A big reason the growth in mobile ad spend hasn't caught up to the growth in mobile audience size is that most mobile ads aren't very appealing to advertisers, particularly brand advertisers. Google is trying to change that.

The search giant has developed four new hard-to-overlook mobile units that aim to make the smaller screen more brand-friendly.

Three of the four new mobile ad units take over a smartphone's or tablet's entire screen, and two do so in a similar fashion to TV commercials.

The interstitial ad formats are "similar to what people are used to on TV" in that they pop up in breaks between what someone is doing in an app, said Jonathan Alferness, Google's director of product management for mobile display ads. "This idea that a user's in the middle of some environment [and] at a logical break point they should see or will see advertising is not particularly jarring to users," he added.

If they are jarred, people will be able to quickly hide or skip past any of the four ads, including its mobile version of TrueView video ads. Google brought the ad unit, which it first popularized on YouTube, to mobile apps last year, but limited it to gaming apps because interstitial videos were considered "perfectly acceptable" in games, Mr. Alferness said.

Since then Google has seen "video becoming an integral part of any experience on mobile," he said. As a result in the next quarter or two the TrueView ads will roll out to non-gaming apps that are part of Google's mobile in-app ad network AdMob.

Google's new mobile lightbox engagement ad throws the interactive kitchen sink at people, but only if they ask for it. The smaller-screen version of a desktop unit Google introduced two years ago, the unit can contain photo slideshows, YouTube videos and Google-powered maps that expand to full-screen when clicked. Advertisers only pay when someone clicks to interact with the expanded ad.

Not all of Google's new mobile ads are designed to overtake a phone's entire screen. An ad Google has been testing for mobile sites called Anchor latches only to the bottom of the screen -- but it persists there even as someone scrolls down a page.

"Being able to anchor a small banner ad at the bottom gives us the ability to bring some visibility, some attention back to ads that can get completely missed in a mobile web environment," Mr. Alferness said.

People will be able to click to hide the ad, but Google's is hoping that people click on the ad to go to the advertiser's landing page since the Anchor ads are priced primarily on a per-click basis, according to Mr. Alferness. The Anchor ad would appear to be an opportunity for Google to join publishers like the Financial Times and charge advertisers based on how long someone is exposed to the ad. But that won't be the case, at least for now.