May 1, 2017

More than a boundary condition…

I’m surprised I need to say this but the land surface is more than just a lower boundary condition. The impression I get from the forecast community is “hey as long as you give me the ‘correct’ fluxes into the atmosphere then we don’t care what else is happening”. Okay fair enough to some extent but I think this why we cannot get over the 1 week-ish forecast hurdle and also miss where convection pops up. Take the second thing, convective initiation. What is convection anyway? Well it begins with density gradients that then cause a bulk motion so that the less dense fluid can lay overtop the more dense fluid. For the atmosphere we are generally talking about sunlight heating the surface which causes a low density heat bubble. The heat bubble rises and if it has enough moisture and cools enough as it rises adiabatically then BAM you […]
December 26, 2016

Time Lapse Cloud Fun

While I wish that picture in the thumbnail was something I saw or took a picture of, it isn’t. But about two months ago I started taking some time lapse videos from my window at NCAR. I mean I spend some portion of my day actually looking at storm systems develop, and plumes of smoke from controlled burns clearly identify the boundary, why should I just take a video. Well here are two such videos. One is just some fair weather cumulus forming right off the front range and the other is a generally overcast sky with some lenticular clouds forming. [one_half]Fair Weather Cumulus [tvideo type=”youtube” clip_id=”y–1rYobyV4″ autoplay=”false” controls=”true” modestbranding=”false”] [/one_half] [one_half_last] Lenticular Clouds [tvideo type=”youtube” clip_id=”f8pa-DG2P6g” autoplay=”false” controls=”true” modestbranding=”false”] [/one_half_last]
December 15, 2016

“If I had the time”: Ground-level Ozone Edition

I know a lot of people feel that if they just had a few more hours in the day they could do this thing they have been meaning to do. Well that’s what this is about. I figure I’d just put the thing I want to do out there and the step-by-step process for how to do it with the hope that someone might just do it. The Problem How do you know whether the ground-level ozone for a given day comes from that days photochemistry or was just pulled down from the prior day’s residual layer? This question is a bit tricky when dealing with observations but I’ve been meaning to test an approach to answer this question. The way is to reframe this heated condensation framework methodology I came up with a few years back. The basic idea is if you have a vertical profile of ozone mixing […]
December 13, 2016

Chicken and Egg: Where to Begin when talking Flux-PBL Feedbacks

I am in the process of coming up with an equation that would isolate the contributions of the surface turbulent fluxes (latent and sensible heat) to convective initiation. The issue is how circular the logic is when talking about boundary layers (BL or PBL) and surface fluxes in general. Specifically sensible heat flux into the boundary layer. The sort of “classical” view is surface heating promotes the growth of a layer close to the Earth’s surface. Well then what’s the problem? There is heat, it grows this boundary layer, and so its easy to determine how sensible heat flux contributes to PBL growth, right? It’s probably easier to explain the issue I’m having rather than talk in vague terms. Background of Problem First take one of my favorite techniques for exploring boundary layer evolution, the mixing diagram (MD) approach. Real quick, the MD approach uses hourly (or 3-hourly) temperature and […]
December 31, 2015

Convective Parameterizations: Fuzzy Terminology

The treatment of clouds in general circulation models was always a bit of a puzzle to me. I remember thinking way back in undergrad “why on Earth would we treat fair weather cumulus, stratiform clouds, and towering cumulus any differently in models? Aren’t they all just RH ~= 100 % means condensation?” Of course the explanation and correct one I got was that if a cloud is much smaller than the resolved model grid size, then we would need some way of putting that into the model so we don’t underestimate that scale. Makes sense. But then why do we call it a convective parameterization? I much prefer the older convention of calling it unresolved cloud parameterization. I don’t think this just a simple quibble over what to call things though. It changes how we fundamentally approach the problem. By calling it a convective parameterization we are conflating a numerical […]
October 27, 2015

Does Surface Energy Partitioning Make Sense?

I’ve been struggling lately with the basic concept of surface energy partitioning of net surface radiation. Mainly the struggle is whether it makes sense that incoming solar gets partitioned instantly into sensible, latent, and ground heat fluxes. Now even if we include some storage term for the land surface it still seems odd that evaporation is related to total incoming solar energy. Before I get into more specifics about my issue with this construct I should say what got me thinking about it in the first place. Aside from it not being sufficiently physical, I remember sitting at a talk by Martin Best from the UK Met office and he was presenting some work from the PLUMBER project (the acronym is too much for me to want to write it). He and co-authors showed that a simple 1-variable linear regression model could out perform all the “physically-based” land surface models […]
March 10, 2015

Moist Convection: initiation versus intensity feedbacks

I was reading/re-reading a few papers today and I noticed this constant interchangeable use of precipitation intensity and convective initiation when trying to quantify soil moisture-precipitation coupling and feedback. This seems odd because the physical mechanism for triggering convection is different than what may determine how intense precipitation is after initiation. The mechanism for triggering convection simply follows the basic meteorology class process, where the surface is heated during the day producing a well mixed atmospheric boundary layer (ABL). The basic structure of the ABL is a temperature maximum at the surface and decreasing at a rate following the dry adiabatic lapse rate (9.8 K/km), and a constant water vapor mixing ratio throughout the ABL. Because the water vapor mixing ratio is constant and the temperature drops with height, there is a relative humidity maximum at the top of the ABL. If the ABL grows high enough relative humidity can […]
August 12, 2014

Issues with using CAPE as a convective trigger

These are just some random thoughts about what using a CAPE-based convective trigger might mean in terms of positive and negative feedbacks. It seems odd to me that using the Community Atmosphere Model uses convective available potential energy (CAPE) as a criterion for whether or not to trigger the deep convection scheme. CAPE is certainly a good measure for convective intensity but as I was taught in my early meteorology courses teach, convective inhibition (CIN) is a more meaningful indicator convective triggering. The simple idea being when CIN is completely eroded (CIN = 0) convection begins. However, in the deep convection scheme in CAM convection is triggered when CAPE is greater than 70 J/kg, or in other words, when the energy available for convection is greater than what is easily found on a fair weather spring or summer day. So what weird behavior could this be causing in the model? […]