to elude accurate simulation in state‐of‐the‐art climate models and seasonal forecast systems. We use first‐of‐their‐kind profile measurements deployed beneath a long‐duration balloon to provide new insights into impacts of fine‐scale waves on equatorial cirrus clouds and the QBO just above the tropopause. Analysis of these balloon‐borne measurements reveals previously uncharacterized fine‐vertical‐scale waves (<1 km) with large horizontal extent (>1000 km) and multiday periods. These waves affect cirrus clouds and QBO winds in ways that could explain current climate model shortcomings in representing these stratospheric influences on climate. Accurately simulating these fine‐vertical‐scale processes thus has the potential to improve sub‐seasonal to near‐term climate prediction. 
    more » « less
  3. ; ; ; (, Atmospheric research)
    The surface wind structure and vertical turbulent transport processes in the eyewall of hurricane Isabel (2003) are investigated using six large-eddy simulations (LESs) with different horizontal grid spacing and three-dimensional (3D) sub-grid scale (SGS) turbulent mixing models and a convection permitting simulation that uses a coarser grid spacing and one-dimensional vertical turbulent mixing scheme. The mean radius-height distribution of storm tangential wind and radial flow, vertical velocity structure, and turbulent kinetic energy and momentum fluxes in the boundary layer generated by LESs are consistent with those derived from historical dropsonde composites, Doppler radar, and aircraft measurements. Unlike the convection permitting simulation that produces storm wind fields lacking small-scale disturbances, all LESs are able to produce sub-kilometer and kilometer scale eddy circulations in the eyewall. The inter-LES differences generally reduce with the decrease of model grid spacing. At 100-m horizontal grid spacing, the vertical momentum fluxes induced by the model-resolved eddies and the associated eddy exchange coefficients in the eyewall simulated by the LESs with different 3D SGS mixing schemes are fairly consistent. Although with uncertainties, the decomposition in terms of eddy scales suggests that sub-kilometer eddies are mainly responsible for the vertical turbulent transport within the boundary layer (~1 km depth following the conventional definition) whereas eddies greater than 1 km become the dominant contributors to the vertical momentum transport above the boundary layer in the eyewall. The strong dependence of vertical turbulent transport on eddy scales suggests that the vertical turbulent mixing parameterization in mesoscale simulations of tropical cyclones is ultimately a scale-sensitive problem. 
    more » « less
  4. ; ; ; (, Journal of Applied Meteorology and Climatology)
    The NOAA G-IV aircraft routinely measures vertical aircraft acceleration from the inertial navigation system at 1 Hz. The data provide a measure of turbulence on a 250-m horizontal scale over a layer from 12.8- to 14.8-km elevation. Turbulence in this layer of tropical cyclones was largest by 35%–40% in the inner 200 km of radius and decreased monotonically outward to the 1000-km radius. Turbulence in major hurricanes exceeded that in weaker tropical cyclones. Turbulence data points were divided among three regions of the tropical cyclone: cirrus canopy; outside the cirrus canopy; and a transition zone between them. Without exception, turbulence was greater within the canopy and weaker outside the canopy. Nighttime turbulence exceeded daytime turbulence for all radii, especially within the cirrus canopy, implicating radiative forcing as a factor in turbulence generation. A case study of widespread turbulence in Hurricane Ivan (2004) showed that interactions between the hurricane outflow channel and westerlies to the north created a region of absolute vorticity of −6 × 10−5s−1in the upper troposphere. Outflow accelerated from the storm center into this inertially unstable region, and visible evidence for turbulence and transverse bands of cirrus appeared radially inward of the inertially unstable region. It is argued that both cloud-radiative forcing and the development of inertial instability within a narrow outflow layer were responsible for the turbulence. In contrast, a second case study (Isabel 2003) displayed strong near-core turbulence in the presence of large positive absolute vorticity and no local inertial instability. Peak turbulence occurred 100 km downwind of the eyewall convection. 
    more » « less
  5. ; ; ; ; ; (, EGUSphere)
    Abstract. Tropical cirrus clouds play a critical role in the climate system and are a major source of uncertainty in our understanding of global warming. Tropical cirrus are affected by processes spanning a wide range of spatial and temporal scales, from ice microphysics on cloud scales to mesoscale convective organization and planetary wave dynamics. This complexity makes tropical cirrus clouds notoriously difficult to model and has left many important questions stubbornly unanswered. At the same time, their multi-scale nature makes them well positioned to benefit from the rise of global, high-resolution simulations of Earth's atmosphere and a growing abundance of remotely sensed and in situ observations. Rapid progress requires coordinated efforts to take advantage of these modern computational and observational abilities. In this Opinion, we review recent progress in cirrus studies, highlight important questions that remain unanswered, and discuss promising paths forward. We find that significant progress has been made in understanding the life cycle of convectively generated ``anvil cirrus and how their macrophysical properties respond to large-scale controls. On the other hand, much work remains to be done to understand how small-scale anvil processes and the climatological anvil radiative effect may respond to global warming. Thin, in situ-formed cirrus are now known to be closely tied to the thermal structure and humidity of the tropical tropopause layer (TTL), but uncertainty at the microphysical scale remains a significant barrier to understanding how these clouds regulate the TTL moisture and temperature budgets, as well as the mixing ratio of water vapor entering the stratosphere. Model representation of ice-nucleating particles, water vapor supersaturation, and ice depositional growth continue to pose great challenges to cirrus modeling. We believe that major advances in the understanding of tropical cirrus can be made through a combination of cross-tool synthesis and cross-scale studies conducted by cross-disciplinary research teams. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email